Hearing aid device incorporating signal processing techniques

• US 6,072,885 A
• Filed: 08/22/1996
• Issued: 06/06/2000
• Est. Priority Date: 07/08/1994
• Status: Expired due to Term

• Current Assignee: Brigham Young University (The Church of Jesus Christ of Latter-Day Saints), Sonic Innovations Incorporated (William Demant Fonden)
• Original Assignee: Sonic Innovations Incorporated (William Demant Fonden)
• Inventors: Stockham, Thomas G. Jr., Mead, Carver A., Chabries, Douglas M.
• Primary Examiner(s): LE, HUYEN D
• Application Number:
  US08/697,412
• Time in Patent Office: 1,384 Days
• Field of Search: 381/231, 381/106, 381/312, 381/320, 381/321, 381/98, 381/99, 455/232.1, 455/234.1, 455/234.2, 455/235.1, 455/247.1, 455/251.1, 455/303, 455/306
• US Class Current: 381/321
• CPC Class Codes: H04R 1/26 Spatial arrangements of separate transducers responsive to two or more frequency ranges
• CPC Class Codes: H04R 25/356 Amplitude, e.g. amplitude shift or compression
• CPC Class Codes: H04R 25/453 electronically

Abstract

A hearing compensation system for the hearing impaired comprises an input transducer for converting acoustical information at an input to electrical signals at an output, an output transducer for converting electrical signals at an input to acoustical information at an output, a plurality of bandpass filters, each bandpass filter having an input connected to the output of said input transducer, a plurality of AGC circuits, each individual AGC circuit associated with a different one of the bandpass filters and having an input connected to the output of its associated bandpass filter and an output connected to the input of the output transducer. The bandpass filters and AGC circuits may be divided into two processing channels, one for low frequencies and one for high frequencies and may drive separate audio transducers, one configured for maximum efficiency at low frequencies and one configured for maximum efficiency at high frequencies.

Claims

1. A hearing compensation system comprising:

an input transducer for converting acoustical information at an input thereof to electrical signals at an output thereof;

a first output transducer for converting electrical signals below a crossover frequency at an input thereof to acoustical information at an output thereof;

a second output transducer for converting electrical signals above said crossover frequency at an input thereof to acoustical information at an output thereof;

a first plurality of bandpass filters, said first plurality of bandpass filters for filtering electrical signals below said crossover frequency, each bandpass filter having an input connected to said output of said input transducer;

a second plurality of bandpass filters, said second plurality of bandpass filters for filtering electrical signals above said crossover frequency, each bandpass filter having an input connected to said output of said input transducer;

a first plurality of AGC circuits, each individual AGC circuit associated with a different one of said first plurality of bandpass filters and having an input connected to said output of its associated bandpass filter and an output summed with said outputs of all other ones of said first plurality of AGC circuits to form a first summed output, said first summed output connected to said input of said first output transducer.

a second plurality of AGC circuits, each individual AGC circuit associated with a different one of said second plurality of bandpass filters and having an input connected to said output of its associated bandpass filter and an output summed with said outputs of all other ones of said second plurality of AGC circuits to form a second summed output, said second summed output connected to said input of said second output transducer.

2. The hearing compensation system of claim 1 wherein said AGC circuits are multiplicative AGC circuits.

3. The hearing compensation system of claim 2 wherein each of said first plurality of said multiplicative AGC circuits and each of said second plurality of said multiplicative AGC circuits comprise:

a first amplifier element having an input and an output, said first amplifier element having a having a gain of 1/e_max, where e_max is the maximum value of an audio envelope to be presented to said AGC circuit for which AGC amplification is to result;

a logarithmic element having an input connected to said output of said first amplifier element, said logarithmic element having a first output carrying a signal indicating the sign of a signal at said input of said logarithmic element and a second output carrying a signal proportional to the logarithm of the absolute value of said signal at said input of said logarithmic element;

a filter element having an input connected to said second output of said logarithmic element and an output, said filter element having a throughput delay;

a delay element having an input connected to said first output of said logarithmic element and an output, said delay element having a delay equal to said throughput delay;

an exponential element having a first input connected to said output of said delay element, a second input connected to said output of said filter element, and an output; and

a second amplifier element having an input and an output, said input connected to said output of said exponential element, said second amplifier element having a gain of e_max.

4. The hearing compensation system of claim 3, wherein said filter element comprises:

a high-pass filter having an input connected to said input of said filter element, and an output;

a low-pass filter having an input connected to the input of said filter element and an output;

an amplifier with gain of less than unity, said amplifier having an input connected to said output of said low-pass filter and an output; and

means for summing said output of said high-pass filter and said output of said amplifier with gain of less than unity to form said output of said filter element.

5. The hearing compensation system of claim 2 wherein each of said first plurality of said multiplicative AGC circuits and each of said second plurality of said multiplicative AGC circuits comprise:

a logarithmic element having an input, a first output carrying a signal indicating the sign of a signal at said input of said logarithmic element and a second output carrying a signal proportional to the logarithm of the absolute value of said signal at said input of said logarithmic element;

a filter element having an input connected to said second output of said logarithmic element and an output, said filter element having a throughput delay;

a delay element having an input connected to said first output of said logarithmic element and an output, said delay element having a delay equal to said throughput delay;

an exponential element having a first input connected to said output of said delay element, a second input connected to said output of said filter element, and an output; and

an amplifier element having an input and an output, said input connected to said output of said exponential element, said amplifier element having a gain of e_max, where e_max is the maximum value of an audio envelope to be presented to said multiplicative AGC circuit for which AGC amplification is to result.

6. The hearing compensation system of claim 5, wherein said filter element comprises:

a high-pass filter having an input connected to said input of said filter element, and an output;

a low-pass filter having an input connected to said input of said filter element and an output;

a subtractor element having an input connected to said output of said low-pass filter, and an output, said subtractor element subtracting log e_max from said output of said low-pass filter;

an amplifier with gain of less than unity, said amplifier having an input connected to said output of said subtractor element, and an output; and

means for summing said output of said high-pass filter and said output of said amplifier with gain of less than unity to form said output of said filter element.

7. The hearing compensation system of claim 2 wherein each of said first plurality of said multiplicative AGC circuits and each of said second plurality of said multiplicative AGC circuits comprise:

an amplifier element having an input and an output, said amplifier element having a having a gain of 1/e_max, where e_max is the maximum value of an audio envelope to be presented to said AGC circuit for which AGC amplification is to result;

a logarithmic element having an input connected to said output of said amplifier element, said logarithmic element having a first output carrying a signal indicating the sign of a signal at said input of said logarithmic element and a second output carrying a signal proportional to the logarithm of the absolute value of said signal at said input of said logarithmic element;

a filter element having an input connected to said second output of said logarithmic element and an output, said filter element having a throughput delay;

a delay element having an input connected to said first output of said logarithmic element and an output, said delay element having a delay equal to said throughput delay; and

an exponential element having a first input connected to said output of said delay element, a second input connected to said output of said filter element, and an output.

8. The hearing compensation system of claim 7, wherein said filter element comprises:

a high-pass filter having an input connected to said input of said filter element, and an output;

a low-pass filter having an input connected to said input of said filter element, and an output;

an amplifier with gain of less than unity, said amplifier having an input connected to said output of said low-pass filter, and an output;

an adder element having an input connected to said output of said low-pass filter, and an output, said adder element adding log e_max to said output of said amplifier with gain of less than unity; and

means for summing said output of said high-pass filter and said output of said adder element to form said output of said filter element.

9. The hearing compensation system of claim 2 wherein each of said first plurality of said multiplicative AGC circuits and each of said second plurality of said multiplicative AGC circuits comprise:

a logarithmic element having an input, a first output carrying a signal indicating the sign of a signal at said input of said logarithmic element and a second output carrying a signal proportional to the logarithm of the absolute value of said signal at said input of said logarithmic element;

a filter element having an input connected to said second output of said logarithmic element and an output, said filter element having a throughput delay;

a delay element having an input connected to said first output of said logarithmic element and an output, said delay element having a delay equal to said throughput delay; and

an exponential element having a first input connected to said output of said delay element, a second input connected to said output of said filter element, and an output.

10. The hearing compensation system of claim 9, wherein said filter element comprises:

a high-pass filter having an input connected to said input of said filter element, and an output;

a low-pass filter having an input connected to the input of said filter element, and an output;

a subtractor element having an input connected to said output of said low-pass filter, and an output, said subtractor element subtracting log e_max from said output of said low-pass filter, where e_max is the maximum value of an audio envelope to be presented to said multiplicative AGC circuit for which AGC amplification is to result;

an amplifier with gain of less than unity, said amplifier having an input connected to said output of said subtractor element and an output;

an adder element having an input connected to said output of said amplifier with gain of less than unity, and an output, said adder element adding log e_max to said output of said amplifier with gain of less than unity; and

means for summing said output of said high-pass filter and said output of said adder element to form said output of said filter element.

11. The hearing compensation system of any one of claims 4, 6, 8 or 10 wherein the gain of said amplifier is equal to 1 minus the ratio of the hearing loss in dB at threshold in a band of frequencies passed by the one of said bandpass filters with which the individual AGC circuit containing said amplifier is associated to a quantity equal to the upper comfort level in dB within said band of frequencies minus the normal hearing threshold in dB within said band of frequencies.

12. The hearing compensation system of claim 2 wherein each of said first plurality of said multiplicative AGC circuits and each of said second plurality of said multiplicative AGC circuits comprise:

a first amplifier element having an input and an output, said input connected to an input node of its AGC circuit, said first amplifier element having a gain of 1/e_max, where e_max is the maximum value of an audio envelope to be presented to said AGC circuit for which AGC amplification is to result;

an envelope detector element having an input connected to said output of said first amplifier element and an output;

a logarithmic element having an input connected to said output of said envelope detector element, said logarithmic element having an output carrying a signal proportional to the logarithm of the value of said signal at said input of said logarithmic element;

a second amplifier element having an input and an output, said input connected to said output of said logarithmic element, said second amplifier having a gain of k-1 where k is a number between zero and one;

an exponential element having an input and an output, said input of said exponential element connected to said output of said second amplifier element; and

a multiplier element having a first input connected to said output of said exponential element, a second input connected to said input node, and an output connected to an output node of its AGC circuit.

13. The hearing compensation system of claim 2 wherein each of said first plurality of said multiplicative AGC circuits and each of said second plurality of said multiplicative AGC circuits comprise:

an envelope detector element having an input and an output, said input of said envelope detector element connected to an input node of its AGC circuit;

a first amplifier element having an input and an output, said input of said first amplifier element connected to said output of said envelope detector element, said first amplifier element having a gain of 1/e_max, where e_max is the maximum value of an audio envelope to be presented to said AGC circuit for which AGC amplification is to result;

a logarithmic element having an input connected to said output of said first amplifier element, said logarithmic element having an output carrying a signal proportional to the logarithm of the value of said signal at said input of said logarithmic element;

a second amplifier element having an input and an output, said input of said second amplifier connected to said output of said logarithmic element, said second amplifier having a gain of k-1 where k is a number between zero and one;

an exponential element having an input and an output, said input of said exponential element connected to said output of said second amplifier element; and

a multiplier element having a first input connected to said output of said exponential element, a second input connected to said input node, and an output connected to an output node of its AGC circuit.

14. The hearing compensation system of claim 2 wherein each of said first plurality of said multiplicative AGC circuits and each of said second plurality of said multiplicative AGC circuits comprise:

an envelope detector element having an input and an output, said input of said envelope detector element connected to an input node of its AGC circuit;

a logarithmic element having an input connected to said output of said envelope detector element, said logarithmic element having an output carrying a signal proportional to the logarithm of the value of said signal at said input of said logarithmic element;

a subtractor element having an input connected to said output of said logarithmic element, and an output, said subtractor element subtracting log e_max from said output of said logarithmic element, where e_max is the maximum value of an audio envelope to be presented to said multiplicative AGC circuit for which AGC amplification is to result;

an amplifier element having an input and an output, said input of said amplifier connected to said output of said subtractor element, said second amplifier having a gain of k-1 where k is a number between zero and one;

an exponential element having an input and an output, said input of said exponential element connected to said output of said amplifier element; and

a multiplier element having a first input connected to said output of said exponential element, a second input connected to said input node, and an output connected to an output node of its AGC circuit.

15. The hearing compensation system of claim 2 wherein each of said first plurality of said multiplicative AGC circuits and each of said second plurality of said multiplicative AGC circuits comprise:

an envelope detector element having an input and an output, said input of said envelope detector element connected to an input node of its AGC circuit;

a first amplifier element having an input and an output, said input of said first amplifier element connected to said output of said envelope detector element, said first amplifier element having a gain of 1/e_max, where e_max is the maximum value of an audio envelope to be presented to said AGC circuit for which AGC amplification is to result;

a logarithmic element having an input connected to said output of said first amplifier element, said logarithmic element having an output carrying a signal proportional to the logarithm of the value of said signal at said input of said logarithmic element;

a second amplifier element having an input and an output, said input of said second amplifier connected to said output of said logarithmic element, said second amplifier having a gain of k-1 where k is a number between zero and one;

an exponential element having an input and an output, said input of said exponential element connected to said output of said second amplifier element;

a soft limiter element having an input connected to said output of said second amplifier element and an output, said soft limiter element having a limiter characteristic selected such that its gain is limited to a maximum value equal to the gain required to compensate for an individual'"'"'s hearing loss at threshold in a frequency band passed by the one of said bandpass filters with which its AGC circuit is associated; and

a multiplier element having a first input connected to said output of said soft limiter element, a second input connected to said input node, and an output connected to an output node of its AGC circuit.

16. The hearing compensation system of claim 2 wherein each of said first plurality of said multiplicative AGC circuits and each of said second plurality of said multiplicative AGC circuits comprise:

a first amplifier element having an input and an output, said input connected to an input node of its AGC circuit, said first amplifier element having a gain of 1/e_max, where e_max is the maximum value of an audio envelope to be presented to said AGC circuit for which AGC amplification is to result;

an envelope detector element having an input connected to said output of said first amplifier element and an output;

a logarithmic element having an input connected to said output of said envelope detector element, said logarithmic element having an output carrying a signal proportional to the logarithm of the value of said signal at said input of said logarithmic element;

a second amplifier element having an input and an output, said input connected to said output of said logarithmic element, said second amplifier having a gain of k-1 where k is a number between zero and one;

an exponential element having an input and an output, said input of said exponential element connected to said output of said second amplifier element;

a soft limiter element having an input connected to said output of said exponential element and an output, said soft limiter element having a limiter characteristic selected such that its gain is limited to a maximum value equal to the gain required to compensate for an individual'"'"'s hearing loss at threshold in a frequency band passed by the one of said bandpass filters with which its AGC circuit is associated; and

a multiplier element having a first input connected to said output of said soft limiter element, a second input connected to said input node, and an output connected to an output node of its AGC circuit.

17. The hearing compensation system of claim 2 wherein each of said first plurality of said multiplicative AGC circuits and each of said second plurality of said multiplicative AGC circuits comprise:

an envelope detector element having an input and an output, said input of said envelope detector element connected to an input node of its AGC circuit;

a logarithmic element having an input connected to said output of said envelope detector element, said logarithmic element having an output carrying a signal proportional to the logarithm of the value of said signal at said input of said logarithmic element;

a subtractor element having an input connected to said output of said logarithmic element, and an output, said subtractor element subtracting log e_max from said output of said logarithmic element, where e_max is the maximum value of an audio envelope to be presented to said multiplicative AGC circuit for which AGC amplification is to result;

an amplifier element having an input and an output, said input of said amplifier connected to said output of said subtractor element, said second amplifier having a gain of k-1 where k is a number between zero and one;

an exponential element having an input and an output, said input of said exponential element connected to said output of said amplifier element;

a soft limiter element having an input connected to said output of said second amplifier element and an output, said soft limiter element having a limiter characteristic selected such that its gain is limited to a maximum value equal to the gain required to compensate for an individual'"'"'s hearing loss at threshold in a frequency band passed by the one of said bandpass filters with which its AGC circuit is associated; and

a multiplier element having a first input connected to said output of said soft limiter element, a second input connected to said input node, and an output connected to an output node of its AGC circuit.

28. The system of any one of claims 15, 16, 17, 19, 20, 21, 25, 26, or 27 further including a noise generator connected to inject a selected amount of noise into said inputs of each of said first plurality of bandpass filters and into said inputs of each of said second plurality of bandpass filters, said noise weighted such that its spectral shape follows the threshold-of-hearing curve of a normal hearing individual as a function of frequency.

32. The hearing compensation system of any one of claims 12, 13, 14, 15, 16, 17 or 31 wherein k is equal to 1 minus the ratio of the hearing loss in dB at threshold in a band of frequencies passed by the one of said bandpass filters with which the individual AGC circuit containing said amplifier is associated to a quantity equal to the upper comfort level in dB within said band of frequencies minus the normal hearing threshold in dB within said band of frequencies.

34. The system of any one of claims 12, 13, 14, 15, 16, 17, 18, 18, 20, 21, 22, 23, 24, 25, 26, 27, 29, 30, 31, or 33 wherein said envelope detector element comprises:

an absolute value element having an input and an output, said input forming the input of said envelope detector element; and

a low-pass filter element having an input, and an output forming the output of said envelope detector element, said input of said low-pass filter element connected to said output of said absolute value element.

35. The system of claim 34 wherein said low-pass filter element has a cutoff frequency which is a monotonic function of the center frequency of said bandpass filter associated with said multiplicative AGC circuit.

38. The hearing compensation system of any one of claims 2 or 37 wherein the number of said first and second pluralities of said bandpass filters, and the number of said first and second pluralities of said multiplicative AGC circuits, is from 12 to 15.

39. The system of any one of claims 1, 2, 3, 5, 7, 9, 12, 13, 14, 15, 16, 17 18, 19, 20, 21, 22, 23, 24, 25, 26, 27, 29, 30, or 36 wherein said first output transducer is an iron-armature transducer.

40. The system of claim 39 wherein said first plurality of bandpass filters pass frequencies in a frequency band approximately below a resonant frequency of said iron-armature transducer.

41. The systems of claim 39 wherein said second plurality of bandpass filters pass frequencies in a frequency band approximately above a resonant frequency of said iron-armature transducer.

42. The system of any one of claims 1, 2, 3, 5, 7, 9, 12, 13, 14, 15, 16, 17 18, 19, 20, 21, 22, 23, 24, 25, 26, 27, 29, 30, or 36 wherein said second output transducer is a moving coil transducer.

43. The system of any one of claims 1, 2, 3, 5, 7, 9, 12, 13, 14, 15, 16, 17 18, 19, 20, 21, 22, 23, 24, 25, 26, 27, 29, 30, or 36 wherein said second output transducer is an electret transducer.

44. The system of any one of claims 1, 2, 3, 5, 7, 9, 12, 13, 14, 15, 16, 17 18, 19, 20, 21, 22, 23, 24, 25, 26, 27, 29, 30, or 31 wherein said crossover frequency is approximately 1 kHz.

18. A sound expander system comprising:

an input transducer for converting acoustical information at an input thereof to electrical signals at an output thereof;

a first output transducer for converting electrical signals below a crossover frequency at an input thereof to acoustical information at an output thereof;

a second output transducer for converting electrical signals above said crossover frequency at an input thereof to acoustical information at an output thereof;

a first plurality of bandpass filters, said first plurality of bandpass filters for filtering electrical signals below said crossover frequency, each bandpass filter having an input connected to said output of said input transducer;

a second plurality of bandpass filters, said second plurality of bandpass filters for filtering electrical signals above said crossover frequency, each bandpass filter having an input connected to said output of said input transducer;

first and second pluralities of multiplicative AGC circuits, each multiplicative AGC circuit of said first plurality of multiplicative AGC circuits associated with a different one of said first plurality of said bandpass filters and each of said first plurality of said multiplicative AGC circuits having an input connected to said output of its associated bandpass filter and an output summed with said outputs of all other ones of said first plurality of multiplicative AGC circuits to form a first summed output, said first summed output connected to said input of said first output transducer, each individual multiplicative AGC circuit of said second plurality of multiplicative AGC circuits associated with a different one of said second plurality of said bandpass filters and each of said second plurality of said multiplicative AGC circuits having an input connected to said output of its associated bandpass filter and an output summed with said outputs of all other ones of said second plurality of multiplicative AGC circuits to form a second summed output, said second summed output connected to said input of said second output transducer, each of said first plurality of said multiplicative AGC circuits and each of said second plurality of said multiplicative AGC circuits comprising a first amplifier element having an input and an output, said input of said first amplifier element connected to an input node of its multiplicative AGC circuit, said first amplifier element having a gain of 1/e_max, where e_max is the maximum value of an audio envelope to be presented to said AGC circuit for which AGC amplification is to result, an envelope detector element having an input connected to said output of said first amplifier element and an output, a logarithmic element having an input connected to said output of said envelope detector element, said logarithmic element having an output carrying a signal proportional to the logarithm of the value of said signal at said input of said logarithmic element, a second amplifier element having an input and an output, said input of said second amplifier element connected to said output of said logarithmic element, said second amplifier having a gain of k-1 where k is a number greater than one, an exponential element having an input and an output, said input of said exponential element connected to said output of said second amplifier element, and a multiplier element having a first input connected to said output of said exponential element, a second input connected to said input node, and an output connected to an output node of its multiplicative AGC circuit.

19. A sound expander system comprising:

an input transducer for converting acoustical information at an input thereof to electrical signals at an output thereof;

a first output transducer for converting electrical signals below a crossover frequency at an input thereof to acoustical information at an output thereof;

a second output transducer for converting electrical signals above said crossover frequency at an input thereof to acoustical information at an output thereof;

a first plurality of bandpass filters, said first plurality of bandpass filters for filtering electrical signals below said crossover frequency, each bandpass filter having an input connected to said output of said input transducer;

a second plurality of bandpass filters, said second plurality of bandpass filters for filtering electrical signals above said crossover frequency, each bandpass filter having an input connected to said output of said input transducer;

a second plurality of bandpass filters, said second plurality of bandpass filters for filtering electrical signals above a crossover frequency, each bandpass filter having an input connected to said output of said input transducer;

first and second pluralities of multiplicative AGC circuits, each multiplicative AGC circuit of said first plurality of multiplicative AGC circuits associated with a different one of said first plurality of said bandpass filters and each of said first plurality of said multiplicative AGC circuits having an input connected to said output of its associated bandpass filter and an output summed with said outputs of all other ones of said first plurality of multiplicative AGC circuits to form a first summed output, said first summed output connected to said input of said first output transducer, each individual multiplicative AGC circuit of said second plurality of multiplicative AGC circuits associated with a different one of said second plurality of said bandpass filters and each of said second plurality of said multiplicative AGC circuits having an input connected to said output of its associated bandpass filter and an output summed with said outputs of all other ones of said second plurality of multiplicative AGC circuits to form a second summed output, said second summed output connected to said input of said second output transducer, each of said first plurality of said multiplicative AGC circuits and each of said second plurality of said multiplicative AGC circuits comprising an envelope detector element having an input and an output, said input of said envelope detector element connected to an input node of its multiplicative AGC circuit, a first amplifier element having an input and an output, said input of said first amplifier connected to said output of said envelope detector element, said first amplifier element having a gain of 1/e_max, where e_max is the maximum value of an audio envelope to be presented to said multiplicative AGC circuit for which AGC amplification is to result, a logarithmic element having an input connected to said output of said first amplifier element, said logarithmic element having an output carrying a signal proportional to the logarithm of the value of said signal at said input of said logarithmic element, a second amplifier element having an input and an output, said input of said second amplifier element connected to said output of said logarithmic element, said second amplifier having a gain of k-1 where k is a number greater than one, an exponential element having an input and an output, said input of said exponential element connected to said output of said second amplifier element, a soft limiter element having an input connected to said output of said exponential element and an output, said soft limiter element having a limiter characteristic selected such that its gain is limited to a maximum value equal to a preselected comfort level in a frequency band passed by the one of said bandpass filters with which its multiplicative AGC circuit is associated, and a multiplier element having a first input connected to said output of said soft limiter element, a second input connected to said input node, and an output connected to an output node of its multiplicative AGC circuit.

20. A sound expander system comprising:

an input transducer for converting acoustical information at an input thereof to electrical signals at an output thereof;

a first output transducer for converting electrical signals below a crossover frequency at an input thereof to acoustical information at an output thereof;

a second output transducer for converting electrical signals above said crossover frequency at an input thereof to acoustical information at an output thereof;

a first plurality of bandpass filters, said first plurality of bandpass filters for filtering electrical signals below said crossover frequency, each bandpass filter having an input connected to said output of said input transducer;

a second plurality of bandpass filters, said second plurality of bandpass filters for filtering electrical signals above said crossover frequency, each bandpass filter having an input connected to said output of said input transducer;

first and second pluralities of multiplicative AGC circuits, each multiplicative AGC circuit of said first plurality of multiplicative AGC circuits associated with a different one of said first plurality of said bandpass filters and each of said first plurality of said multiplicative AGC circuits having an input connected to said output of its associated bandpass filter and an output summed with said outputs of all other ones of said first plurality of multiplicative AGC circuits to form a first summed output, said first summed output connected to said input of said first output transducer, each individual multiplicative AGC circuit of said second plurality of multiplicative AGC circuits associated with a different one of said second plurality of said bandpass filters and each of said second plurality of said multiplicative AGC circuits having an input connected to said output of its associated bandpass filter and an output summed with said outputs of all other ones of said second plurality of multiplicative AGC circuits to form a second summed output, said second summed output connected to said input of said second output transducer, each of said first plurality of said multiplicative AGC circuits and each of said second plurality of said multiplicative AGC circuits comprising an envelope detector element having an input and an output, said input of said envelope detector element connected to an input node of its multiplicative AGC circuit, a logarithmic element having an input connected to said output of said envelope detector, said logarithmic element having an output carrying a signal proportional to the logarithm of the value of said signal at said input of said logarithmic element, a subtractor element having an input connected to said output of said logarithmic element, and an output, said subtractor element subtracting log e_max from said logarithmic element, where e_max is the maximum value of an audio envelope to be presented to said multiplicative AGC circuit for which AGC amplification is to result, an amplifier element having an input and an output, said input of said amplifier element connected to said output of said subtractor element, said amplifier element having a gain of k-1 where k is a number greater than one, an exponential element having an input and an output, said input of said exponential element connected to said output of said amplifier element, a soft limiter element having an input connected to said output of said exponential element and an output, said soft limiter element having a limiter characteristic selected such that its gain is limited to a maximum value equal to a preselected comfort level in a frequency band passed by the one of said bandpass filters with which its multiplicative AGC circuit is associated, and a multiplier element having a first input connected to said output of said soft limiter element, a second input connected to said input node, and an output connected to an output node of its multiplicative AGC circuit.

21. A sound expander system comprising:

an input transducer for converting acoustical information at an input thereof to electrical signals at an output thereof;

a first output transducer for converting electrical signals below a crossover frequency at an input thereof to acoustical information at an output thereof;

a second output transducer for converting electrical signals above said crossover frequency at an input thereof to acoustical information at an output thereof;

a first plurality of bandpass filters, said first plurality of bandpass filters for filtering electrical signals below said crossover frequency, each bandpass filter having an input connected to said output of said input transducer;

a second plurality of bandpass filters, said second plurality of bandpass filters for filtering electrical signals above said crossover frequency, each bandpass filter having an input connected to said output of said input transducer;

first and second pluralities of multiplicative AGC circuits, each multiplicative AGC circuit of said first plurality of multiplicative AGC circuits associated with a different one of said first plurality of said bandpass filters and each of said first plurality of said multiplicative AGC circuits having an input connected to said output of its associated bandpass filter and an output summed with said outputs of all other ones of said first plurality of multiplicative AGC circuits to form a first summed output, said first summed output connected to said input of said first output transducer, each individual multiplicative AGC circuit of said second plurality of multiplicative AGC circuits associated with a different one of said second plurality of said bandpass filters and each of said second plurality of said multiplicative AGC circuits having an input connected to said output of its associated bandpass filter and an output summed with said outputs of all other ones of said second plurality of multiplicative AGC circuits to form a second summed output, said second summed output connected to said input of said second output transducer, each of said first plurality of said multiplicative AGC circuits and each of said second plurality of said multiplicative AGC circuits comprising a first amplifier element having an input and an output, said input of said first amplifier element connected to an input node of its multiplicative AGC circuit, said first amplifier element having a gain of 1/e_max, where e_max is the maximum value of an audio envelope to be presented to said AGC circuit for which AGC amplification is to result, an envelope detector element having an input connected to said output of said first amplifier element and an output, a logarithmic element having an input connected to said output of said envelope detector element, said logarithmic element having an output carrying a signal proportional to the logarithm of the value of said signal at said input of said logarithmic element, a second amplifier element having an input and an output, said input of said second amplifier element connected to said output of said logarithmic element, said second amplifier having a gain of k-1 where k is a number greater than one, an exponential element having an input and an output, said input of said exponential element connected to said output of said second amplifier element, a soft limiter element having an input connected to said output of said exponential element and an output, said soft limiter element having a limiter characteristic selected such that its gain is limited to a maximum value equal to a preselected comfort level in a frequency band passed by the one of said bandpass filters with which its multiplicative AGC circuit is associated, and a multiplier element having a first input connected to said output of said soft limiter element, a second input connected to said input node, and an output connected to an output node of its multiplicative AGC circuit.

22. A sound expander system comprising:

an input transducer for converting acoustical information at an input thereof to electrical signals at an output thereof;

a first output transducer for converting electrical signals below a crossover frequency at an input thereof to acoustical information at an output thereof;

a second output transducer for converting electrical signals above said crossover frequency at an input thereof to acoustical information at an output thereof;

a first plurality of bandpass filters, said first plurality of bandpass filters for filtering electrical signals below said crossover frequency, each bandpass filter having an input connected to said output of said input transducer;

a second plurality of bandpass filters, said second plurality of bandpass filters for filtering electrical signals above said crossover frequency, each bandpass filter having an input connected to said output of said input transducer;

first and second pluralities of multiplicative AGC circuits, each multiplicative AGC circuit of said first plurality of multiplicative AGC circuits associated with a different one of said first plurality of said bandpass filters and each of said first plurality of said multiplicative AGC circuits having an input connected to said output of its associated bandpass filter and an output summed with said outputs of all other ones of said first plurality of multiplicative AGC circuits to form a first summed output, said first summed output connected to said input of said first output transducer, each individual multiplicative AGC circuit of said second plurality of multiplicative AGC circuits associated with a different one of said second plurality of said bandpass filters and each of said second plurality of said multiplicative AGC circuits having an input connected to said output of its associated bandpass filter and an output summed with said outputs of all other ones of said second plurality of multiplicative AGC circuits to form a second summed output, said second summed output connected to said input of said second output transducer, each of said first plurality of said multiplicative AGC circuits and each of said second plurality of said multiplicative AGC circuits comprising an envelope detector element having an input and an output, said input of said envelope detector element connected to an input node of its multiplicative AGC circuit, a first amplifier element having an input and an output, said input of said first amplifier connected to said output of said envelope detector element, said first amplifier element having a gain of 1/e_max, where e_max is the maximum value of an audio envelope to be presented to said multiplicative AGC circuit for which AGC amplification is to result, a logarithmic element having an input connected to said output of said first amplifier element, said logarithmic element having an output carrying a signal proportional to the logarithm of the value of said signal at said input of said logarithmic element, a second amplifier element having an input and an output, said input of said second amplifier element connected to said output of said logarithmic element, said second amplifier having a gain of k-1 where k is a number greater than one, an exponential element having an input and an output, said input of said exponential element connected to said output of said second amplifier element, and a multiplier element having a first input connected to said output of said exponential element, a second input connected to said input node, and an output connected to an output node of its multiplicative AGC circuit.

23. A sound expander system comprising:

an input transducer for converting acoustical information at an input thereof to electrical signals at an output thereof;

a first output transducer for converting electrical signals below a crossover frequency at an input thereof to acoustical information at an output thereof;

a second output transducer for converting electrical signals above said crossover frequency at an input thereof to acoustical information at an output thereof;

a first plurality of bandpass filters, said first plurality of bandpass filters for filtering electrical signals below said crossover frequency, each bandpass filter having an input connected to said output of said input transducer;

a second plurality of bandpass filters, said second plurality of bandpass filters for filtering electrical signals above said crossover frequency, each bandpass filter having an input connected to said output of said input transducer;

first and second pluralities of multiplicative AGC circuits, each multiplicative AGC circuit of said first plurality of multiplicative AGC circuits associated with a different one of said first plurality of said bandpass filters and each of said first plurality of said multiplicative AGC circuits having an input connected to said output of its associated bandpass filter and an output summed with said outputs of all other ones of said first plurality of multiplicative AGC circuits to form a first summed output, said first summed output connected to said input of said first output transducer, each individual multiplicative AGC circuit of said second plurality of multiplicative AGC circuits associated with a different one of said second plurality of said bandpass filters and each of said second plurality of said multiplicative AGC circuits having an input connected to said output of its associated bandpass filter and an output summed with said outputs of all other ones of said second plurality of multiplicative AGC circuits to form a second summed output, said second summed output connected to said input of said second output transducer, each of said first plurality of said multiplicative AGC circuits and each of said second plurality of said multiplicative AGC circuits comprising an envelope detector element having an input and an output, said input of said envelope detector element connected to an input node of its multiplicative AGC circuit, a logarithmic element having an input connected to said output of said envelope detector, said logarithmic element having an output carrying a signal proportional to the logarithm of the value of said signal at said input of said logarithmic element, a subtractor element having an input connected to said output of said logarithmic element, and an output, said subtractor element subtracting log e_max from said logarithmic element, where e_max is the maximum value of an audio envelope to be presented to said multiplicative AGC circuit for which AGC amplification is to result, an amplifier element having an input and an output, said input of said amplifier element connected to said output of said subtractor element, said amplifier element having a gain of k-1 where k is a number greater than one, an exponential element having an input and an output, said input of said exponential element connected to said output of said amplifier element, and a multiplier element having a first input connected to said output of said exponential element, a second input connected to said input node, and an output connected to an output node of its multiplicative AGC circuit.

24. A sound discriminator system comprising:

an input transducer for converting acoustical information at an input thereof to electrical signals at an output thereof;

a first output transducer for converting electrical signals below a crossover frequency at an input thereof to acoustical information at an output thereof;

a second output transducer for converting electrical signals above said crossover frequency at an input thereof to acoustical information at an output thereof;

a first plurality of bandpass filters, said first plurality of bandpass filters for filtering electrical signals below said crossover frequency, each bandpass filter having an input connected to said output of said input transducer;

a second plurality of bandpass filters, said second plurality of bandpass filters for filtering electrical signals above said crossover frequency, each bandpass filter having an input connected to said output of said input transducer;

first and second pluralities of multiplicative AGC circuits, each multiplicative AGC circuit of said first plurality of multiplicative AGC circuits associated with a different one of said first plurality of said bandpass filters and each of said first plurality of said multiplicative AGC circuits having an input connected to said output of its associated bandpass filter and an output summed with said outputs of all other ones of said first plurality of multiplicative AGC circuits to form a first summed output, said first summed output connected to said input of said first output transducer, each individual multiplicative AGC circuit of said second plurality of multiplicative AGC circuits associated with a different one of said second plurality of said bandpass filters and each of said second plurality of said multiplicative AGC circuits having an input connected to said output of its associated bandpass filter and an output summed with said outputs of all other ones of said second plurality of multiplicative AGC circuits to form a second summed output, said second summed output connected to said input of said second output transducer, each of said first plurality of said multiplicative AGC circuits and each of said second plurality of said multiplicative AGC circuits comprising a first amplifier element having an input and an output, said input of said first amplifier connected to an input node of its multiplicative AGC circuit, said first amplifier element having a gain of 1/e_max, where e_max is the maximum value of an audio envelope to be presented to said AGC circuit for which AGC amplification is to result, an envelope detector element having an input connected to said output of said first amplifier element and an output, a logarithmic element having an input connected to said output of said envelope detector element, said logarithmic element having an output carrying a signal proportional to the logarithm of the value of said signal at said input of said logarithmic element , a second amplifier element having an input and an output, said input of said second amplifier element connected to said output of said logarithmic element, said second amplifier having a gain of k-1 where k is a number between zero and -1, an exponential element having an input and an output, said input of said exponential element connected to said output of said second amplifier element, and a multiplier element having a first input connected to said output of said exponential element, a second input connected to said input node, and an output connected to an output node of its multiplicative AGC circuit.

25. A sound discriminator system comprising:

an input transducer for converting acoustical information at an input thereof to electrical signals at an output thereof;

a first output transducer for converting electrical signals below a crossover frequency at an input thereof to acoustical information at an output thereof;

a second output transducer for converting electrical signals above said crossover frequency at an input thereof to acoustical information at an output thereof;

a first plurality of bandpass filters, said first plurality of bandpass filters for filtering electrical signals below said crossover frequency, each bandpass filter having an input connected to said output of said input transducer;

a second plurality of bandpass filters, said second plurality of bandpass filters for filtering electrical signals above said crossover frequency, each bandpass filter having an input connected to said output of said input transducer;

first and second pluralities of multiplicative AGC circuits, each multiplicative AGC circuit of said first plurality of multiplicative AGC circuits associated with a different one of said first plurality of said bandpass filters and each of said first plurality of said multiplicative AGC circuits having an input connected to said output of its associated bandpass filter and an output summed with said outputs of all other ones of said first plurality of multiplicative AGC circuits to form a first summed output, said first summed output connected to said input of said first output transducer, each individual multiplicative AGC circuit of said second plurality of multiplicative AGC circuits associated with a different one of said second plurality of said bandpass filters and each of said second plurality of said multiplicative AGC circuits having an input connected to said output of its associated bandpass filter and an output summed with said outputs of all other ones of said second plurality of multiplicative AGC circuits to form a second summed output, said second summed output connected to said input of said second output transducer, each of said first plurality of said multiplicative AGC circuits and each of said second plurality of said multiplicative AGC circuits comprising an envelope detector element having an input and an output, said input of said envelope detector connected to an input node of its multiplicative AGC circuit, a first amplifier element having an input and an output, said input of said first amplifier connected to said output of said envelope detector element, said first amplifier element having a gain of 1/e_max, where e_max is the maximum value of an audio envelope to be presented to said multiplicative AGC circuit for which AGC amplification is to result, a logarithmic element having an input connected to said output of said first amplifier element, said logarithmic element having an output carrying a signal proportional to the logarithm of the value of said signal at said input of said logarithmic element, a second amplifier element having an input and an output, said input of said second amplifier element connected to said output of said logarithmic element, said second amplifier having a gain of k-1 where k is a number between zero and -1, an exponential element having an input and an output, said input of said exponential element connected to said output of said second amplifier element, a soft limiter element having an input connected to said output of said second amplifier element and an output, said soft limiter element having a limiter characteristic selected to limit its gain to a maximum value equal to a preselected comfort level in a frequency band passed by the one of said bandpass filters with which its multiplicative AGC circuit is associated, and a multiplier element having a first input connected to said output of said soft limiter element, a second input connected to said input node, and an output connected to an output node of its multiplicative AGC circuit.

26. A sound discriminator system comprising:

an input transducer for converting acoustical information at an input thereof to electrical signals at an output thereof;

a first output transducer for converting electrical signals below a crossover frequency at an input thereof to acoustical information at an output thereof;

a second output transducer for converting electrical signals above said crossover frequency at an input thereof to acoustical information at an output thereof;

a first plurality of bandpass filters, said first plurality of bandpass filters for filtering electrical signals below said crossover frequency, each bandpass filter having an input connected to said output of said input transducer;

a second plurality of bandpass filters, said second plurality of bandpass filters for filtering electrical signals above said crossover frequency, each bandpass filter having an input connected to said output of said input transducer;

first and second pluralities of multiplicative AGC circuits, each multiplicative AGC circuit of said first plurality of multiplicative AGC circuits associated with a different one of said first plurality of said bandpass filters and each of said first plurality of said multiplicative AGC circuits having an input connected to said output of its associated bandpass filter and an output summed with said outputs of all other ones of said first plurality of multiplicative AGC circuits to form a first summed output, said first summed output connected to said input of said first output transducer, each individual multiplicative AGC circuit of said second plurality of multiplicative AGC circuits associated with a different one of said second plurality of said bandpass filters and each of said second plurality of said multiplicative AGC circuits having an input connected to said output of its associated bandpass filter and an output summed with said outputs of all other ones of said second plurality of multiplicative AGC circuits to form a second summed output, said second summed output connected to said input of said second output transducer, each of said first plurality of said multiplicative AGC circuits and each of said second plurality of said multiplicative AGC circuits comprising an envelope detector element having an input and an output, said input of said envelope detector connected to an input node of its multiplicative AGC circuit, a logarithmic element having an input connected to said output of said envelope detector, said logarithmic element having an output carrying a signal proportional to the logarithm of the value of said signal at said input of said logarithmic element, a subtractor element having an input connected to said output of said logarithmic element, and an output, said subtractor element subtracting log e_max from said logarithmic element, where e_max is the maximum value of an audio envelope to be presented to said multiplicative AGC circuit for which AGC amplification is to result, an amplifier element having an input and an output, said input of said amplifier element connected to said output of said subtractor element, said amplifier element having a gain of k-1 where k is a number between zero and -1, an exponential element having an input and an output, said input of said exponential element connected to said output of said amplifier element, a soft limiter element having an input connected to said output of said second amplifier element and an output, said soft limiter element having a limiter characteristic selected to limit its gain to a maximum value equal to a preselected comfort level in a frequency band passed by the one of said bandpass filters with which its multiplicative AGC circuit is associated, and a multiplier element having a first input connected to said output of said soft limiter element, a second input connected to said input node, and an output connected to an output node of its multiplicative AGC circuit.

27. A sound discriminator system comprising:

an input transducer for converting acoustical information at an input thereof to electrical signals at an output thereof;

a first output transducer for converting electrical signals below a crossover frequency at an input thereof to acoustical information at an output thereof;

a second output transducer for converting electrical signals above said crossover frequency at an input thereof to acoustical information at an output thereof;

a first plurality of bandpass filters, said first plurality of bandpass filters for filtering electrical signals below said crossover frequency, each bandpass filter having an input connected to said output of said input transducer;

a second plurality of bandpass filters, said second plurality of bandpass filters for filtering electrical signals above said crossover frequency, each bandpass filter having an input connected to said output of said input transducer;

first and second pluralities of multiplicative AGC circuits, each multiplicative AGC circuit of said first plurality of multiplicative AGC circuits associated with a different one of said first plurality of said bandpass filters and each of said first plurality of said multiplicative AGC circuits having an input connected to said output of its associated bandpass filter and an output summed with said outputs of all other ones of said first plurality of multiplicative AGC circuits to form a first summed output, said first summed output connected to said input of said first output transducer, each individual multiplicative AGC circuit of said second plurality of multiplicative AGC circuits associated with a different one of said second plurality of said bandpass filters and each of said second plurality of said multiplicative AGC circuits having an input connected to said output of its associated bandpass filter and an output summed with said outputs of all other ones of said second plurality of multiplicative AGC circuits to form a second summed output, said second summed output connected to said input of said second output transducer, each of said first plurality of said multiplicative AGC circuits and each of said second plurality of said multiplicative AGC circuits comprising a first amplifier element having an input and an output, said input of said first amplifier element connected to an input node of its multiplicative AGC circuit, said first amplifier element having a gain of 1/e_max, where e_max is the maximum value of an audio envelope to be presented to said AGC circuit for which AGC amplification is to result, an envelope detector element having an input connected to said output of said first amplifier element and an output, a logarithmic element having an input connected to said output of said envelope detector element, said logarithmic element having an output carrying a signal proportional to the logarithm of the value of said signal at said input of said logarithmic element, a second amplifier element having an input and an output, said input of said second amplifier element connected to said output of said logarithmic element, said second amplifier having a gain of k-1 where k is a number between zero and -1, an exponential element having an input and an output, said input of said exponential element connected to said output of said second amplifier element, a soft limiter element having an input connected to said output of said exponential element and an output, said soft limiter element having a limiter characteristic selected to limit its gain to a maximum value equal to a preselected comfort level in a frequency band passed by the one of said bandpass filters with which its multiplicative AGC circuit is associated, and a multiplier element having a first input connected to said output of said soft limiter element, a second input connected to said input node, and an output connected to an output node of its multiplicative AGC circuit.

29. A sound discriminator system comprising:

an input transducer for converting acoustical information at an input thereof to electrical signals at an output thereof;

a first output transducer for converting electrical signals below a crossover frequency at an input thereof to acoustical information at an output thereof;

a second output transducer for converting electrical signals above said crossover frequency at an input thereof to acoustical information at an output thereof;

a first plurality of bandpass filters, said first plurality of bandpass filters for filtering electrical signals below said crossover frequency, each bandpass filter having an input connected to said output of said input transducer;

a second plurality of bandpass filters, said second plurality of bandpass filters for filtering electrical signals above said crossover frequency, each bandpass filter having an input connected to said output of said input transducer;

first and second pluralities of multiplicative AGC circuits, each multiplicative AGC circuit of said first plurality of multiplicative AGC circuits associated with a different one of said first plurality of said bandpass filters and each of said first plurality of said multiplicative AGC circuits having an input connected to said output of its associated bandpass filter and an output summed with said outputs of all other ones of said first plurality of multiplicative AGC circuits to form a first summed output, said first summed output connected to said input of said first output transducer, each individual multiplicative AGC circuit of said second plurality of multiplicative AGC circuits associated with a different one of said second plurality of said bandpass filters and each of said second plurality of said multiplicative AGC circuits having an input connected to said output of its associated bandpass filter and an output summed with said outputs of all other ones of said second plurality of multiplicative AGC circuits to form a second summed output, said second summed output connected to said input of said second output transducer, each of said first plurality of said multiplicative AGC circuits and each of said second plurality of said multiplicative AGC circuits comprising an envelope detector element having an input and an output, said input of said envelope detector connected to an input node of its multiplicative AGC circuit, a first amplifier element having an input and an output, said input of said first amplifier element connected to said output of said envelope detector element, said first amplifier element having a gain of 1/e_max, where e_max is the maximum value of an audio envelope to be presented to said multiplicative AGC circuit for which AGC amplification is to result, a logarithmic element having an input connected to said output of said first amplifier element, said logarithmic element having an output carrying a signal proportional to the logarithm of the value of said signal at said input of said logarithmic element, a second amplifier element having an input and an output, said input of said second amplifier element connected to said output of said logarithmic element, said second amplifier having a gain of k-1 where k is a number between zero and -1, an exponential element having an input and an output, said input of said exponential element connected to said output of said second amplifier element, and a multiplier element having a first input connected to said output of said exponential element, a second input connected to said input node, and an output connected to an output node of its multiplicative AGC circuit.

30. A sound discriminator system comprising:

an input transducer for converting acoustical information at an input thereof to electrical signals at an output thereof;

a first output transducer for converting electrical signals below a crossover frequency at an input thereof to acoustical information at an output thereof;

a second output transducer for converting electrical signals above said crossover frequency at an input thereof to acoustical information at an output thereof;

a first plurality of bandpass filters, said first plurality of bandpass filters for filtering electrical signals below said crossover frequency, each bandpass filter having an input connected to said output of said input transducer;

a second plurality of bandpass filters, said second plurality of bandpass filters for filtering electrical signals above said crossover frequency, each bandpass filter having an input connected to said output of said input transducer;

first and second pluralities of multiplicative AGC circuits, each multiplicative AGC circuit of said first plurality of multiplicative AGC circuits associated with a different one of said first plurality of said bandpass filters and each of said first plurality of said multiplicative AGC circuits having an input connected to said output of its associated bandpass filter and an output summed with said outputs of all other ones of said first plurality of multiplicative AGC circuits to form a first summed output, said first summed output connected to said input of said first output transducer, each individual multiplicative AGC circuit of said second plurality of multiplicative AGC circuits associated with a different one of said second plurality of said bandpass filters and each of said second plurality of said multiplicative AGC circuits having an input connected to said output of its associated bandpass filter and an output summed with said outputs of all other ones of said second plurality of multiplicative AGC circuits to form a second summed output, said second summed output connected to said input of said second output transducer, each of said first plurality of said multiplicative AGC circuits and each of said second plurality of said multiplicative AGC circuits comprising an envelope detector element having an input and an output, said input of said envelope detector connected to an input node of its multiplicative AGC circuit, a logarithmic element having an input connected to said output of said envelope detector, said logarithmic element having an output carrying a signal proportional to the logarithm of the value of said signal at said input of said logarithmic element, a subtractor element having an input connected to said output of said logarithmic element, and an output, said subtractor element subtracting log e_max from said logarithmic element, where e_max is the maximum value of an audio envelope to be presented to said multiplicative AGC circuit for which AGC amplification is to result, an amplifier element having an input and an output, said input of said amplifier element connected to said output of said subtractor element, said amplifier element having a gain of k-1 where k is a number between zero and -1, an exponential element having an input and an output, said input of said exponential element connected to said output of said amplifier element, and a multiplier element having a first input connected to said output of said exponential element, a second input connected to said input node, and an output connected to an output node of its multiplicative AGC circuit.

31. A hearing compensation system comprising:

an input transducer for converting acoustical information at an input thereof to electrical signals at an output thereof;

an output transducer for converting electrical signals at an input thereof to acoustical information at an output thereof;

a plurality of bandpass filters, each bandpass filter having an input connected to said output of said input transducer;

a plurality of multiplicative AGC circuits, each individual multiplicative AGC circuit of said first plurality of multiplicative AGC circuits associated with a different one of said bandpass filters and having an input connected to said output of its associated bandpass filter and an output summed with said outputs of all other ones of said multiplicative AGC circuits to form a summed output, said summed output connected to said input of said output transducer, wherein each of said multiplicative AGC circuits comprises an envelope detector element having an input forming the input node of its multiplicative AGC circuit, and an output, a first amplifier element having an input, said input of said first amplifier element connected to said output of said envelope detector element, said first amplifier element having a gain of 1/e_max, where e_max is the maximum value of an audio envelope to be presented to said multiplicative AGC circuit for which AGC amplification is to result, a logarithmic element having an input connected to said output of said first amplifier element, said logarithmic element having an output carrying a signal proportional to the logarithm of the value of said signal at said input of said logarithmic element, a second amplifier element having an input and an output, said input of said second amplifier element connected to said output of said logarithmic element, said second amplifier having a gain of k-1 where k is a number between zero and one, an exponential element having an input and an output, said input of said exponential element connected to said output of said second amplifier element, a soft limiter element having an input connected to said output of said exponential element and an output, said soft limiter element having a limiter characteristic selected such that its gain is limited to a maximum value equal to the gain required to compensate for an individual'"'"'s hearing loss at threshold in a frequency band passed by the one of said bandpass filters with which its multiplicative AGC circuit is associated, and a multiplier element having a first input connected to said output of said soft limiter element, a second input connected to said input node of said multiplicative AGC circuit, and an output forming the output node of its multiplicative AGC circuit.

33. A sound expander system comprising:

an input transducer for converting acoustical information at an input thereof to electrical signals at an output thereof;

an output transducer for converting electrical signals at an input thereof to acoustical information at an output thereof;

a plurality of bandpass filters, each bandpass filter having an input connected to said output of said input transducer;

a plurality of multiplicative AGC circuits, each individual multiplicative AGC circuit of said first plurality of multiplicative AGC circuits associated with a different one of said bandpass filters and having an input connected to said output of its associated bandpass filter and an output summed with said outputs of all other ones of said multiplicative AGC circuits to form a summed output, said summed output connected to said input of said output transducer, wherein each of said multiplicative AGC circuits comprises an envelope detector element having an input forming the input node of its multiplicative AGC circuit, and an output, a first amplifier element having an input, said input of said first amplifier element connected to said output of said envelope detector element, said first amplifier element having a gain of 1/e_max, where e_max is the maximum value of an audio envelope to be presented to said multiplicative AGC circuit for which AGC amplification is to result, a logarithmic element having an input connected to said output of said first amplifier element, said logarithmic element having an output carrying a signal proportional to the logarithm of the value of said signal at said input of said logarithmic element, a second amplifier element having an input and an output, said input of said second amplifier element connected to said output of said logarithmic element, said second amplifier having a gain of k-1 where k is a number greater than one, an exponential element having an input and an output, said input of said exponential element connected to said output of said second amplifier element, a soft limiter element having an input connected to said output of said exponential element and an output, said soft limiter element having a limiter characteristic selected such that its gain is limited to a maximum value equal to a preselected comfort level in a frequency band passed by the one of said bandpass filters with which its multiplicative AGC circuit is associated, and a multiplier element having a first input connected to said output of said soft limiter element, a second input connected to said input node of said multiplicative AGC circuit, and an output forming the output node of its multiplicative AGC circuit.

36. A hearing compensation system comprising:

an input transducer for converting acoustical information at an input thereof to electrical signals at an output thereof;

a first output transducer for converting electrical signals below a crossover frequency at an input thereof to acoustical information at an output thereof;

a second output transducer for converting electrical signals above said crossover frequency at an input thereof to acoustical information at an output thereof;

a first plurality of bandpass filters, said first plurality of bandpass filters for filtering electrical signals below said crossover frequency, each bandpass filter having an input connected to said output of said input transducer;

a second plurality of bandpass filters, said second plurality of bandpass filters for filtering electrical signals above said crossover frequency, each bandpass filter having an input connected to said output of said input transducer;

a noise generator connected to inject a selected amount of noise into said inputs of each of said first plurality of bandpass filters and into said inputs of each of said second plurality of bandpass filters, said noise weighted such that its spectral shape follows the threshold-of-hearing curve of a normal hearing individual as a function of frequency; and

a first plurality of AGC circuits, each individual AGC circuit associated with a different one of said first plurality of bandpass filters and having an input connected to the output of its associated bandpass filter and an output summed with the outputs of all other ones of said first plurality of AGC circuits to form a first summed output, said first summed output connected to the input of said first output transducer;

a second plurality of AGC circuits, each individual AGC circuit associated with a different one of said second plurality of bandpass filters and having an input connected to the output of its associated bandpass filter and an output summed with the outputs of all other ones of said second plurality of AGC circuits to form a second summed output, said second summed output connected to the input of said second output transducer.

37. The hearing compensation system of claim 36 wherein said AGC circuits are multiplicative AGC circuits.

45. A hearing compensation system comprising:

an input transducer for converting acoustical information at an input thereof to electrical signals at an output thereof;

a first plurality of signal compression circuits for processing said electrical signals below a crossover frequency, each signal compression circuit having an input connected to said output of said input transducer, and an output;

a second plurality of signal compression circuits for processing said electrical signals above said crossover frequency, each signal compression circuit having an input connected to said output of said input transducer, and an output;

a first output transducer having an input connected to a summation of said outputs of said first plurality of signal compression circuits for converting said electrical signals below said crossover frequency at said input thereof to acoustical information at an output thereof;

a second output transducer having an input connected to a summation of said outputs of said first plurality of signal compression circuits for converting said electrical signals above said crossover frequency at said input thereof to acoustical information at an output thereof.

46. The system of claim 45 wherein said first output transducer is an iron-armature transducer.

47. The system of claim 45 wherein said second output transducer is a moving coil transducer.

48. The system of claim 45 wherein said second output transducer is an electret transducer.

49. The system of claim 45 wherein said crossover frequency is approximately 1 kHz.