Noise cancelling systems

• US 5,937,070 A
• Filed: 10/02/1995
• Issued: 08/10/1999
• Est. Priority Date: 09/14/1990
• Status: Expired due to Fees

• Current Assignee: Chris Todter, Paul Bremner, Scott Clifton
• Original Assignee: Chris Todter, Paul Bremner, Scott Clifton
• Inventors: Bremner, Paul, Todter, Chris, Clifton, Scott
• Primary Examiner(s): Isen, Forester W.
• Application Number:
  US08/538,077
• Time in Patent Office: 1,408 Days
• Field of Search: 381/71, 381/94, 381/71.6, 381/71.12, 381/71.11, 381/71.8, 381/92, 381/94.7
• US Class Current: 381/71.6
• CPC Class Codes: G10K 11/17823 Reference signals, e.g. ambient acoustic environment
• CPC Class Codes: G10K 11/17833 by using a self-diagnostic function or a malfunction prevention function, e.g. detecting abnormal output levels
• CPC Class Codes: G10K 11/17853 of the filter
• CPC Class Codes: G10K 11/17857 Geometric disposition, e.g. placement of microphones
• CPC Class Codes: G10K 11/17875 using an error signal without a reference signal, e.g. pure feedback
• CPC Class Codes: G10K 11/17885 additionally using a desired external signal, e.g. pass-through audio such as music or speech
• CPC Class Codes: G10K 2210/108 Communication systems, e.g. where useful sound is kept and noise is cancelled
• CPC Class Codes: G10K 2210/1081 Earphones, e.g. for telephones, ear protectors or headsets
• CPC Class Codes: G10K 2210/1082 Microphones, e.g. systems using "virtual" microphones
• CPC Class Codes: G10K 2210/3011 Single acoustic input
• CPC Class Codes: G10K 2210/3028 Filtering, e.g. Kalman filters or special analogue or digital filters
• CPC Class Codes: H04M 1/19 Arrangements of transmitters, receivers, or complete sets to prevent eavesdropping, to attenuate local noise or to prevent undesired transmission; Mouthpieces or receivers specially adapted therefor circuit arrangements for preventing eavesdropping H04M1/68; telephone cabinets E04H1/14

Abstract

A first major aspect of the invention relates to an active noise cancelling system which detects ambient noise and applies electro-accoustic processing thereto to produce an acoustic signal for cancelling out the ambient noise. The active noise cancelling system may be used to cancel all noise but an audio signal which is desired to be heard by the user. A second major aspect of the invention relates particularly to a system for electronically cancelling noise input to a user microphone, by utilizing known characteristics of speech signal and ambient noise. The first major aspect of the invention can be used in concert with the second major aspect in a communication system at the earpiece of a telephone and the mouthpiece of a telephone.

Claims

1. A system for attenuating acoustic noise energy at a listener'"'"'s ear comprising:

(a) sum signal transducer means, responsive to a sum signal, for producing acoustic energy corresponding to said sum signal, the sum signal transducer being closely coupled acoustically with the ear to enable the produced acoustic energy to attenuate noise energy at said listener'"'"'s ear;

(b) the sum signal transducer means being part of a feedforward circuit which further comprises;

(1) first transducer means, disposed external to the close acoustical coupling, for converting ambient noise energy to a corresponding first signal, and

(2) means for applying compensation to the first signal resulting in a second signal, said compensation including compensation for undesirable feedback from the sum signal transducer to the first transducer means;

(c) the sum signal transducer also being in a closed loop feedback circuit which further comprises;

(1) second transducer means, internal to the close acoustical coupling, for converting impinging acoustic energy to a corresponding third signal, and

(2) means for applying compensation to the third signal resulting in a fourth signal; and

(d) means for summing the second and fourth signals to produce said sum signal, the compensations of both circuits cooperating to cause an overall transfer function to attenuate noise energy at the listener'"'"'s ear over a predetermined frequency range.

2. System according to claim 1 further comprising means for actuating energization for the system by physical contact with a listener'"'"'s ear.

3. The system according to claim 1 wherein the feedback circuit further comprises compensation means for inhibiting negative feedback instability.

4. The system according to claim 3 wherein the compensation means comprises a notch filter having a center frequency that is a first flexural resonance frequency of the sum signal transducer means.

5. The system according to claim 3 wherein the compensation means for inhibiting negative feedback instability comprises a low pass filter which attenuates the gain and maintains a positive phase margin above 180°

.

6. The system according to claim 1 wherein the predetermined frequency range comprises the frequencies at and below a first flexural resonant frequency of the sum signal transducer means, and wherein the closed loop transfer function of the feedback circuit attenuates acoustic noise energy within a range at the low frequency end of said predetermined frequency range, and wherein the transfer function of the feedforward circuit attenuates acoustic noise energy within a range at the high frequency end of said predetermined frequency range.

7. The system according to claim 6 further comprising phase compensation means for frequencies above the first flexural resonance frequency of the sum signal transducer means to inhibit audio feedback instability.

8. The system according to claim 1 further comprising means for adapting the at least one means for applying compensation to variations in operating characteristics comprising:

(a) means for comparing predetermined parameters of the input and output signals of said at least one means for applying compensation, and

(b) means for adjusting the said at least one means for applying compensation in accordance with the comparison.

9. The system according to claim 8 wherein the means for comparing the predetermined parameters comprises means for low pass filtering each of the input and output signals and means for comparing the phase of the filtered signals, and wherein the means for adjusting the transfer function comprises means responsive to the phase comparison for adjusting phase compensation of the at least one means for applying compensation, and means responsive to both the phase comparison and the root mean square of the input signal for adjusting gain compensation of the at least one means for applying compensation.

10. The system according to claim 1 further comprising a microphone apparatus for converting desired acoustic energy to an audio signal for communication elsewhere, the apparatus comprising:

(a) a plurality of spaced microphones at known positions in relation to a source of the desired acoustic energy, each microphone being disposed for receiving the desired acoustic energy, at least one of the microphones comprising the first transducer, and

(b) means for processing the signals from each microphone to attenuate ambient noise from the audio signal, the processing being according to an algorithm utilizing known propagation characteristic differences between ambient acoustic noise energy and the desired acoustic energy.

11. The system according to claim 10 further comprising means for applying one such algorithm adapted to high frequencies, frequencies having wave lengths generally shorter than the microphone spacing, and another such algorithm adapted to low frequencies, frequencies having wave lengths generally on the order of the microphone spacing or longer, and means for summing the resultant signals.

12. The system according to claim 11 further comprising a housing for mounting the plurality of microphones, said housing being shaped to emphasize said differences between ambient acoustic noise energy propagation and the desired acoustic energy propagation.

13. A device for canceling acoustic noise energy in the vicinity of a listener'"'"'s ear comprising:

(a) first transducer means, disposed proximate the listener'"'"'s ear and responsive to a first signal, for producing corresponding acoustic energy,

(b) second transducer means for converting the acoustic noise energy to a corresponding second signal, the first and second transducer means being in a closed loop system wherein the acoustic energy produced by the first transducer means feeds back and impinges the second transducer means, the acoustic noise energy being an input to the closed loop, the acoustic energy produced by the first transducer means also feeding forward to impinge the listener'"'"'s ear, and

(c) means for notch filtering the second signal to produce the first signal, said means having a relatively sharp gain roll-off to the notch center frequency which is the first flexural resonance frequency of the first transducer means, and further having an increasing phase characteristic at the notch center frequency to provide a phase margin for frequencies above the first flexural resonance frequency, a total transfer function of the closed loop attenuating noise energy over a predetermined frequency range, the two acoustic energies being summed by the listener'"'"'s ear.

14. Device according to claim 13 further comprising means for actuating energization for the device by physical contact with a listener'"'"'s ear.

15. For an acoustic noise energy cancelling system having a noise detecting microphone electrically connected through a compensation circuit to drive a speaker to produce acoustic energy for attenuating the noise energy, a device for adapting the system to variations in operating characteristics at frequencies within a predetermined range, the device comprising means for adapting gain and phase characteristics of the compensation circuit based on a priori knowledge of the transfer functions of the system components, and in particular a two-pole model of the speaker'"'"'s suspension resonance characteristics.

16. The system according to claim 15 further comprising:

(a) first transducer means, responsive to a first signal, for producing corresponding acoustic energy,

(b) means for closely coupling acoustically the first transducer means with the listener'"'"'s ear,

(c) second transducer means for converting the noise energy to a corresponding second signal, the second transducer means being disposed in close proximity to the listener'"'"'s ear but sufficiently external to the close acoustical coupling for the second signal to contain phase leading information related to the noise energy, and

(d) means for applying compensation to the second signal resulting in the first signal, the compensation causing a transfer function to attenuate noise energy at the listener'"'"'s ear for frequencies at or below the first flexural resonance of the first transducer means, the compensation being derived at least in part from a mathematical model of the fundamental resonance characteristics of the first transducer means.

17. A device for canceling ambient acoustic noise both at a listener'"'"'s ear and in transmitted audio comprising:

(a) first transducer means, disposed proximate the listener'"'"'s ear and responsive to a first signal, for producing corresponding acoustic energy;

(b) second transducer means for converting the acoustic noise energy to a corresponding second signal, the first and second transducer means being in a closed loop system wherein the acoustic energy produced by the first transducer means feeds back and impinges the second transducer means, the acoustic noise energy being an input to the closed loop, the acoustic energy produced by the first transducer means also feeding forward to impinge the listener'"'"'s ear;

(c) means within the closed loop for applying compensation to the first signal resulting in the second signal, the compensation causing the closed loop transfer function to attenuate noise energy at the listener'"'"'s ear over a predetermined frequency range, the two acoustic energies being summed by the listener'"'"'s ear; and

(d) an apparatus for converting desired acoustic energy to an audio signal for communication elsewhere comprising;

(1) a plurality of spaced microphones at known positions in relation to a source of the desired acoustic energy, each microphone being disposed for receiving the desired acoustic energy, at least one of the microphones comprising the second transducer means, and

(2) means for processing the signals from each microphone to attenuate ambient noise from the audio signal, the processing being according to an algorithm which uses known differences between the directivity and spatial correlation of the ambient acoustic noise energy and the desired acoustic energy.

18. For an acoustic noise energy cancelling system including a noise detecting microphone connected through a compensation circuit to drive a speaker to produce acoustic energy for attenuating the noise energy at a listener'"'"'s ear, an apparatus for converting desired acoustic energy to an audio signal for communication elsewhere, the apparatus comprising:

(a) a plurality of spaced microphones at known positions relative to a source of the desired acoustic energy, each microphone being disposed for receiving the desired acoustic energy, at least one of the spaced microphones comprising the noise detecting microphone, and

(b) means for processing the signals from each microphone to attenuate ambient noise from the audio signal, the processing being according to an algorithm which uses known differences between the directivity and spatial correlation of the ambient acoustic noise energy and the desired acoustic energy.

19. The device according to claim 18 further comprising a housing for mounting the plurality of microphones, said housing being shaped to emphasize said differences between ambient acoustic noise energy propagation and the desired acoustic energy propagation.

20. The device according to claim 18 further comprising means for applying one such algorithm adapted to high frequencies, frequencies having wave lengths generally shorter than the microphone spacing, and another such algorithm adapted to low frequencies, frequencies having wave lengths generally on the order of the microphone spacing or longer, and means for summing the resultant signals.

21. The device according to claim 20 wherein the algorithm adapted to high frequencies comprises:

(a) amplification of each microphone signal by an amount sufficient to offset the known attenuation of the source audio signal emanating from known source location,

(b) phase-shifting of each microphone signal by an amount equal to, but opposite in phase to the known lag of the source signal at each microphone, and

(c) summing of the amplified and phase-shifted microphone signals so as to cancel the noise through interference of multiple uncorrelated signals and improve the signal-to-noise ratio of the desired source audio signal.

22. The device according to claim 20 wherein the algorithm adapted to low frequencies comprises:

(a) selection of two or more microphone signals which for the case of known source location are also known to experience the greatest difference in magnitude of signal from the desired acoustic source;

(b) application of known or measurable gain and phase-shifting of the two or more microphone signals to achieve approximately the same amplitude and in-phase representation of the known or assumed specific unwanted noise field at low frequencies;

(c) differencing of two or more amplified and phase-shifted signals to cancel the unwanted noise common to these signals and thus leave a residual signal proportional to the desired audio source signal at low frequencies; and

(d) amplification of the difference signal to recover a single audio source signal with signal-to-noise ratio better than any of the individual microphone signals.

23. A device for canceling ambient acoustic noise energy present at a microphone to prevent the noise from becoming part of the signal being transmitted from the microphone comprising:

(a) a plurality of spaced microphones at known positions relative to a source of desired acoustic energy, each microphone being disposed for receiving the desired acoustic energy, and

(b) means for processing the signals from each microphone to attenuate ambient noise from the audio signal, the processing being according to an algorithm which uses known differences between the directivity and spatial correlation of the ambient acoustic noise energy and the desired acoustic energy.

24. The device according to claim 23 further comprising means for applying one such algorithm adapted to high frequencies, frequencies having wave lengths generally shorter than the microphone spacing, and another such algorithm adapted to low frequencies, frequencies having wave lengths generally on the order of the microphone spacing or longer, and means for summing the resultant signals.

25. The device according to claim 24 wherein the algorithm adapted to high frequencies comprises:

(a) amplification of each microphone signal by an amount sufficient to offset the known attenuation of the source audio signal emanating from known source location,

(b) phase-shifting of each microphone signal by an amount equal to, but opposite in phase to the known lag of the source signal at each microphone, and

(c) summing of the amplified and phase-shifted microphone signals so as to cancel the noise through interference of multiple uncorrelated signals and improve the signal-to-noise ratio of the desired source audio signal.

26. The device according to claim 24 wherein the algorithm adapted to low frequencies comprises:

(a) selection of two or more microphone signals which for the case of known source location are also known to experience the greatest difference in magnitude of signal from the desired acoustic source;

(b) application of known or measurable gain and phase-shifting of the two or more microphone signals to achieve approximately the same amplitude and in-phase representation of the known or assumed specific unwanted noise field at low frequencies;

(c) differencing of two or more amplified and phase-shifted signals to cancel the unwanted noise common to these signals and thus leave a residual signal proportional to the desired audio source signal at low frequencies; and

(d) amplification of the difference signal to recover a single audio source signal with signal-to-noise ratio better than any of the individual microphone signals.

27. The device according to claim 23 further comprising a housing for mounting the plurality of microphones, said housing being shaped to emphasize said differences between ambient acoustic noise energy propagation and the desired acoustic energy propagation.