Acoustic measurement

• US 3,912,880 A
• Filed: 07/05/1974
• Issued: 10/14/1975
• Est. Priority Date: 07/06/1973
• Status: Expired due to Term

• Current Assignee: Deceased Huth, Edwin John Powter, Robert Humphrey Benton
• Original Assignee: Deceased Huth, Edwin John Powter, Robert Humphrey Benton
• Inventors: Powter, Edwin John, Benton, Robert Humphrey, Huth, deceased
• Primary Examiner(s): Cooper, William G.
• Assistant Examiner(s): Olms, Douglas W.
• Application Number:
  US05/486,240
• Time in Patent Office: 466 Days
• Field of Search: 179/175.1A,175.1R,175
• US Class Current: 379/27.08
• CPC Class Codes: G01H 3/06 by electric means
• CPC Class Codes: H04R 29/001 for loudspeakers H04R29/007 takes precedence
• CPC Class Codes: H04R 29/004 for microphones H04R29/007 takes precedence

Abstract

The frequency response of a microphone can be measured by feeding an acoustic spectrum generated from a pseudo-random bit sequence generator into the microphone and analysing the output using a Fourier analyser. The acoustic spectrum generator is portable and battery operated. A phase locked oscillator system is used to enable the output of a microphone under test to be analysed even after recording on magnetic tape. The device ensures that the sampling rate of the analogue to digital converter feeding the Fourier analyser is locked onto the drive frequency of the pseudo-random bit sequence generator.

Claims

1. A system for measuring a frequency response of an acousticelectric transducer and associated circuitry including an acoustic generator adapted to be sealed to an acoustic-electric transducer housing, said acoustic generator arranged to generate an acoustic signal having a plurality of discrete sinusoidal frequencies;

receiver means electrically connected to said acoustic-electric transducer and associated circuitry for receiving an electrical output from said acoustic-electric transducer and associated circuitry and feeding a signal related to said electrical output to an analogue to digital converter having a first sampling means operable to sample a recEived signal at a variable sampling rate;

control means connected to said sampling means for controlling said variable sampling rate in response to variations in frequency of said received signal;

a Fourier analyser means, connected to an output of said analogue to digital converter and arranged to derive the frequency response of said acoustic electric transducer.

2. A system as claimed in claim 1 wherein said receiver means includes a recorder for recording said electrical output on a recording medium and playback means for regenerating said electric output from said recording medium.

3. A system as claimed in claim 2 wherein said acoustic generator includes a pseudo-random binary generator having a finite, sequence repetition frequency, electrically coupled to an electro-acoustic transducer.

4. A system as claimed in claim 3 wherein said frequency of said received signal is said finite sequence repetition frequency.

5. A system as claimed in claim 4 wherein said control means includes a comparator having a first input connected to an output of a second sampling means, a second input connected to a reference level generator means, an output of said comparator connected to a voltage controlled oscillator means coupled to both said first and second sampling means and arranged to control said variable sampling rate.

6. A system as claimed in claim 5 wherein said recording medium is magnetic tape.

7. A system as claimed in claim 1 wherein said acoustic generator is powered by a battery.

8. A system as claimed in claim 7 wherein said acoustic-electric transducer and associated circuitry is a telephone microphone and associated connections of said telephone microphone to a telephone exchange.

9. A method of measuring a frequency response of an acoustic-electric transducer and associated circuitry comprising the steps of:

a. feeding an acoustic signal comprising a plurality of discrete sinusoidal frequencies into said acoustic electric transducer, b. sampling an electric signal derived from said acoustic electric transducer at a sampling rate controlled by a first signal derived from said electric signal, to produce a sequential series of samples of said electric signal, c. converting said sequential series of samples into a sequence of numbers, each number representative of an amplitude of a sample, d. Fourier analysing said digital sequence of numbers to produce a sequence of numbers representing said frequency response.

10. A method as claimed in claim 9 wherein an electric output of said acoustic-electric transducer is recorded on a recording medium, and said electric signal is derived from said recording medium.

11. A method as claimed in claim 10 wherein said acoustic signal comprises a pseudo-random binary sequence repeating with a substantially constant frequency.

12. A method as claimed in claim 11 wherein said first signal is related to said substantially constant frequency.

13. A method as claimed in claim 12 wherein each number of said sequence of numbers representing said frequency response is multiplied, in a digital computer, by a weighting factor, to produce a sequence of weighted numbers, and integrating said sequence of weighted numbers to produce a number representing a send loudness rating for said acoustic electric transducer.

14. A method as claimed in claim 13 wherein said recording medium is magnetic tape.

15. A method as claimed in claim 14 wherein said acoustic-electric transducer and associated circuitry is a telephone microphone and associated connections of said telephone microphone to a telephone exchange.

16. A portable acoustic generator for use with the system claimed in claim 1 said portable acoustic generator including spacer means for positioning an electric-acoustic transducer at a predetermined distance from an acoustic-electric transducer to be tested, generator means electrically connected to said acoustic-electric transducer for generating a plurality of discrete Sinusoidal frequencies, and a battery power supply for powering said generator means.

17. A portable acoustic generator as claimed in claim 16 wherein said spacer means is arranged to seal said portable acoustic generator to a housing holding said acoustic-electric transducer.

18. A portable acoustic generator as claimed in claim 17 wherein said generator means comprises a pseudo-random binary sequence generator.

19. A portable acoustic generator as claimed inn claim 18 enclosed in a case so dimensioned as to be manually manipulatable.

20. A frequency analyser for use with the system claimed in claim 1 said frequency analyser comprising an analogue to digital converter having a first sampling means operable to sample a received signal at a variable sampling rate, control means connected to said sampling means for controlling said variable sampling rate in response to variations in frequency of said received signal, and a Fourier analyser means connected to an output of said analogue to digital converter.

21. A frequency analyser as claimed in claim 20 wherein said control means includes a comparator having a first input connected to an output of a second sample means, a second input of said comparator connected to a reference level generator means, an output of said comparator connected to a voltage controlled oscillator means, said voltage controlled oscillator means coupled to both said first and second sampling means and arranged to control said variable sampling rate.