Apparatus and method for the measurement of acoustic absorption and impedance

Claim 2

Apparatus for measuring the acoustical properties of a nonlinear test specimen, comprising: a standing-wave chamber having a sound-input end and a specimenreceiving end; means for disposing said specimen at said specimen-receiving end of said chamber, first sound generator means for propagating an acoustic broadband signal into said sound-input end of said chamber at a level sufficient to reach the non-linear regime of said specimen; second sound generator means for propagating a single-frequency acoustical tracer signal into said sound-input end of said chamber, said tracer signal having an intensity less than the intensity of said broadband signal; pickup means for sensing the maxima and minima of the standingwave pattern generated within said chamber and providing corresponding output signals; filter means for extracting said tracer signal from said output signals; and, means for providing an indication of the amplitude of said tracer signal as a function of the position of said pickup means within said chamber.

Claim 3

Apparatus as defined in claim 1 wherein said chamber comprises an elongated tube and wherein said pickup means is adapted to sense said standing-wave pattern along the major axis of said chamber.

Claim 4

Apparatus as defined in claim 1 wherein said second sound generator means comprises a sinewave signal source, and including an electroacoustical transducer operatively connected to said sources.

Claim 5

Apparatus as defined in claim 2 wherein the surface area of said specimen is substantially coextensive with the cross-sectional area of said chamber.

Claim 6

Apparatus as defined in claim 1 wherein said first sound generator means propagates broadband noise into said chamber at an overall level greater than 100 decibels, where 0 decibels equals 0.0002 dyne per square centimeter.

Claim 7

Apparatus as defined in claim 5 wherein the spectrum and level of said broadband noise is shaped to be substantially the same as that encountered in the normal operating environment of said specimen.

Claim 8

Apparatus as defined in claim 1 wherein said second sound generator means propagates a sinewave tone into said chamber at a level lower than the level of the acoustic energy from said first sound generator means.

Claim 9

Apparatus as defined in claim 1 wherein said pickup means comprises: a pressure-responsive microphone having a movable probe connected thereto.

Claim 10

Apparatus as defined in claim 1 wherein said filter means comprises: a 1/3 -octave electrical filter; and a narrow-band-pass tracking filter connected in cascade with said 1/3 -octave electrical filter, the center frequency of which is selectively adjustable.

Claim 11

Apparatus as defined in claim 9 including: means connecting said tracking filter to said second sound generaTor means to cause said center frequency to correspond to the frequency of said tracer signal.

Claim 12

Apparatus as defined in claim 1 including: drive means connected to said pickup means and to said indication means for synchronous movement thereof.

Claim 13

Apparatus as defined in claim 1 wherein said first sound generator means comprises: an electroacoustical transducer.

Claim 14

Apparatus as defined in claim 1 wherein said second sound generator means comprises: an electroacoustical transducer.

Claim 15

Apparatus as defined in claim 1 wherein said first and second sound generators each comprise: an electroacoustical transducer.

Claim 16

Apparatus as defined in claim 14 including: first and second waveguides interposed between respective ones of said transducers and said sound input end of said chamber, said waveguides converging at an angle less than approximately 30* from the major axis of said chamber.

Claim 17

Apparatus for measuring acoustical properties of a non-linear test specimen, comprising: an elongated standing-wave tube having a sound input end and a specimen receiving end; means for disposing said specimen at said specimen-receiving end of said tube; a broadband noise generator; spectrum shaper means connected to said noise generator for selectively adjusting the spectrum and level of the output therefrom; a first electroacoustic transducer for propagating high-intensity sound into said tube in response to an electrical input; first power amplifier means responsive to the output of said shaper means to provide an electrical input to said first transducer; a sinewave oscillator; a second electroacoustic transducer for propagating a low-intensity sound into said tube in response to an electrical input; second power amplifier means responsive to the output of said oscillator to provide an electrical input to said second transducer; movable pressure microphone means translatable along the axis of said tube for sensing the maximum and minimum sound pressure in the standing wave generated therein and generating a corresponding output signal; filter means connected to the output of said microphone means for extracting from said output signal that portion which corresponds to said low-intensity sound; and indicator means responsive to the extracted signal output from said filter means to provide an indication of sound pressure versus position of said microphone means.

Claim 18

Apparatus for measuring acoustical properties of a non-linear test specimen, comprising: an elongated standing-wave tube having a sound input end and a specimen receiving end; means for disposing said specimen at said specimen-receiving end of said tube; a broadband noise generator; spectrum shaper means connected to said noise generator for selectively adjusting the spectrum and level of the output therefrom; a first electroacoustic transducer for propagating high-intensity sound into said tube in response to an electrical input; first power amplifier means responsive to the output of said shaper means to provide an electrical input to said first transducer; a sinewave oscillator; a second electroacoustic transducer for propagating a low-intensity sound into said tube in response to an electrical input; second power amplifier means responsive to the output of said oscillator to provide an electrical input to said second transducer; movable pressure microphone means translatable along the axis of said tube for sensing the maximum and minimum sound pressure in the standing wave generated therein and generating a corresponding output signal; filter means connected to the output of said microphone means for extracting from said output signal that portion which corresponds to said low-intensity sound; and indicator means responsive to the extracted signal output from said filter means to provide an indication of sound pressure versus position of said microphone means.

Claim 19

Apparatus as defined in claim 16 including: means connecting said filter means to said oscillator to cause the frequency of the extracted signal output to correspond to the frequency of said low-intensity sound propagated by said second transducer.

Claim 22

The method defined in claim 18 wherein said biasing noise signal comprises a broadband noise signal having a sound pressure level greater than 100 decibels, where 0 decibels equals 0.0002 dyne per square centimeter.

Claim 23

The method defined in claim 18 wherein said biasing noise signal has a predetermined spectrum shape and sound pressure level.

Claim 24

The method defined in claim 18 wherein the spectrum and level of said broadband noise signal is shaped to be substantially the same as that encountered in the normal operating environment of said test specimen.