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Research Papers

An Improved Technique for Measuring the Vibration Sensitivity of Subminiature Microphones

[+] Author and Article Information
Jonathan D. Walsh

Department of Mechanical Engineering,
Binghamton University,
Binghamton, NY 13902
e-mail: jwalsh3@binghamton.edu

Quang T. Su

Department of Mechanical Engineering,
Binghamton University,
Binghamton, NY 13902
e-mail: qsu@binghamton.edu

Daniel Warren

GN Advanced Science,
Glenview, IL 60026
e-mail: dwarren@gnresound.com

Contributed by the Noise Control and Acoustics Division of ASME for publication in the JOURNAL OF VIBRATION AND ACOUSTICS. Manuscript received August 10, 2017; final manuscript received December 3, 2017; published online January 25, 2018. Assoc. Editor: Theodore Farabee.

J. Vib. Acoust 140(3), 031008 (Jan 25, 2018) (7 pages) Paper No: VIB-17-1363; doi: 10.1115/1.4038681 History: Received August 10, 2017; Revised December 03, 2017

Presented is a test methodology for characterizing the vibration sensitivity of miniature microphones. An ordinary vibration sensitivity experiment becomes difficult because vibrating surfaces are also sources of sound. This sound is picked up by the microphone being tested, changing the result. The sound pressure will be correlated with the vibration signal such that averaging will not serve to increase the accuracy of that result.The previously described techniques reduce the correlated pressure using custom experimental equipment and have geometric limitations. In the improved technique, the microphone is treated like a linear two-input-one-output system. The two input signals (vibration and acoustic pressure) are measured, and the vibration sensitivity is determined using two different spectral analysis techniques. These techniques have good agreement between one another, and the measured values fit well into a simple acoustic model of the microphone. A technique for estimating the major source of measurement error indicates that this error is small enough for a reasonable estimate of vibration sensitivity to be made.

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References

Rule, E. , Suellentrop, F. J. , and Perls, T. A. , 1960, “ Vibration Sensitivity of Condenser Microphones,” J. Acoust. Soc. Am., 32(7), pp. 821–823. [CrossRef]
Friis, L. , 2009, “ Investigation of Internal Feedback in Hearing Aids,” Ph.D. thesis, Technical University of Denmark, Lyngby, Denmark. http://orbit.dtu.dk/en/publications/investigation-of-internal-feedback-in-hearing-aids(3c5ffce2-9828-4a4f-93ba-51e7eb0508ff).html
Killion, M. C. , 1974, “ Vibration Sensitivity Measurements on Subminiature Condenser Microphones,” 49th Convention of the Audio Engineering Society, New York, Sept. 9–12. https://www.etymotic.com/media/publications/erl-0057-1975.pdf
Koukias, S. , 2011, “ Vibration Sensitivity of Miniature Microphones,” Master's thesis, Technical University of Denmark, Lyngby, Denmark.
Bendat, J. S. , and Piersol, A. G. , 2011, Random Data: Analysis and Measurement Procedures, 4th ed., John Wiley & Sons, New York.
Bendat, J. S. , and Piersol, A. G. , 1980, Engineering Applications of Correlation and Spectral Analysis, John Wiley & Sons, New York.

Figures

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Fig. 1

The experimental setup includes a shaker, speaker, probe microphone, and accelerometer/vibrometer

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Fig. 2

Experimental setup diagram

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Fig. 3

A microphone's signal is a combination of an acoustic response and the vibration response

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Fig. 4

The vibration measurement. The microphone behaves like a two-input-one-output system.

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Fig. 5

A speaker-driven measurement. The microphone does not vibrate and therefore the acoustic sensitivity is measured.

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Fig. 6

Simultaneous uncorrelated speaker and shaker signals combine. This meets the requirement, γvp2≠0, in Eq. (8).

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Fig. 7

Diagram of the sound pressure verification test. The microphone's motion is fixed near a vibrating shaker surface.

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Fig. 8

The sound pressure verification test. The microphone is held in place on a clamped metal surface near to the shaker.

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Fig. 9

Analytical model of a microphone. The diaphragm mass m at l = 0 separates the back-volume from the front-volume. Sound pressure enters the device at l = Lf.

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Fig. 10

The vibration caused by the speaker is similar to the experimental background noise

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Fig. 11

The two techniques for estimating vibration sensitivity and the decibel difference between them

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Fig. 12

The cavity technique in Killion [4] (using data from Friis [3]), compared to the technique in Sec. 2.2. Both curves are from similar electret microphone devices.

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Fig. 13

Coherence for the techniques of Secs. 2.2 (Technique 1) and 2.3 (Technique 2)

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Fig. 14

The vibration sensitivity estimate compared with the estimated error caused by sound pressure measurement

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Fig. 15

The difference between the (dBSPL) vibration sensitivity estimate and the estimated error due to sound pressure measurement

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Fig. 16

The pressure sensitivity of a more massive (thicker) diaphragm decreases at low frequencies and increases at high frequencies

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Fig. 17

The vibration sensitivity of a more massive (thicker) diaphragm increases at all frequencies

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Fig. 18

The input referred vibration sensitivity of the experimental measurement is predicted by the analytical model for diaphragm thickness hd = 1 μm. A thinner or less massive diaphragm tends to decrease the (Pa/g) vibration sensitivity.

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