Technical Brief

Acoustically Coupled Microphone Arrays

[+] Author and Article Information
R. N. Miles

Department of Mechanical Engineering,
Binghamton University,
State University of New York,
Binghamton, NY 13902-6000

Contributed by the Noise Control and Acoustics Division of ASME for publication in the JOURNAL OF VIBRATION AND ACOUSTICS. Manuscript received February 24, 2016; final manuscript received July 22, 2016; published online September 8, 2016. Assoc. Editor: Theodore Farabee.

J. Vib. Acoust 138(6), 064503 (Sep 08, 2016) (9 pages) Paper No: VIB-16-1088; doi: 10.1115/1.4034332 History: Received February 24, 2016; Revised July 22, 2016

An analysis is presented of the performance benefits that can be achieved by introducing acoustic coupling between the diaphragms in an array of miniature microphones. The introduction of this coupling is analogous to the principles employed in the ears of small animals that are able to localize sound sources. Measured results are shown, which indicate a dramatic improvement in acoustic sensitivity, and noise performance can be achieved by packaging a pair of small microphones so that their diaphragms share a common back volume of air. This is also shown to reduce the adverse effects on directional response of mismatches in the mechanical properties of the microphones.

Copyright © 2016 by ASME
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Fig. 1

Schematic representation of uncoupled and coupled microphone arrays. The upper figure shows a schematic of the diaphragms, equivalent mechanical spring stiffnesses, and back volumes of a typical uncoupled microphone array consisting of four microphones. The lower panel illustrates the same set of microphones in which their diaphragm motions are coupled by having them share a common back volume of air.

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Fig. 2

Uncoupled Knowles omnidirectional TO38-30886-B21 microphone array. The microphones are assembled so that the distance between them is d = 12 mm. A Bruel and Kjaer 4138 1/8 in. microphone is also shown. This microphone was used as a calibrated reference microphone in the measurements. If we take y to be measured in the direction parallel to the line joining the microphones, the two test microphones are then located at y = ±6 mm, and the reference microphone is at y = 0. The sound source was located at approximately y = 1.5 m so that the sound propagated in the y direction. The measurements were performed in the anechoic chamber at the Binghamton University, which is anechoic at all the frequencies above approximately 80Hz.

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Fig. 3

Predicted and measured sensitivity of a Knowles TO38-30886-B21 microphone. The predictions are the result of evaluating Eq. (4) using the parameters given in Eq. (5).

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Fig. 4

Predicted and measured sensitivity of the difference in output voltage of two Knowles TO microphones configured as shown in Fig. 2. The sensitivity is shown due to a plane sound wave incident in the direction parallel to the line joining the two microphones. The separation distance between the microphones is d = 0.012 m. When measuring the difference in pressure, the sensitivity is reduced substantially over much of the frequency range of interest, owing to the fact that the difference in the pressures at the two microphones will be approximately ΔP≈eι̂ωt(eι̂ωd/(2c)−e−ι̂ωd/(2c))≈−Peι̂ωtι̂kd, where ω is the frequency in rad/s, and c ≈ 344 m/s is the speed of sound propagation. The data are shown normalized to the reference pressure measured by a calibrated microphone at a point equidistant between the microphones in the direction of sound propagation.

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Fig. 5

Measured directivity of uncoupled Knowles TO microphones at 1 kHz configured as shown in Fig. 2. Adding the outputs of the nondirectional microphones produces a nondirectional signal as shown in the polar plot in the lower left while subtracting the outputs produces a figure eight polar pattern as shown in the lower right.

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Fig. 6

Coupled Knowles TD-24621 microphone array. A schematic of the configuration is shown in the upper portion of the figure, and a photograph of the actual array is shown below the schematic. A differential pressure at the two sound inlets results in both diaphragms moving such that there is minimal compression of air within the microphones. The essential components of these TD microphones, including the diaphragm, backplate, and electronic interface, are identical to those used in the uncoupled TO series microphone array shown in Fig. 2. The microphones were oriented so that the sound wave propagated parallel to the line joining the two microphones.

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Fig. 7

Measured microphone sensitivities for difference signals in uncoupled and coupled two microphone arrays. The coupled microphone array provides substantially greater sensitivity at low frequencies than the uncoupled array. This increased sensitivity is caused by the fact that as the diaphragms move in response to pressure differences at the sound inlets, the uncoupled array does not result in significant air compression within the package, unlike what occurs in the uncoupled array.

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Fig. 8

Measured directivity patterns at 1 kHz obtained using the coupled two microphone array shown in Fig. 6. Appropriate processing of the outputs of the coupled microphone array provides both a directional (shown in the lower right) and nondirectional output (shown in the lower left) as in the uncoupled array.

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Fig. 9

The measured noise floor of coupled microphones is superior to that of uncoupled microphones. The measured improvement in noise performance is roughly 25 dB at low frequencies. The difference of the output signals from the uncoupled omnidirectional microphones was obtained using a Stanford Research Systems SR560 Low Noise Preamplifier. When the output signals from the coupled array of Fig. 6 are added to obtain a differential pressure signal, the noise floor achieved is very similar to that of a single TD differential microphone. When their signals are subtracted, the result is an omnidirectional output which has a noise floor that is very similar to that of a single omnidirectional microphone.




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