US3715500A - Unidirectional microphones - Google Patents

Unidirectional microphones Download PDF

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US3715500A
US3715500A US3715500DA US3715500A US 3715500 A US3715500 A US 3715500A US 3715500D A US3715500D A US 3715500DA US 3715500 A US3715500 A US 3715500A
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tubes
acoustic
microphone
cavities
line
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G Sessler
J West
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Nokia Bell Labs
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Nokia Bell Labs
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    • HELECTRICITY
    • H04ELECTRIC COMMUNICATION TECHNIQUE
    • H04RLOUDSPEAKERS, MICROPHONES, GRAMOPHONE PICK-UPS OR LIKE ACOUSTIC ELECTROMECHANICAL TRANSDUCERS; DEAF-AID SETS; PUBLIC ADDRESS SYSTEMS
    • H04R1/00Details of transducers, loudspeakers or microphones
    • H04R1/20Arrangements for obtaining desired frequency or directional characteristics
    • H04R1/32Arrangements for obtaining desired frequency or directional characteristics for obtaining desired directional characteristic only
    • H04R1/34Arrangements for obtaining desired frequency or directional characteristics for obtaining desired directional characteristic only by using a single transducer with sound reflecting, diffracting, directing or guiding means
    • H04R1/38Arrangements for obtaining desired frequency or directional characteristics for obtaining desired directional characteristic only by using a single transducer with sound reflecting, diffracting, directing or guiding means in which sound waves act upon both sides of a diaphragm and incorporating acoustic phase-shifting means, e.g. pressure-gradient microphone

Abstract

A second-order unidirectional microphone is constructed with two pairs of acoustic tubes arranged to sample a sound field at four different points on a straight line. Acoustic signals from two diametrically opposed tubes, one short and one long, are summed in a first cavity and signals from two other opposed tubes, one short and one long, are summed in another cavity. Signals developed in the two cavities are differentially combined by an electret or other transducer interposed between the cavities. Necessary signal delay is provided directly by differences in tube lengths.

Description

United States Patent 11 1 1111 3,715,500 i Sessler et al. 1 51 Feb. 6, 1973 [5 UNIDIRECTIONAL MICROPHONES 2,699,473 1 1955 Kettler ..179/1211) [75] manta; Gerhard Mama sealer, Summit; 2,301,744 11/1942 Olson ..179 1 DM gag gg west plamfield Primary Examiner-Ralph D. Blakeslee Assistant Examiner-Jon Bradford Leaheey [73] Assignee: Bell Telephone Laboratories, lncortt n y-R. Gl nt er et a porated, Murray Hill, NJ.

[22] Filed: July 21, 1971 [2]] Appl. No.: 164,507

[57] ABSTRACT A second-order unidirectional microphone is constructed with two pairs of acoustic tubes arranged to sample a sound field at four different points on a 5 s CL "179/1 BM 179/121 D, 79/111 E straight line. Acoustic signals from two diametrically [51] Int. Cl. ..H04r 1/32 opposed tubes one short and one long are summed m [58] me at Sam. 179/12 D lDM 121R 1115 a first cavity and signals from two other opposed tubes, one short and one long, are summed in another cavity. Signals developed in the two cavities are dif- [56] References cued ferentially combined by an electret or other trans- UNITED STATES PATENTS ducer interposed between the cavities. Necessary signal delay is provided directly by differences in tube 3,573,400 4/1971 Sessler ..179/121 D 1 lengths, 2,228,886 l/l94l Olson ..l79/l DM 2,793,255 5/ I957 Schlenker ..l79/l DM 10 Claims, 4 Drawing Figures UNIDIRECTIONAL MICROPHONES This invention relates to electroacoustic transducers and more particularly to a directional microphone with a unidirectional directivity pattern.

Backgroundof the Invention Description of the Prior Art Unidirectional microphones, e.g., either first-order gradient (cardioid), or second-order gradient microphones, respond predominantly to sound incident from one direction. A first-order cardioid pattern is achieved by combining in phase opposition the output of a pressure-sensitive element with the delayed output of a second pressure-sensitive element separated from the first by a distance that is small compared to a wavelength. A second-order gradient pattern is achieved by combining in phase opposition the output of a pressure-gradient element with the delayed output of another pressure-gradient element separated from the first by a distance that is small compared to a wavelength. In some microphones the delay is provided by an acoustical network integrally associated with the microphone structure, e.g., by' cavity arrangements exposed to a sound field at prescribed locations or by arrangements of acoustic tubes spaced to open at selected points in a sound field. In others, electrical delay networks are interposed between the two transducer elements to achieve a desired directional pattern.

Either arrangement requires a complex structure. In

addition, an electrical delay system often requires auxiliary energizing power. It is evident, of course, that a microphone, to be generally useful in a wide variety of applications should be relatively simple in construc- Summary of the Invention In accomplishing this and other objects and in accordance with the invention, a unidirectional microphone is constructed by combining two firstorder gradient microphones. Directionality is achieved by adding the output of a first-order gradient microphone to the delayed output of another, spatially displaced, but in-line, gradient microphone-of opposite polarity. In accordance with the invention, the requisite delay is produced by means of two pairs of sensing tubes of different lengths. The tubes are arranged to sample a sound field at four different points on a straight line. Acoustic signals from two diametricallyv opposed tubes, one short and one long, are summed in one cavity, and acoustic signals from two other diametrically opposed tubes, one short and one long, are summed in another cavity. Signals developed in the two cavities are differentially combined by an electret transducer interposed between the cavities. By an appropriate selection of tube lengths and placement, the signal delay necessary to achieve a unidirectional characteristic is obtained directly without other electrical or mechanically means.

Brief Description of the Drawing The invention will be more fully understood from the following description of a preferred embodiment thereof taken in connection with 'the appended drawings, wherein:

FIG. 1 depicts a unidirectional microphone constructed in accordance with the invention;

FIG. 2 is a schematic cross section of the unidirectional electret microphone shown in FIG. 1;

FIG. 3 is a schematic representation of the design principle of the invention which illustrates the combination of two first-order gradient microphones to achieve a directional characteristic, and

FIG. 4 is the directivity pattern of a unidirectional microphone in accordance with the invention for different physical dimensions.

Detailed Description A microphone which embodies the principles of the invention in a compact, durable configuration suitable for use in numerous applications is illustrated in FIG. 1. The microphone of FIG. 1 comprises a structure which includes two first-order gradient transducers of opposite polarities spaced apart from one another, a delay system, and an arrangement for adding the output of one of the gradient transducers to the delayed output of the other. It will be recognized that the addition of signals from twogradient microphone systems in this manner gives rise, in accordance with the well-known gradient principle, to a unidirectional sensitivity response pattern. Both signal delay and signal addition, however, is accomplished, in accordance with this invention by virtue of the structural arrangement employed; no additional electrical components are needed.

A sound field is sampled at four selected points in a straight line by means of two pairs of acoustic tubes, 10 and 12, and 11 and 13, which feed acoustic signals into cylindrical casing 14. Tubes 10 and 12 are of equal length and feed sound from two separated points on a straight line passing through a diameter of cylindrically shaped casing 14. Signals from tubes 10 and 12 are brought, respectively, into upper and lower cavities in' casing 14, and serve differentially to excite opposite sides of transducer 20 placed within cavity 14 in a plane perpendicular to its axis. Transducer 20 in fact divides the easing into two separate cavities. The system of tubes 10 and 12, the two cavities, and transducer 20 together constitute a first-order gradient microphone.

Tubes 11 and 13 are likewise of equal length but the length of the pair of tubes 11 and 13 is different from the length of the pair of tubes and 12. Tubes 11 and 13 sample a sound field at two points on the same straight line, passing through a diameter of casing 14'. Signals from the tubes are brought respectively into the upper and lower cavity portion of the casing and serve differentially to excite opposite sides of transducer 20. The system of tubes 11 and 13, the two cavities, and the transducer constitute another first-order gradient microphone.

The internal construction of the transducer of FIG. 1 is illustrated in FIG. 2. It will be observed that the easing 14 is divided into two internal cavities l5 and 16 by a transducer arrangement supported perpendicular to the axis of casing 14. The transducer is formed of a perforated backplate l7 and a foil electret 18 held in close proximity to perforated backplate 14. Foil electret l8 seals the two cylindrical cavities from each other. Cavity receives acoustic energy from tube 10, a longer tube, and from tube 11, a shorter tube, both arranged to sample a sound field on a straight line, e.g., on a diameter of the cylinder. Tubes 10 and 11 form parts of two different gradient microphone systems; their sound signals are effectively added together in cavity 15. Lower cavity 16 is fed by tube 12, a longer'tube, and tube 13, a shorter tube, both arranged to sample the field at points on the same straight line as the sampling points of tubes 10 and 11. Tubes 12 and 13 form parts of two different gradient microphone systems; their sound signals are added together in cavity 16.

The principle of operation of the unidirectional microphone of the invention is schematically indicated in FIG. 3. In the figure, d represents the separation of the open outer ends of acoustic tubes 10 and 12, which together form one first-order gradient microphone, and also represents the separation of the open outer ends of acoustic tubes 11 and 13, which together form another first-order gradient microphone. Distance d, indicates the separation between the tubes of the two gradient systems. The short tubes 11 and 13 belong to a gradient with a small delay whereas the long tubes 10 and 12 belong to a gradient with a greater delay. If the difference in the length of the two pairs of tubes is denoted d the delay 1- between the two gradient transducers is given by d le, where c is the velocity of sound. The required delay r for the directional transducer is established entirely by the system of acoustic tubes. It is represented in FIG. 3 as element 30 solely to illustrate the relationships involved.

In a conventional system signals from two gradient microphones are individually summed, signals from one are delayed, e.g., in delay system 30, and the two resultant signals are then added, e.g., in adder 31. In accordance with this invention, to the contrary, addition of pairs of signals takes place directly in the respective cavities, and the necessary delay is achieved directly through the selection, dimensioning, and placement of the system of acoustic tubes.

Details of the construction ofa cavity transducer employing acoustic tubes are described in detail in Sessler- West US. Pat. No. 3,573,400, issued Apr. 6, I971. The directional microphone described in that patent achieves a toroidal sensitivity characteristic. Yet, structural elements similar to those used in fabricating the unidirectional microphone of this invention are identical. Accordingly, details of fabrication are omitted here.

In a typical transducer unit 20 employed in practice, a backplate 17 was constructed from a brass disc 4 cm in diameter with holes 0.08 cm in diameter, and with four circular ridges on one surface, each 25.4p.m high. Electret foil 18 consisted of a 25.4p.m layer of fluoroethylenepropylene, marketed commercially under the trademark Teflon FEP, which was metalized on one side and charged preferably using an electron beam method. The transducer formed by the backplate and the electret foil is mounted so that the two chambers formed within casing 14 are sealed from each other. In practice, one of the two chambers is adjustable in volume, e.g., using a screw plunger or the like to permit tuning of the acoustic (Helmholtz) resonances of the tube cavity system. Electrostatic transducers employing perforated backplates and foil electret diaphragm members are known to those skilled in the art and described, for example, in Sessler-West U.'S. Pat. No. 3,118,022, granted Jan. 14, 1964. Details of backplate preparation, ridge structure, and the like are similarly known and described in the art, for example in Sessler-West U. S. Pat. No. 3,118,979, granted Jan. 21, 1964.

In the example of practice, tubes 10 and 12 were 5 cm long and tubes 11 and 13 were each 3.5 cm long. The two sets of tubes had inner and outer diameters of 0.22 and 0.32 cm, respectively. All tubes were open at their ends and were filled with approximately 60 mg. of steel wool to damp acoustic resonances.

Each cavity and its associated tubes thus form a Helmholz resonator or an acoustic low-pass filter. By placing the resonance frequency at the lower end of the frequency range of interest, a compensation of the m dependence, discussed below, of the sensitivity of the system is achieved.

Since all four sensors are, in this invention, arranged to sample a sound field at points in a straight line, the directivity pattern of the transducer is rotationally symmetric. Addition of the output voltages of both gradient systems thus yields a sensitivity pattern S given by S=A(iFd [exp(iFd,)+exp(ikd (I) where A is a constant of proportionality independent of frequency and angle of incidence 0 relative to the rotational axis of the system, i represents the imaginary operator, k represents wave number, I is equal to (k cos 0), and d d and (I, represent spacings, in cm, as illustrated in FIG. 3. For Ed, 1 and kd 1, this may be written as =A(kd cos 0)(d +d, cos 6) (2) The sensitivity S of the system, for a constant angle ofincidence, follows from Equation (2) and is given by (3) where B is a constant of proportionality independent of frequency and 0) represents angular frequency. Sensitivity is thus proportional to the square of frequency.

For constant frequency and for d,=d sensitivity 8, also following from Equation (2), is given by S=Dcos0(l+cos0), (4) where D is a constant of proportionality independent of angle of incidence. The directivity pattern is thus characterized by a maximum at 0 0, zeros at 0 90 and 180, and two sidelobes at 0 Theoretical directivity patterns of the transducer for a number of ratios 11 /11 are tabulated in the patterns of FIG. 4.

Frequency response measurements on the transducer constructed in practice and described above indicate that sensitivity for 0 0 is within 2 db from 250 Hz to 3 kHz. The response for 0 90 and 180 is -20 db lower than the response for 0 0 over most of the telephone band. The Helmholz resonance of the system aids materially in compensating and equalizing the system. The measured directivity index for the microphone described in detail above is about 8 db as compared with a calculated value of 8.7 db.

It is evident that the exact sensitivity and directivity patterns for the transducer may be altered by varying the overall size of the unit, the relative sizes of the cavities and the lengths of the tubes. With such modifications, however, the relationships among the various elements must, of course, be maintained to achieve the results described herein. Moreover, it will be recognized that the unidirectional characteristic of the microphone of the invention reduces to a cardioid 'pattern if only two of the tubes are used to supply signal samples to the system, one short tube to feed one cavity and one long tube to feed the other cavity.

What is claimed is:

l. A unidirectional microphone, which comprises,

first and second selectively dimensioned acoustic chambers,

electroacoustic means positioned to separate said chambers for differentially converting acoustic signals in said chambers into electrical signals,

first and second acoustic tubes of first and second lengths, respectively, mated into said first chamber and extending outward therefrom to sample a sound field at first and second points on a straight line, and

third and fourth acoustic tubes of said first and second lengths, respectively, mated into said second chamber and extending outward therefrom to sample said sound field at third and fourth points on said straight line,

the sampling end of said first tube and the sampling end of said fourth tube being spaced apart from one another on said straight line by the same distance as the sampling end of said second tube and the sampling end of said third tube are spaced apart on said straight line, and the sampling ends of said first and said second tubes being spaced farther apart from one another than the sampling ends of said third and said fourth tubes.

2. A unidirectional microphone as defined in-claim 1, wherein, said electroacoustic means comprises a foilelectret transducer.

3. A unidirectional microphone as defined in claim 1, wherein,

said tubes of said first length are selected to be longer than the tubes of said second length by a length related to the spacing on said straight line between the sampling point of one of said first length tubes feeding said first chamber and one of said second length tubes feeding said second chamber.

4. A unidirectional microphone as defined in claim 3, wherein,

the ratio of the difference in length between said tubes of said first and second lengths to the spacing on said straight line between the sampling point of that of said first length tubes feeding said first chamber and one of said second length second tubes feeding said second chamber is unity. 5. A directional electrostatic transducer,'which comprises,

first and second selectively dimensioned cylindrical acoustic cavities, transducer means separating said cavities for differentially converting acoustic signals supplied to said cavities into electrical signals, two acoustic tubes of first and second lengths opening, respectively, into said first and second cavities and extending outward therefrom to sample a sound field at first points separated from one another by a selected distance on a straight line parallel to a diameter of said cavity, and two acoustic tubes of said first and second lengths opening, respectively, into said second and first cavities and extending outward therefrom to sample said sound field at second points separated from one another by said selected distance and diametrically opposed to said first points on said line, the sampling ends of said tubes opening into said first cavity being spaced farther from one another than the sampling ends of said tubes opening into said second cavity. 6. A directional transducer as defined in claim 5, wherein,

said electrostatic transducer means comprises, a foilelectret transducer supported between said cavities in a plane perpendicular to an axis thereof. 7. A unidirectional second-order gradient microphone, which comprises,

first and second selectively dimensioned acoustic cavities, transducer means intermediate said differentially converting acoustic signals in said cavities into electrical signals, two acoustic tubes of first and second lengths opening, respectively, into said first and second cavities and extending outward therefrom in a first direction to sample a sound field at points on a straight line separated from one another by a selected distance, and two acoustic tubes of said first and second lengths opening, respectively, into said second and first cavities and extending outward therefrom in a second direction opposite to said first direction to sample said sound field at points on said straight line separated from one another by said selected distance. 8. A second-order gradient microphone, as defined in claim 7, wherein,

all of said tubes are equipped with means for damping acoustic resonances. 9. A second-order gradient microphone, as defined in claim 7, wherein,

said cavities are proportioned to be resonant at the lower end of the frequency range to be accommodated by said microphone. 10. A second-order gradient microphone which comprises, in combination,

a cylindrical casing,

chambers for ward therefrom to sample said sound field, respectively, at third and fourth points on said straight line,

said first and said second sampling points being spaced apart from one another by a greater distance than said third and said fourth sampling points, and the spacing between said first and said fourth sampling points being equal to the spacing between said second and said third sampling points.

Claims (10)

1. A unidirectional microphone, which comprises, first and second selectively dimensioned acoustic chambers, electroacoustic means positioned to separate said chambers for differentially converting acoustic signals in said chambers into electrical signals, first and second acoustic tubes of first and second lengths, respectively, mated into said first chamber and extending outward therefrom to sample a sound field at first and second points on a straight line, and third and fourth acoustic tubes of said first and second lengths, respectively, mated into said second chamber and extending outward therefrom to sample said sound field at third and fourth points on said straight line, the sampling end of said first tube and the sampling end of said fourth tube being spaced apart from one another on said straight line by the same distance as the sampling end of said second tube and the sampling end of said third tube are spaced apart on said straight line, and the sampling ends of said first and said second tubes being spaced farther apart from one another than the sampling ends of said third and said fourth tubes.
1. A unidirectional microphone, which comprises, first and second selectively dimensioned acoustic chambers, electroacoustic means positioned to separate said chambers for differentially converting acoustic signals in said chambers into electrical signals, first and second acoustic tubes of first and second lengths, respectively, mated into said first chamber and extending outward therefrom to sample a sound field at first and second points on a straight line, and third and fourth acoustic tubes of said first and second lengths, respectively, mated into said second chamber and extending outward therefrom to sample said sound field at third and fourth points on said straight line, the sampling end of said first tube and the sampling end of said fourth tube being spaced apart from one another on said straight line by the same distance as the sampling end of said second tube and the sampling end of said third tube are spaced apart on said straight line, and the sampling ends of said first and said second tubes being spaced farther apart from one another than the sampling ends of said third and said fourth tubes.
2. A unidirectional microphone as defined in claim 1, wherein, said electroacoustic means comprises a foil-electret transducer.
3. A unidirectional microphone as defined in claim 1, wherein, said tubes of said first length are selected to be longer than the tubes of said second length by a length related to the spacing on said straight line between the sampling point of one of said first length tubes feeding said first chamber and one of said second length tubes feeding said second chamber.
4. A unidirectional microphone as defined in claim 3, wherein, the ratio of the difference in length between said tubes of said first and second lengths to the spacing on said straight line between the sampling point of that of said first length tubes feeding said first chamber and one of said second length second tubes feeding said second chamber is unity.
5. A directional electrostatic transducer, which comprises, first and second selectively dimensioned cylindrical acoustic cavities, transducer means separating said cavities for differentially converting acoustic signals supplied to said cavities into electrical signals, two acoustic tubes of first and second lengths opening, respectively, into said first and second cavities and extending outward therefrom to sample a sound field at first points separated from one another by a selected distance on a straight line parallel to a diameter of said cavity, and two acoustic tubes of said first and second lengths opening, respectively, into said second and first cavities and extending outward therefrom to sample said sound field at second points separated from one another by said selected distance and diametrically opposed to said first points on said line, the sampling ends of said tubes opening into said first cavity being spaced farther from one another than the sampling ends of said tubes opening into said second cavity.
6. A directional transducer as defined in claim 5, wherein, said electrostatic transducer means comprises, a foil-electret transducer supported between said cavities in a plane perpendicular to an axis thereof.
7. A unidirectional second-order gradient microphone, which comprises, first and second selectively dimensioned acoustic cavities, transducer means intermediate said chambers for differentially converting acoustic signals in said cavities into electrical signals, two acoustic tubes of first and second lengths opening, respectively, into said first and second cavities and extending outward therefrom in a first direction to samplE a sound field at points on a straight line separated from one another by a selected distance, and two acoustic tubes of said first and second lengths opening, respectively, into said second and first cavities and extending outward therefrom in a second direction opposite to said first direction to sample said sound field at points on said straight line separated from one another by said selected distance.
8. A second-order gradient microphone, as defined in claim 7, wherein, all of said tubes are equipped with means for damping acoustic resonances.
9. A second-order gradient microphone, as defined in claim 7, wherein, said cavities are proportioned to be resonant at the lower end of the frequency range to be accommodated by said microphone.
US3715500A 1971-07-21 1971-07-21 Unidirectional microphones Expired - Lifetime US3715500A (en)

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Cited By (27)

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US4156800A (en) * 1974-05-30 1979-05-29 Plessey Handel Und Investments Ag Piezoelectric transducer
US4555598A (en) * 1983-09-21 1985-11-26 At&T Bell Laboratories Teleconferencing acoustic transducer
EP0186996A2 (en) * 1984-12-20 1986-07-09 AT&T Corp. Unidirectional second order gradient microphone
US5007091A (en) * 1987-04-23 1991-04-09 Utk Uuden Teknologian Keskus Oy Procedure and device for facilitating audiovisual observation of a distant object
US5121426A (en) * 1989-12-22 1992-06-09 At&T Bell Laboratories Loudspeaking telephone station including directional microphone
US5226076A (en) * 1993-02-28 1993-07-06 At&T Bell Laboratories Directional microphone assembly
EP0711095A2 (en) * 1994-11-03 1996-05-08 AT&T Corp. Baffled microphone assembly
US5633935A (en) * 1993-04-13 1997-05-27 Matsushita Electric Industrial Co., Ltd. Stereo ultradirectional microphone apparatus
US5692060A (en) * 1995-05-01 1997-11-25 Knowles Electronics, Inc. Unidirectional microphone
US5745588A (en) * 1996-05-31 1998-04-28 Lucent Technologies Inc. Differential microphone assembly with passive suppression of resonances
US5848172A (en) * 1996-11-22 1998-12-08 Lucent Technologies Inc. Directional microphone
EP0985327A1 (en) * 1998-01-20 2000-03-15 Shure Brothers Incorporated Flush mounted uni-directional microphone
US20020131228A1 (en) * 2001-03-13 2002-09-19 Potter Michael D. Micro-electro-mechanical switch and a method of using and making thereof
US20020182091A1 (en) * 2001-05-31 2002-12-05 Potter Michael D. Micro fluidic valves, agitators, and pumps and methods thereof
US20030008676A1 (en) * 2001-07-03 2003-01-09 Baumhauer John Charles Communication device having a microphone system with optimal acoustic transmission line design for improved frequency and directional response
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US20050044955A1 (en) * 2003-08-29 2005-03-03 Potter Michael D. Methods for distributed electrode injection and systems thereof
US20050205966A1 (en) * 2004-02-19 2005-09-22 Potter Michael D High Temperature embedded charge devices and methods thereof
US7116792B1 (en) * 2000-07-05 2006-10-03 Gn Resound North America Corporation Directional microphone system
US20070074731A1 (en) * 2005-10-05 2007-04-05 Nth Tech Corporation Bio-implantable energy harvester systems and methods thereof
US7211923B2 (en) 2001-10-26 2007-05-01 Nth Tech Corporation Rotational motion based, electrostatic power source and methods thereof
US7217582B2 (en) 2003-08-29 2007-05-15 Rochester Institute Of Technology Method for non-damaging charge injection and a system thereof
US20090059724A1 (en) * 2007-09-04 2009-03-05 Scanlon Michael V Systems and Methods for Analyzing Acoustic Waves
US20090161900A1 (en) * 2007-12-21 2009-06-25 Tandberg Telecom As Microphone assembly for minimizing acoustic feedback from a loudspeaker
US20130028440A1 (en) * 2011-07-26 2013-01-31 Akg Acoustics Gmbh Noise reducing sound reproduction system

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Cited By (45)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US4156800A (en) * 1974-05-30 1979-05-29 Plessey Handel Und Investments Ag Piezoelectric transducer
US4555598A (en) * 1983-09-21 1985-11-26 At&T Bell Laboratories Teleconferencing acoustic transducer
EP0186996A2 (en) * 1984-12-20 1986-07-09 AT&T Corp. Unidirectional second order gradient microphone
EP0186996A3 (en) * 1984-12-20 1987-12-02 American Telephone And Telegraph Company Unidirectional second order gradient microphone
US4742548A (en) * 1984-12-20 1988-05-03 American Telephone And Telegraph Company Unidirectional second order gradient microphone
US5007091A (en) * 1987-04-23 1991-04-09 Utk Uuden Teknologian Keskus Oy Procedure and device for facilitating audiovisual observation of a distant object
US5121426A (en) * 1989-12-22 1992-06-09 At&T Bell Laboratories Loudspeaking telephone station including directional microphone
US5226076A (en) * 1993-02-28 1993-07-06 At&T Bell Laboratories Directional microphone assembly
US5633935A (en) * 1993-04-13 1997-05-27 Matsushita Electric Industrial Co., Ltd. Stereo ultradirectional microphone apparatus
EP0711095A3 (en) * 1994-11-03 2001-01-10 AT&T Corp. Baffled microphone assembly
EP0711095A2 (en) * 1994-11-03 1996-05-08 AT&T Corp. Baffled microphone assembly
US5692060A (en) * 1995-05-01 1997-11-25 Knowles Electronics, Inc. Unidirectional microphone
US5745588A (en) * 1996-05-31 1998-04-28 Lucent Technologies Inc. Differential microphone assembly with passive suppression of resonances
US5848172A (en) * 1996-11-22 1998-12-08 Lucent Technologies Inc. Directional microphone
EP0985327A1 (en) * 1998-01-20 2000-03-15 Shure Brothers Incorporated Flush mounted uni-directional microphone
EP0985327B1 (en) * 1998-01-20 2009-06-03 Shure Acquisition Holdings, Inc. Flush mounted uni-directional microphone
US7116792B1 (en) * 2000-07-05 2006-10-03 Gn Resound North America Corporation Directional microphone system
US20020131228A1 (en) * 2001-03-13 2002-09-19 Potter Michael D. Micro-electro-mechanical switch and a method of using and making thereof
US7280014B2 (en) 2001-03-13 2007-10-09 Rochester Institute Of Technology Micro-electro-mechanical switch and a method of using and making thereof
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Also Published As

Publication number Publication date Type
DE2235169C3 (en) 1974-02-21 grant
DE2235169B2 (en) 1973-07-26 application
CA948305A1 (en) grant
GB1407266A (en) 1975-09-24 application
CA948305A (en) 1974-05-28 grant
DE2235169A1 (en) 1973-02-01 application
JPS5219966B1 (en) 1977-05-31 grant

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