Unidirectional Microphones

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

This invention relates to electroacoustic transducers and more particularly to a directional microphone with a unidirectional directivity pattern.

Background of the Invention

Directional microphones are commonly employed in a variety of audio communications systems to emphasize weak signals in a noisy environment and to reduce the effects of reverberations. Such microphones also find application in situations where such sounds have a degrading influence on the quality of sound transmission. Microphones are available with numerous diverse sensitivity patterns ranging from the omnidirectional to the unidirectional patterns. In sound studio applications and the like, the unidirectional microphone is widely used.

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 construction, reasonably small in size and devoid of all unnecessary electronic apparatus and power-consuming elements.

Thus, it is an object of the present invention to overcome the faults of prior microphone arrangements in a simple, compact electroacoustic transducer which exhibits a unidirectional directivity pattern.

Summary of the Invention

In accomplishing this and other objects and in accordance with the invention, a unidirectional microphone is constructed by combining two first-order 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 diametrically 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 two gradient 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 casing 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 10 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 casing 14 is divided into two internal cavities 15 and 16 by a transducer arrangement supported perpendicular to the axis of casing 14. The transducer is formed of a perforated backplate 17 and a foil electret 18 held in close proximity to perforated backplate 14. Foil electret 18 seals the two cylindrical cavities from each other. Cavity 15 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.sub.2 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.sub.1 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.sub.3, the delay .tau. between the two gradient transducers is given by d.sub.3 /c, where c is the velocity of sound. The required delay .tau. 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 of a cavity transducer employing acoustic tubes are described in detail in Sessler-West U.S. Pat. No. 3,573,400, issued Apr. 6, 1971. 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 100 holes 0.08 cm in diameter, and with four circular ridges on one surface, each 25.4.mu.m high. Electret foil 18 consisted of a 25.4.mu.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 .omega..sup.2 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(ikd.sub.2) [exp(ikd.sub.1) + exp(ikd.sub.3) ], (1)

where A is a constant of proportionality independent of frequency and angle of incidence .theta. relative to the rotational axis of the system, i represents the imaginary operator, k represents wave number, k is equal to (k cos .theta.), and d.sub.1, d.sub.2, and d.sub.3 represent spacings, in cm, as illustrated in FIG. 3. For kd.sub.1 <<1 and kd.sub.3 <<1, this may be written as

S = A(ikd.sub.2) (ikd.sub.1 + ikd.sub.3)

= A(k.sup.2 d.sub.2 cos .theta.) (d.sub.3 + d.sub.1 cos .theta.) (2)

The sensitivity S of the system, for a constant angle of incidence, follows from Equation (2) and is given by

S = Bk.sup.2 = B(.omega./c).sup.2 , (3)

where B is a constant of proportionality independent of frequency and .omega. represents angular frequency. Sensitivity is thus proportional to the square of frequency.

For constant frequency and for d.sub.1 =d.sub.3, sensitivity S, also following from Equation (2), is given by

S = D cos .theta. (1 + cos .theta.) , (4)

where D is a constant of proportionality independent of angle of incidence. The directivity pattern is thus characterized by a maximum at .theta. = 0.degree., zeros at .theta. = 90.degree. and 180.degree., and two sidelobes at .theta. = 120.degree.. Theoretical directivity patterns of the transducer for a number of ratios d.sub.3 /d.sub.1 are tabulated in the patterns of FIG. 4.

Frequency response measurements on the transducer constructed in practice and described above indicate that sensitivity for .theta. = 0.degree. is within 2 db from 250 Hz to 3 kHz. The response for .theta. = 90.degree. and 180.degree. is 10-20 db lower than the response for .theta. = 0.degree. 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.

Claims

What is claimed is:

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.

10. A second-order gradient microphone which comprises, in combination,

a cylindrical casing,

electroacoustic transducer means coextensive with the internal cross section of said casing and positioned to separate said casing into two selectively dimensioned acoustic chambers,

a first pair of acoustic tubes with open ends of first and second lengths, respectively, opening into one of said chambers and extending outward therefrom to sample a sound field, respectively, at first and second points on a straight line, and,

a second pair of acoustic tubes with open ends of said first and second lengths, respectively, opening into the other one of said chambers and extending outward 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.