Directional Microphone

Abstract

A directional microphone with toroidal or truncated toroidal sensitivity characteristics is constructed from a plurality of concentric transducer elements, the outputs of which are combined in accordance with a predetermined formula.

Description

This relates generally to directional transducers and more specifically to electroacoustic transducers having a toroidal or truncated toroidal sensitivity pattern.

BACKGROUND OF THE INVENTION

1. Field of the Invention

Directional microphones, which are microphones designed to be more sensitive to sounds impinging from one direction than from another, are commonly employed in numerous aspects of audio communications. In situations where it is desirable to emphasize acoustic signals emanating from sound sources located approximately in a single plane while rejecting spurious and reflected acoustic signals, a microphone with approximately toroidal sensitivity characteristics is appropriate. Such microphones have uniformly high sensitivity for sound impinging in a selected single plane and low sensitivity for sound impinging normal to this plane. Situations requiring microphones of this type are quite prevalent. For example, in conference room telephone arrangements where several talkers are located around a conference table, it is desirable that the talkers' voices be received by a single microphone unit. Such a microphone must be sensitive to sounds in the plane of the talkers and must reject spurious acoustic energy, such as energy reflected from the conference table and other objects located within the conference room. Additionally, such a microphone must be durable and, in some cases, it may be desirable to have a constant phase response in the plane of maximum sensitivity.

2. Prior Art

In the past, microphones with toroidal directional characteristics have generally employed two identical pressure gradient receivers oriented at right angles to each other. By adding the output of one such receiver to the 90.degree. phase shifted output of the other, one obtains an almost uniform response in the plane of the two gradients and a sensitivity which decreases in proportion to the cosine of the angle between the direction of incidence of the sound wave and the gradient plane. One such toroidal microphone is disclosed in H. F. Olson, U.S. Pat. No. 2,539,671 issued Jan. 30, 1951. Olson employs two perpendicular ribbon microphone elements in conjunction with appropriate phase shifting networks to achieve toroidal sensitivity. However, microphones such as that described by Olson have several features which may be significant shortcomings in certain situations. Firstly, the sensitivity of such a microphone decreases only as the cosine of the angle between the plane of the pressure gradients and the direction of incidence of the sound. Secondly, such a microphone requires a broadband 90.degree. phase shifting network. Finally, the phase response in the plane of maximum sensitivity is a function of the angle of incidence of the received sound wave.

It is thus an object of the present invention to eliminate these prior shortcomings in electroacoustic transducers having a toroidal or truncated toroidal sensitivity pattern.

SUMMARY OF THE INVENTION

In accomplishing this and other objectives and in accordance with the invention, a second order or higher even order directional microphone with toroidal sensitivity characteristics is constructed with a plurality of concentric, electroacoustic transducer elements electrically interconnected according to a predetermined weighing formula. In a preferred embodiment of the invention, a second order microphone is constructed with a pair of concentric cylindrical foil electret transducer elements electrically interconnected so that the output of the interior receiving element is subtracted from the output of the exterior element.

BRIEF DESCRIPTION OF THE DRAWING

The invention will be more fully apprehended from the following detailed description of illustrative embodiments thereof, taken in conjunction with the appended drawings wherein:

FIG. 1 is a perspective view partially in section of a full toroidal directional microphone constructed in accordance with an embodiment of the invention;

FIG. 2 is a perspective view partially in section of a truncated toroidal microphone constructed in accordance with another embodiment of the invention; and

FIG. 3 shows one possible directional sensitivity pattern of a microphone constructed in accordance with the invention.

DETAILED DESCRIPTION

The microphone shown in a partially sectioned perspective view in FIG. 1 is a second order microphone which is highly sensitive to sound waves impinging is a given plane and tends to reject acoustic signals impinging from a direction normal to that plane. The microphone is designed to sample the surrounding sound field impinging on two concentric annular electroacoustic transducers 10 and 11 of equal sensitivity located in the desired plane of maximum sensitivity. The electrical outputs of the two annular transducers are combined in a differential amplifier or other subtractive circuit 12 which produces an output signal proportional to the difference between the signal produced by the inner transducer 10 and the signal produced by the outer transducer 11. Such differential or subtractive circuits are well known in the electronic art and need not be described in detail here. In an alternative arrangement, oppositely polarized or oppositely biased transducers are employed in which case their outputs are added.

In the particular embodiment of the invention shown in FIG. 1, the concentric circular transducers 10 and 11 are strips of foil electret material formed into sections of two concentric cylinders. Such foil electret transducers are particularly desirable in this situation because they are highly sensitive and easily formed into a circular configuration. The theory underlying the operation of foil electret transducers is described in detail in an article entitled "Electrostatic Microphones with Electret Foil" by G. M. Sessler which will be found in the Journal of the Acoustical Society of America Vol. 35 No. 9, pp. 1,354--1,357, Sept. 1963, and in U.S. Pat. No. 3,118,022 issued Jan. 14, 1964 to G. M. Sessler and J. E. West and thus will not be discussed here.

In the arrangement of FIG. 1, the outer annular foil electret transducer comprises a cylindrical layer of metallic foil bonded to a cylindrical layer of thin plastic material, for example, of the type known commercially by the name Teflon FEP, or other dielectric material which has been prepolarized in an electrostatic field at an elevated temperature. The bonded foil and dielectric layer 14 is stretched across a perforated annular backplate 15 which is secured to an annular metallic chamber 16 having a generally U-shaped cross section opening to the outer dimension of the transducer 11. The metallic chamber 16 is electrically insulated from an annular structural outer casing 17 by a layer of insulating material 18. Electret foil layer 14 is electrically connected to the outer casing 17 by means of an electrically conductive clamp 13.

The construction of the inner transducer element 10 is similar to that of the outer element and so the inner element has not been shown in cross section. It should be noted that the inner element is typically greater in height than the outer element, making the surface area of the two transducers approximately equal. This arrangement is chosen because, as indicated above, the signals produced by the two annular transducers 10 and 11 must be balanced in order to provide a toroidal sensitivity characteristic. Equal area foil electret transducers have the requisite equal sensitivity and equal capacitance.

Both annular transducer elements are fitted with appropriate electrical connectors 19 and 20. These connectors include a conductive inner terminal 21, an insulating central layer 22 and a conductive outer sheath 23. In a preferred arrangement, the outer sheath is electrically connected to the outer casing 17 of the transducer, which is made of a conductive material. This establishes electrical contact between the outer sheath 23 and the foil layer 14 which is maintained in contact with the outer casing 17 by clamp 13. The conductive inner terminal 21 is electrically connected through the transducer's insulating layer 18 to the annular metallic chamber 16. Signals from the inner connector 19 and the outer connector 20 are applied to a subtractive circuit 12 which provides an output signal on lead 26 proportional to the difference between the two applied signals.

In a preferred embodiment of the invention, the outer annular transducer 11 includes an annular scattering element 30 of triangular cross section located on its rear or internal face between the outer and inner transducers. This element is included to suppress standing waves between the inner sensor and the back surface of the casing 17 of the outer sensor. Without this arrangement, selected frequencies might be emphasized by the inner sensor 10 and the system might not function properly for these frequencies.

The two annular transducers 10 and 11 are secured together in any desired fashion, for example, by a plurality of radially extending support elements 27, 28 and 29.

In practice, the microphone arrangement described above has been found to produce a full toroidal sensitivity pattern of the type shown in FIG. 3. The microphone's highest sensitivity occurs in the plane including the two annular transducers 10 and 11. For a sound impinging from above or below this plane at an angle .phi. with the plane, the microphone's sensitivity falls off in proportion to (cos .phi.).sup.2. Further, the microphone's phase response is uniform in all directions. Additionally, no phase shifting networks are required.

FIG. 2 shows a half-toroidal microphone constructed in accordance with a second alternative embodiment of the invention. Like the prior arrangement, this microphone includes two concentric circular transducing devices with the signals they produce being subtractively combined in a differential amplifier or other appropriate subtractive circuit in the fashion shown at 12 in FIG. 1. This embodiment of the invention differs from the embodiment described in conjunction with FIG. 1 in that the inner and outer circular transducer elements are coplanar, concentric surfaces. This structure thus employs area sensors in place of the line sensors employed in FIG. 1. Since in this arrangement, the transducer is exposed to sounds impinging from its underside by diffraction only, a half-toroidal sensitivity pattern is created. Again equal area foil electret transducers are shown, but it is to be understood that numerous other types of electroacoustic transducers may be employed.

The arrangement shown in FIG. 2 employs a single foil electret sheet 31, of the type described in conjunction with FIG. 1, having a layer of foil 32 on its outer side bonded to a layer of dielectric material 33 on its inner side. The foil-dielectric sheet 31 is mounted above a perforated metallic backplate which is split into a circular central region 34 and an annular outer region 35 separated by an annular ring of electrical insulating material 36. The outer backplate ring 35 is surrounded by a second annular ring of electrical insulation 37 and the entire insulator-backplate assembly is fitted over a cylindrical acoustic cavity 38 defined by a metallic cylindrical casing 39. The foil-dielectric sheet is secured across the casing above the backplate by an annular securing ring 40 secured to the casing 39, for example, by a plurality of screws such as 41.

With this arrangement, a first electrical contact is made to the external casing 39 which is in electrical contact with the foil layer 32. This first contact is made by way of the outer sheath of jacks 42 and 43 which abut the casing. A second electrical contact is made with the interior circular backplate section 34, by way of wire 44 and a third contact is made with the outer annular backplate section 35 by way of wire 45. The signal taken between the inner backplate 34 and the casing 39 is subtractively combined with the signal taken between the outer annular backplate 35 and the casing 39 in a subtractive network similar to that shown in FIG. 1. This combined signal reflects a transducer with a truncated toroidal sensitivity pattern.

It is to be understood that in this second embodiment, as in the first, it is often desirable to equalize signals from the inner and outer circular elements before they are combined. Again, this is preferably accomplished by employing inner and outer transducers of equal sensitivity and of equal area. Note, however, that if the sensitivity of the transducers is not naturally balanced, an external weighting circuit of a type well-known in the electronic arts may be employed to correct this deficiency. By use of such an arrangement, output signal levels may be equalized to compensate for unequal output signal amplitudes produced by transducer elements of unequal sensitivities or areas. Additionally, while the inner backplate element 34 has been shown in FIG. 2 as a solid circular area, it may also be a ring of backplate material similar to but of smaller radius than ring 35.

It should also be noted that other alternative arrangements similar to that described above will provide the requisite inner and outer transducer effect. For example, in the foil electret microphone described in conjunction with FIG. 2, a single backplate may be employed in conjunction with a sheet of electret foil which has been oppositely polarized in inner and outer circular regions.

In the discussion above, only second order toroidal and second order truncated toroidal microphones have been discussed in detail. However, it is to be understood that higher, even order toroidal and truncated toroidal microphones, constructed in accordance with the invention, employ more than two concentric transducer elements in an arrangement similar to that described above. The signals produced by such transducers are selectively combined to create a toroidal sensitivity pattern, for example, by differentially combining them or by deriving signals with opposite polarities from adjacent transducer elements and by adding all of the signals together. Furthermore, the signals produced by the individual transducer elements may be weighted to equalize their amplitudes or otherwise to establish a selected schedule of amplitudes. Thus, a fourth order toroidal or truncated toroidal microphone is constructed with an inner transducer element and two concentric annular ring transducers, where the inner transducer, the first ring transducers and the second or outer ring transducers are weighted by adjusting the amplitudes of individual output signals produced by the transducers in the ratio +3:-4:+1, respectively. In general, a toroidal or truncated toroidal transducer of order 2n, where n is an integer, is constructed with an inner transducer and n concentric outer transducer rings where the inner transducer is weighted in proportion to

and the outer transducer rings, corresponding to r=1, ...,n, are weighted in proportion to

The sensitivity of a higher order toroidal or truncated toroidal microphone constructed in accordance with the invention is proportional to (cos .phi.).sup.2n, where .phi. is the angle between the direction of incidence of a sound wave and the plane containing the transducer and n is the number of concentric outer transducer rings employed. At the same time the amplitude-frequency response of such a microphone rises with approximately 6n db./octave.

It is to be understood that the above-described arrangements are merely illustrative of the application of the principles of the invention. Other arrangements may be devised by those skilled in the art without departing from the spirit and scope of the invention. For example, while in the detailed description above only concentric circular transducers are considered, rectangular or square concentric transducers may be employed in accordance with the invention. Furthermore, in view of the principle of reciprocity, it is obvious that the transducers of the present invention may also be employed as loudspeakers, which exhibit toroidal or other directional patterns, to convert voltage variations into corresponding sound pressure variations. The term "transducer" has, for this reason, been employed herein to designate the units structurally, regardless of whether they effect a conversion from acoustic energy into electrical energy, or vice versa.

Claims

We claim:

1. A directional microphone of order 2n comprising, an inner electroacoustic transducer element and n concentric outer electroacoustic transducer elements, each of said transducer elements producing individual electrical signals, and means for selectively combining said individual electrical signals.

2. A directional microphone as defined in claim 1 wherein said individual electrical signals from adjacent ones of said inner and said outer transducer elements are combined with opposite signal polarities.

3. A directional microphone as defined in claim 1 wherein said means for combining said individual electrical signals comprises for adjusting the amplitudes of said individual electrical signals so that the relative weight of the signal produced by said inner transducer element is proportional to

and so that the relative weight of the signal produced by each annular outer element r=1 through r=n is proportional to

and means for summing said weighted individual electrical signals.

4. A second order directional transducer comprising, an inner electroacoustic transducer element, an outer electroacoustic transducer element approximately concentric with said inner element, and circuit means for differentially interconnecting said inner and outer transducer elements.

5. A directional transducer as defined claim 4, said transducer further including scattering means located intermediate said first and second transducer elements for preventing standing waves from forming between said first and second transducer elements.

6. A directional transducer as defined in claim 4 wherein said inner and outer transducer elements are circular, said outer element having a radius greater than the radius of said inner element.

7. A directional transducer as defined in claim 6 wherein said first and second circular transducer elements comprise sections of two concentric cylinders.

8. A directional transducer as defined in claim 6 wherein said first and second annular transducer elements lie horizontally in a single plane.

9. A directional foil electret microphone having a layer of foil-dielectric material maintained in close proximity to a perforated metallic backplate which encloses an acoustic cavity, said microphone being characterized in that said metallic backplate is divided into a central region and at least one electrically insulated concentric annular region, said regions being interconnected so as to generate a truncated toroidal sensitivity pattern.

10. A directional transducer comprising, an inner annular foil electret transducer element for converting acoustic signals into electrical signals, an outer annular foil electret transducer element for converting acoustic signals into electrical signals, an annular scattering element of approximately triangular cross section intermediate said inner transducer element and said outer transducer element, subtractive circuit means, and electrical interconnecting means for conducting signals from said inner annular transducer and said outer annular transducer to said subtractive circuit means.