Unidirectional microphone

June 25, 1963

3,095,484

W. A. BEA VERSON ET AL

UNIDIRECTIONAL MICROPHONE

Filed Oct. 22, 1959

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United States Patent Office

3,095,484

Patented June 25, 1963

3,095,484

UNIDIRECTIONAL MICROPHONE

Wayne A. Beaverson, Buchanan, and Robert C. Ramsey, Niles, Mich., assignors to Electro-Voice, Incorporated, Buchanan, Mich., a corporation of Indiana

Filed Oct. 22, 1959, Ser. No. 848,120

Claims. (CI. 179—115.5)

The present invention relates generally to devices for translating acoustical energy into electrical energy, and more particularly to directional microphones.

There are many occasions, particularly in sound recording for motion pictures, sound pickup of television programs, and large stage productions for radio broadcasts, when it is desirable to keep the microphone out of the field of action, and it is therefore necessary on such occasions to place the microphone at a considerable distance from the point at which sound originates. Directional microphones are particularly advantageous for such uses in order to reduce background noises and avoid picking up reflected sounds.

Microphones having cardioid response patterns are well known, and these microphones produce negligible response to sounds impinging on the microphone from the rear of the microphone. The patent application of Alpha M. Wiggins, entitled “Unidirectional Microphone,” No. 403,099, filed January 11, 1954, is an example of a microphone with a cardioid response pattern.

A microphone with a cardioid response pattern does not have sharp directivity, that is, the microphone will respond to sounds occurring on either side of the microphone as well as in front of the microphone so that it discriminates principally against those sounds originating at the rear of the microphone. It is one of the objects of the present invention to provide a microphone with greater directivity than conventional cardioid microphones.

The patents of Harry F. Olson, No. 2,228,886 and No. 2,299,342, disclose another type of directional microphone which may be referred to as a line microphone. In this type of microphone, a plurality of different length acoustical paths extend in front of the microphones so that only those sounds traveling directly toward the microphone will arrive at the electroacoustical transducer of the microphone in proper phase. Modified forms of line microphones are shown in the patents of Fred Daniels No. 2,739,659 and No. 2,789,651. The line microphones previously known to the art have been too large and cumbersome for commercial sound, radio, and television use, the Daniels patents disclosing such a microphone with a ‘ length of 1980 feet, although it is to be understood that this particular construction represents the longest known line microphone at that time. It is therefore a further object of the present invention to provide a directional microphone of the line type which is substantially smaller than those heretofore known to the art for a given directional frequency range.

The novel features which are characteristic of the present invention are set forth with particularity in the appended claims. The invention itself, both as to its organization and method of operation, together with additional objects and advantages thereof, particularly will best be understood from the following description of a preferred embodiment thereof, when read in connection 05 with the accompanying drawings, in which:

.FIGURE 1 is a side elevational view of a microphone with a preferred construction of the present invention;

FIGURE 2 is an enlarged fragmentary sectional view of a portion of the microphone of FIGURE 1 taken along 70 the line 2—2 thereof;

FIGURE 3 is an enlarged fragmentary sectional view of the microphone of FIGURES 1 and 2 taken on the same plane as FIGURE 2;

FIGURE 4 is a sectional view taken along the line view taken along the line view taken along the line

FIGURE 5 is a sectional 5—5 of FIGURE 3; and

FIGURE 6 is a sectional 6—6 of FIGURE 1.

The following description of the microphone illustrated in the figures will set forth a pressure gradient electroacoustical transducer with a diaphragm coupled to the atmosphere at its front side by a plurality of acoustical paths of different lengths, and coupled at its rear side through an acoustical resistance. As a result of this construction, the microphone has much greater directivity than a line microphone of comparable dimensions with a pressure electroacoustical transducer, or a cardioid microphone.

The Olson patent, No. 2,228,886, referred to above, includes a disclosure of a line microphone with two forwardly directed acoustical transmission lines coupled to opposite sides of a pressure gradient transducer, each of the acoustical transmission lines having a plurality of acoustical paths of different lengths. The pressure gradient line microphone of the present invention achieves substantially the same directivity as the Olson microphone without the complexity and space requirements of a second multiple path acoustical transmission line, and also facilitates cancellation of sounds originating behind the microphone.

FIGURE 1 illustrates the side view of the microphone and shows a multipath acoustical transmission line 10 in the form of a cylindrical tube extending from the transducer housing 12. The housing 12 is provided with an outwardly extending circular plate 14 which is adapted to be journaled within a pair of bifurcations of a mounting stand of conventional construction.

The acoustical transmission line 10 must provide a plurality of different length acoustical paths to the transducer within the housing 12. In the construction shown in FIGURES 1 and 2, this is achieved by providing a sound permeable window in the form of a plurality of holes 16 equally spaced along a line parallel with the axis of a hollow cylindrical tube 18. One end of the tube 18 is sealed within the housing 12 for the transducer, and the other end of the tube 18 is sealed by an end plug 20. The end plug 20 is also provided with a bore 22 extending therethrough to permit sound waves to enter into the tube from the end thereof. The bore 22 particularly aids the response and performance of the microphone when used for close talking.

The acoustical transmission line 10 also has a second hollow cylindrical tube or sleeve 24 coaxially disposed about the tube 18 and spaced therefrom. The sleeve 24 is provided with a number of elongated slots 26 equally spaced along two lines disposed on opposite sides of the sleeve and displaced from the line of the holes in the tube by 90 degrees relative to the common axis. The purpose of the sleeve 24 is to keep dirt and foreign particles out of the tube 18, and otherwise protect the tube. For this reason, the slots 26 are provided with sound permeable windows 28, such as a cloth layer.

The holes 16 in the tube 18 are provided with a layer of material 30, as illustrated in FIGURE 6, which forms with the holes 16 an acoustical impedance. The acoustical impedance of the holes adjacent to the housing 12 is greater than that of the remote holes 16 and layers 30 so that the holes 16 act as an extended opening parallel to the axis of the tube, and sound pressures exerted at all points along the tube 18 result in equal sound pressures in the housing 12.

The plug 20 has an axial threaded channel 32 which ac3,095,484 accommodate the leads to the voice coil 74 from the transformer 84.

The slot 92, interior of the cover 86, bores 96 in the disc 82, and holes 99 in the plate 76 form a path for sound pressures to the side of the diaphragm 72 opposite the acoustical transmission line 10. This acoustical path is provided with a first acoustical impedance formed by the bores 96 and layers 98, and a second acoustical impedance formed by the holes 99 and layers 100. This construction is designed to perform three separate functions.

First, the acoustical path is provided with the proper phase shift and amplitude attenuation to provide a cardioid polar response pattern for the lower frequencies of the response range of the microphone. The first accoustical impedance and the chamber between the plate. 76 and disc 82 are most prominent in providing the desired phase shift and amplitude attenuation for this purpose.

Second, the first and second acoustical impedances provide a relatively sharp frequency cut-off for the acoustical path at a frequency from 200 cycles per second to 800 cycles per second, thus preventing sound pressures of frequencies above the cut-off frequency from impinging on the side of the diaphragm opposite the transmission line 10. For frequencies above the cut-off frequency, the microphone operates as a line microphone.

Third, the second acoustical impedance and the chamber between the plate 76 and disc 82, and to a lesser extent the first acoustical impedance and the chamber within the cap 86, damp the mechanical resonance of the diaphragm 72.

In one particular construction of a microphone according to the teachings of this invention, the tube 18 has an outer diameter of ^x/z inch and has a length of 11%2 inches. Thirty-two holes 16 are spaced by a distance of iy₃₂ inch along the tube, and the diameter of the holes is approximately 0.094 inch. The sleeve 24 is spaced from the tube 18 by approximately % inch, and four slots 26 are disposed in each side of the sleeve 24 with a width of ⁵/ic inch and a length of 2½ inches. The tube 18 is constructed of steel and the sleeve 24 is constructed of brass, although it is to be understood that other materials are also suitable.

Operation of the microphone may be described as follows. The acoustical transmission line 10 provides a commodates a mounting screw 34 for a sound permeable cap 36. The cap 36 has a circular rim 37 with an indentation 38 which fits about the exterior surface of the sleeve 24, and an axial hub 40 which is attached to the rim 37 by a disc portion 42. The disc portion has an opening 44 extending therethrough and aligned with the bore 22 in the plug 20, and a sound permeable screen 46 extends from the hub 40 to the rim 37 to keep particles out of the tube 18.

FIGURES 3, 4 and 5 illustrate the details of the housing 12 and the electrocoustical transducer disposed within the housing, designated 48. The housing 12 has a cylindrical cross section and an axial opening 50 at one end which engages the end of the sleeve 24 opposite the cap 36. The housing 12 also has a circular shoulder 52 which extends inwardly from the opening 50 to engage the end of the tube 18 opposite the cap 36. The transducer 48 has a cylindrical permanent magnet 54 which has a channel 56 therethrough which has the same diameter as the tube 18 and is aligned with the tube 18. The transducer 48 also has a yoke 58 which has a leg 60 secured to the housing 12 and disposed between the magnet 54 and the housing 12, the leg 60 being integral with a second leg 62 normal thereto extending parallel to the magnet 54. A magnetically impermeable hollow sleeve 64 is disposed about the magnet 54 and extends from the leg 60 of the yoke 58 to a plate 66 of magnetically permeable material which completes the yoke.

The plate 65 has a cylindrical opening therein confronting the magnet 54 and spaced therefrom to form a circular gap 70. A domed diaphragm 72 is mounted at its periphery on the plate 65 and is positioned to seal the channel 56 of the magnet, so that the diaphragm 72 is coupled on one side to the acoustical transmission line 10. The diaphragm 72 supports a voice coil 74 which is disposed within the gap 70 and the voice coil is translatable parallel to the axis of the magnet responsive to sound pressures from the acoustical transmission line 10. It is to be understood that some other electromechanical transducer may be employed with the diaphragm 72 rather than the electromagnetic transducer disclosed, such as a piezoelectric electromechanical transducer.

A plate 76 is mounted to the peripheral portions of the plate 66 of the magnetic yoke, and the plate 76 is pro- -vided with an indentation 78 confronting the dome of the 45 plurality of paths t o toe diaphragm 72^ of differentjength, diaphragm 72. The plate 76 is constructed of nonmagnetic material, and it is provided with two legs 80 on opposite sides thereof which extend normal thereto. The legs 80 abut opposite sides of a disc 82 which is secured t__________\ and thus the legs 80 transmit pressure to maintain the transducer 48 in position.

A transformer 84 for matching the impedance of the transducer voice coil to an electrical transmission line is each path being defined by one of the holes 16. For sounds traveling along the axis of the microphone from the front thereof, the total path length to the diaphragm is the same for all holes 16 through which the sound Therefore, each of the acoustical paths delivers sound energy to the diaphragm which is of the same phase, and there is no cancellation of sound on the front side of the diaphragm. Sound energy directed toward the microphone from any mounted to the side of the disc 82 opposite the transducer 55 °+^Γ direction than the front, passes through the phi48, and a hollow cover 86 is disposed about the transformer 84. The cover 86 is cup shaped and sealed to the housing and maintained in position by a bolt 88 extending through an axial opening 89 in the cover 88 and anchored in a threaded bore 90 on the axis of the disc 82. As 60 illustrated in FIGURE 4, the cover 86 is provided with a slot shaped opening 92 on its surface adjacent to the circular plate 14 of the mounting means for the microphone, and a sound permeable covering 94 is disposed over the slot 92.

Sound pressures are free to enter into the interior of the cover 86 through the slot 92. The disc 82 then permits the sound pressures to pass through two small bores 96 in the disc, shown in FIGURE 4. The bores 96 are spaced from each other and located remote from the transformer. A layer 98 of doth is disposed over the bores 96. The plate 76 is also provided with a plurality of holes 99 disposed in an arc which confront the periphery of the diaphragm dome. The holes 99 are also covered with a cloth legs OU iWUl UPMVSllC Siura VI a vuov vivu ω -- . . ,.

to the housing 12 by screws 83 anchored in the housing, 50 ^enter transmission line 10.

° . . ., tha nafMictioQl rsathc ziPllVATC C/MIfli rality of acoustical paths of the transmission line 10 with different total length paths, which results in phase differences and cancellation at the front of the diaphragm provided the line is at least a half wavelength along.

If the transmission line 10 is less than a half wavelength in length, there will be incomplete cancellation at the diaphragm of sounds originating off of the forward axis of the microphone. For this reason, the cut-off frequency of the path to the rear of the microphone is selected to be approximately equal to the frequency at which the transmission line 10 is a half wavelength. As a result, the microphone is directional above the cut-off frequency by virtue of the directivity of the acoustical transmission line, and directional below the cut-off fre-0 quency by virtue of the cardioid response resulting from the sound path to the rear of a diaphragm. Hence, the microphone is directional throughout its entire response range, even though the acoustical transmission line is too short to produce significant directivity at the lower freMill Cl^lJ 1 UV111V. * »JW - --------- -., . I* &lt;1 _ layer 100. FIGURE 5 also illustrates two holes 102 which 75 quencies of the response range.

3,085,484

In the frequency range near the cut-off frequency, both the cardioid response from the sound path to the rear of the diaphragm and the directional charcteristics of the acoustical transmission line 10 occur, and these are combined at the diaphragm to maintain the directivity of the microphone. In this frequency range, sounds originating at the rear of the microphone are not completely cancelled by the multi-path transmission line 10, and sound pressure passing through the path to the rear of the diaphragm is beneficial to provide this cancellation.

From the foregoing disclosure, those skilled in the art will readily devise many modifications and improvements upon the microphone described herein which are within the spirit of this invention. For example, the multi-path transmission line can clearly be formed by a single slot in the tube 18, rather than a plurality of holes 16 spaced along the tube; or the transmission line could be- constructed by a plurality of separate tubes of different length, as described in the art. It is therefore intended that the scope of the present invention be not limited by the foregoing disclosure, but rather only by the appended claims.