US3378649A - Pressure gradient directional microphone - Google Patents

Description

A ril 16, 1968 s. MAWBY 3,378,649

PRESSURE GRADIENT DIRECTIONAL MICROPHONE Filed Sept. 4, 1964 5 Sheets$heei INVENTOR. HAROLD 5: MAWBY L ;n zr '4 TTORNE Y5 April 16, 1968 1-1.5. MAWBY 3,378,649

PRESSURE GRADIENT DIRECTIONAL MfCROPHONE Filed Sept. L, 1964 3 Sheets-Sheet 1? arm/qwe- Y5 H. S. MAWBY PRESSURE GRADIENT DIRECTIONAL MICROPHONE April 16, 1968 5 Sheets-Sheet 5 Filed Sept. l, 1964 ATTORNEYS United States Patent 3,378,549 PRESSURE GRADIENT DIRECTIUNAL MllCEtOil-IONE Harold S. Mawhy, Niles, Mich, assignor to Electro- Voice, Incorporated, Buchanan, Mich, a corporation of Indiana Filed Sept. 4, 1%4, Ser. No. 394,598 Claims. (Cl. 179-121) ABETRACT OF THE DISCLOSURE The specification discloses a pressure gradient unidirectional microphone which utilizes an electromagnetic transducer. The transducer has a diaphragm with one side exposed to the sound field surrounding the microphone and a cavity on the other side of the diaphragm. Two spaced parallel tubes extend from the cavity and communicate with the cavity. Each of the tubes is provided with a sound window along its length, and a layer of acoustical resistance material is disposed over the sound windows. The tubes and resistance material layers form acoustical transmission lines.

The specification also discloses an acoustical transmission line formed by a tubular screen with a layer of acoustical resistance material in the form of a winding of thread extending along the exterior surface of the tubular screen and a plug of solid material within the tubular screen provided with an axial channel.

The present invention relates generally to microphones of the pressure gradient type.

Pressure gradient microphones employ a sound responsive element such as a ribbon or a diaphragm mounted within a casing which is provided with at least two openings, one opening in communication with each side of the sound responsive element. Such microphones exhibit directional properties, since the phase of the sound waves impinging upon the sound responsive element is partially determined by the direction of the sound wave relative to the axis between the openings in the casing. Such microphones may be bidirectional, that is, produce a minimum electrical response for sound waves impinging upon the microphone from two directions. More often, such microphones are used with a cardioid response pattern, that is, a minimum electrical response only for sound waves impinging in one particular direction on the microphone casing.

Wiggins Patent No. 3,115,207 entitled, Unidirectional Microphone, is an example of a pressure gradient microphone which achieves a cardioid response pattern. In this microphone, a diaphragm is utilized in a dynamic type transducer, and the microphone casing is provided with a front opening facing one side of this diaphragm. There are two openings on the opposite side of the diaphragm, one located near the diaphragm and the other located at a further distance from the diaphragm. The opening nearer the diaphragm provides a cardioid response pattern for the microphone for frequencies in the higher portion of the response range of the microphone, while the opening remote from the rear side ofthe diaphragm provides a cardioid response pattern for frequencies in the lower portion of the response range of the microphone.

For all cardioid microphones, sounds originating from the rear of the diaphragm and aligned with the axis between the effective opening to the rear of the diaphragm and the front opening of the microphone will produce a minimum electrical response. This is true also of the Wiggins microphone described in Patent No. 3,115,207. For sounds within the low frequency portion of the reice sponse range of the microphone, those sounds originating to the rear of the diaphragm on the axis between the remote opening to the rear of the diaphragm and the opening to the front of the diaphragm produce a minimum electrical response. In like manner, the Wiggins microphone produces a minimum electrical response for sounds in the high frequency portion of the response range impinging on the rear of the microphone along the axis of the opening to the front of the diaphragm and the effective opening to the rear of the diaphragm. In the Wiggins microphone, this effective opening to the rear of the diaphragm is located on the mechanical axis of the microphone casing, that is, on the axis perpendicular to thediaphragm and located at the center of the diaphragm, because of the fact that two separate openings disposed on opposite sides of this axis at equal distance therefrom are utilized for the high frequency opening to the rear of the diaphragm. As a result, the axis of minimal electrical response to acoustical sounds directed at the rear of the diaphragm shifts with frequency because of the fact that the effective high frequency opening, and the front opening are not on a common axis. It is an object of the present invention to provide a pressure gradient microphone which is unidirectional over a wide range of frequencies and which exhibits but a single axis for sounds which will produce a minimal electrical response throughout the entire frequency range of the microphone.

Unidirectional microphones are generally utilized because it is desirable to eliminate background noises, feedback in a public address system, or other undesirable acoustical events. If the acoustical axis of minimal electrical response varies with frequency, the microphone may produce undesirable characteristics originating at certain locations for certain frequencies and not all frequencies. It is therefore an object of the Present invention to provide a unidirectional microphone which has essentially the same response pattern throughout the entire frequency range of the microphone and which has an acoustical axis of minimal electrical response which is the same relative to the casing of the microphone throughout the entire response range of the microphone.

These and further objects of the present invention will be more readily understood from a further consideration of this specification, particularly when viewed in the light of the drawings, in which:

FIGURE 1 is a graph illustrating the electrical response of a microphone of the type set forth in Patent No. 3,115,207 for sound waves impinging on the microphone from different directions, the waves being directed toward the intersection of the two axes and the electrical output of the microphone being indicated by the distance between the intersection of the two axes and the point on the curve along the axis of the wave, the solid line curve indicating the electrical output for sound waves in the high frequency portion of the frequency response range of the microphone and the dash line curve indicating the electrical output for sound waves located in the low frequency portion of the frequency response range of the microphone;

FIGURE 2 is a sectional view, partly in elevation, of a microphone constructed according to the teachings of the present invention, the section being taken along the central axis of the microphone;

FIGURE 3 is a sectional view taken along the line 3-3 of FIGURE 2;

FIGURE 4 is an exploded view of the elements located within the casing of the microphone of FIGURES 2 and 3, some elements being omitted for clarity of illustration;

FIGURE 5 is a longitudinal sectional view of a modified form of acoustical transmission line which may be substituted for the transmission lines using the slotted tubes 78 and 88 in the embodiment of FIGURES 1 through 4;

FIGURE 6 is a transverse sectional view taken along the line 66 of FIGURE and FIGURE 7 is a sectional view of still another form of transmission line taken on a plane similar to that of FIGURE 6.

FIGURE 1 assumes the microphone to be located with its central axis along the Y axis at the intersection of the X axis. The solid line curve is the desired cardioid electrical response and represents the high frequency response for the microphone of FIGURES 10 and 11 of Wiggins Patent No. 3,115,207, and also the electrical response of the microphone illustrated in FIGURES 2 through 4 of this application. The dash line curve illustrates the undesirable deviation of the axis of minimum response which occurs at the low frequency end of the response range of the microphone of the Wiggins patent. FIGURE 1 iliustrates that the axis of minimal response of the Wiggins microphone at low frequencies differs from the axis of minimal response at high frequencies by the angle on.

The present microphone employs a casing 16 which is provided with a first set 12 of slots 14 disposed in and normal to a line parallel to the central axis of the casing 10, the central axis being indicated at 16. The casing 10 also has a second set 18 of slots 14 disposed in and normal to a second line on the opposite side of the casing 10 from the set 12 and also disposed in a line parallel to the central axis 16 of the casing. The casing 10 is also provided with a front opening 20 located symmetrically about the central axis 16 at one end of the casing 10.

The exploded view of FIGURE 4 illustrates the elements disposed within the microphone casing 10 and may be viewed in conjunction with FIGURES 2 and 3. The microphone has a face plate 22 disposed normal to the central axis 16 and adjacent to the front opening 20. The face plate 22 has a central opening 24 coaxial with the axis 16, and a diaphragm 26 is mounted on the face plate 22 confronting the opening 20. The diaphragm 26 has a cylindrical voice coil 28 which is disposed translatably and coaxially within the opening 24 of the face plate 22. The diaphragm 26 is constructed in the manner illustrated in Patent No. 3,258,853 of Harold F. Moiser entitled Dynamic Microphone, filed Oct. 1, 1962, and will not be further described.

A U-shaped yoke 30 has a pair of parellel legs 32 and 34 which extend normally from a flat base 36, and the ends of the legs 32 and 34 remote from the base 36 are mounted on the face plate 22 by means of a pair of screws 38. A magnet support member 40 is also mouted on the face plate 22 by means of a pair of screws 42, and the magnet support member 40 has a circular opening 44 of smaller diameter than the opening 24 and coaxial about the axis 16. A cylindrical magnet 46 is wedged within the opening 44 and has one end abutting the base 36 of the yoke 30 and the other end terminating on the plane of the surface of the front plate 22 confronting the front opening 20. The voice coil 28 is translatably disposed between the cylindrical magnet 46 and the opening 24 in the front plate 22, this region being a circular magnetic gap.

The cylindrical magnet 46 is provided with a cylindrical axial channel 43 which terminates at one end in alignment with an aperture 56 in the base 36 of the yoke and at the other end confronting the diaphragm 26. The end confronting the diaphragm is provided with a layer 52 of cloth and a mass 53 of packed fiber glass is disposed within the channel 48 to provide an acoustical damping resistance. The aperture of the yoke communicates with a cylindrical hollow tube 54 which is closed except for an aperture 56 which confronts the aperture 51) of the yoke. The tube 54 thereby provides a terminating cavity for the diaphragm 26 and is acoustically coupled thereto through the channel 43 in the magnet 46, the layer 52 of cloth, and the cavity 58 formed between the diaphragm 26 and the magnet 46. The tube 54 is secured in position by three screws 61) which are anchored in the yoke base 36. The end of the tube 54 remote from the yoke 36 is disposed and sealed in a recess 62 in a rear plate 64 which extends normal to the axis 16 and is mounted on the casing 10.

The face plate 22 is provided with two apertures 66 and 68 located at opposite sides of the axis 16. The magnet support member 46 is also provided with two apertures 76 and 72 which confront the apertures 66 and 68, respectively. Additionally, the magnetic support member 46 is provided with slots 74 and 76 which extend between the aperture and the voice coil magnetic gap and between the aperture 72 and the voice coil magnetic gap, respectively.

A first cylindrical tube 78 is sealed within the aperture 70 of the magnetic support member 40 and extends parallel to the central axis 16 confronting the first set 12 of slots 14. The tube 78 is open at its end disposed within the aperture 71), and is sealed within a recess 30 of the rear plate 64. The tube 78 has an elongated slot 82 parallel to the axis 16 and disposed confronting the first set 12 of slots 14, and a layer 64 of cloth which forms an acoustical impedance is disposed over the slot 82. A cloth grill 36 is also mounted on the interior surface of the casing 10 covering the slots 14 of the first set I12.

A second tube 38 has one open end mounted in the aperture 72 of the magnetic support member 40 and the other end mounted in a recess 91) in the rear plate 64. The second tube 88 is parallel to the first tube 73 and disposed on the opposite side of the axis 16 as the first tube 78 and spaced from the axis 16 by the same distance as the first tube 78. The second tube 38 is also provided with a slot 92 parallel to the axis 16 and confronting the second set 18 of slots 14. A layer @4- of cloth is mounted on the second tube 88 in a manner identical to the layer 84 on the first tube 78, and a cloth grill 96 is mounted on the interior surface of the casing 10 confronting the second set 18 of slots 14.

The open ends of the tubes 78 and 88 are in communication with both the voice coil gap and the apertures 66 and 68 of the face plate 22, and hence the rear side of the diaphragm 26. Further, the slots 82 and 92 commence at the same distance from the diaphragm 26 and terminate at the same distance from the diaphragm 26. The slots 82 and 92 are parallel to each other and equally spaced from the central axis 16 of the casing. As a result, sound entering on any plane normal to the axis 16 through the slots 14, and subsequently the slots 82 and 92, is effectively entering the cavity 58 of the diaphragm 26 at a point on the axis 16 of the microphone. As a result, the microphone of FIGURES 2 through 4 achieves a cardioid response pattern represented by the solid line of FIG- URE 1 when the microphone is located at the intersection of the X and Y axes with the front opening disposed upwardly.

A layer 52 of cloth in the mouth of the channel confronting the diaphragm chamber 58 is selected with the volume of the tube 54 to provide a terminating impedance for the acoustical transmission lines formed by the two slotted tubes 78 and 88. In like manner, the cloth layers 34 and 94 which cover the slots 82 and 92 in the tubes 78 and S3 are selected for the purpose of providing the desired carioid response throughout the entire range of the microphone. In addition, the diameter of the tubes 7 3 and 83, the acoustical impedance of the slots 32 and 92, and the acoustical resistance of the cloth layers and 94 are selected to attenuate sound waves proportional to frequency. The grills 86 and 96 are selected to exclude foreign particles, such as dust and dirt, from the microphone, and are selected to produce a minimum acoustical effect.

Sound waves travelling along the axis 16 toward the grill 5T8 impinge upon the front side of the diaphragm 2-6 through the opening 26 in the microphone casing 16. These same sound waves pass along the casing of the microphone and enter the tubes 7% an be through the slots 14 in the casing 10' and the slots 32 and 912. in the tubes '78 and 38, respectively. if the sound waves are of a frequency near the upper limit of the response range of the microphone, the portion of the sound waves entering through the slots 14 near the rear plate 64, that is, remote from the back side of the diaphragm 26, are substantially attenuated as they travel along the tubes 73 and 88 toward the diaphragm. The tubes 78 and 88, the slots 82 and 92, and the layers 84 and 94 of cloth, are selected to provide attenuation for sound waves proportional to frequency, and hence for the higher frequencies the effective distance between the back side of the diaphragm and the openings in the tubes '78 and $8 to the sound fields surrounding the microphone is approximately the distance between the back side of the diaphragm 2t: and the portion of the sound permeable regions of the tubes 78 and 88 immediately adjacent to the diaphragm. This effective distance for each of the sound paths formed by the tubes 78 and 88 and the acoustical impedance provided by the layers 84 and 94 of cloth and the acoustical inductance of the slots 82 and 92, provides a phase shift for sound waves in each of the two paths to the rear side of the diaphragm which is approximately equal to the shift in phase of a sound wave of the same frequency travelling between the front side of the diaphragm and the regions of the tubes '78 and 88 which effectively permit sound waves of the high frequency end of the response range of the microphone to pass to the rear side of the diaphragm. Hence, the high frequency sound waves travelling along the axis 16 from the back of the microphone are cancelled in this manner, providing a cardioid response pattern.

Low frequency sounds, that is, sound waves having a frequency near the lower end of the response range of the microphone, are not appreciably attenuated as they travel through the tubes 78 and 83. However, only those sound waves entering the tubes 78 and 88 through the sound permeable regions thereof adjacent to the end plate 64, that is, remote from the rear side of the diaphragm, have an adequate phase difference from sound waves inipinging on the front of the diaphragm to provide an adequate pressure gradient on the microphone diaphragm. The lengths of these long paths to the rear side of the microphone through the two tubes '73 and 83 for sound waves having frequencies at the low end of the frequency response range of the microphone and the acoustical impedance provided by the layers 84 and M of cloth and the slots 82 and 92 are selected to produce the desired phase shift at the diaphragm for sound waves traveling on axis toward the front of the diaphragm. This structure also produces a phase shift for low frequency sound waves on the axis traveling toward the rear of the diaphragm to cancel sound waves striking the front of the diaphragm, thereby producing a cardioid response pattern. for the low frequency end of the microphone. Intermediate frequencies effectively enter the microphone through the sound permeable regions of the tubes 73 and 38 intermediate the elfective openings for the high and low frequency sound waves described above, and a proper phase shift is also provided for these sound waves in order to produce cardioid response pattern for all sound waves within the frequency response range of the microphone.

FIGURE 2 also illustrates a cup shaped circular grill 98 mounted over the front opening of the microphone. The grill 98 is supported by a circular spider 160 which threadedly engages the mouth 102 of the casing 10. A screw 104 threadedly engages the spider 160 to maintain the grill in position. The electrical output from the microphone is taken from the voice coil 28. In FIGURE 6 4, electrical leads 106 and 1% which extend from the voice coil 28 are for this purpose.

The microphone illustrated in FIGURES 1 through 4 has two parallel sound permeable transmission lines to the rear side of the diaphragm 26 formed by the tubes 78 and 88, slots 82 and 92, and layers of cloth 84 and 9 d. FIGURES 5 and 6 illustrate a modified form of sound permeable transmission line which may be utilized for the two acoustical transmission lines of the microphone of FIGURES 1 through 4.

A cylindrical wire screen 110 forms a support for a layer 112 of cloth which entirely surrounds the cylindrical tube 11%) of screen. The tube 110 corresponds in function to the tube '78 with its slot 82, or the tube 38 with its slot 92 in that it supports the layer 112 of cloth and is sound permeable, but the cylindrical screen tube lid lacks the acoustical impedance of the slot 82 or 92 of the tubes 78 and 8d, and the acoustical impedance is necessary to provide the proper phase in shift to achieve a cardioid response pattern, as indicated in FIGURE 1.

One or more plugs 114 are disposed in the tubular screen 11!) and are provided with central channels 116 which have the proper diameter to produce the desired acoustical impedance, largely inductive. A plurality of plugs may be used at spaced intervals within the screen tube 119 to provide the desired phase shift. The layer of cloth 112 forms an acoustical resistance, in exactly the same manner as the layers of cloth $4 and 94 in the microphone of FIGURES 1 through 4, to produce the desired attenuation for the acoustical wave impinging upon the rear side of the microphone diaphragm. FIGURE 5 illustrates two plugs 11 i disposed along the length of the screen tube 110.

The plugs 114 may also be used in the tubes '78 and 88 of the embodiment of the invention shown in FIGURES 1 through 4, and the slots E52 and 92 may be wider than the optimum width for producing the desired acoustical inductance. There is also an advantage to using a plurality of spaced plugs 114 in solid tubes, such as the tubes 78 and 855 in that the slot or other apertures utilized to permit entry of the sound waves into the tube will not be controlled to the same dimensional accuracy required of the construction of FIGURES 1 through 4.

FIGURE 7 illustrates a modified transmission line from that illustrated in FIGURES 5 and 6. A sound permeable tube, such as the screen tube 114) is utilized to support a layer 118 of thread which is wound about the exterior surface of the tube 110 in helical form and in as many layers as required in order to provide the necessary acoustical resistance. One or more plugs 114 may also be disposed on the interior of the porous tube 110 to provide the desired acoustical impedance. One of the advantages of the acoustical transmission line of FIGURE 7 is that the desired acoustical resistance for the layer 118 is adjustable, since the resistance increases with the number ,of layers of thread. It is also to be recognized that the layer 118 which comprises a plurality of turns of thread may be substituted for the layer of cloth 84 or 94 on the acoustical transmission lines of the microphone illustrated in FIGURES 1 through 4.

From the foregoing disclosure, those skilled in the art will readily devise many modifications for the present invention. Further, the present invention may be adapted to applications other and in addition to those herein set forth. 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.

The invention claimed is:

1. A pressure gradient microphone having a continuous response range comprising, in combination: an electroacoustical transducer having a vibratile member, a casing attached to the vibratile member and mounting the vibratile member with one side in communication with the sound field surrounding the microphone, said casing having a recess confronting the other side of the vibratile member forming a cavity, a pair of straight parallel tubes open at one end mounted on the casing with the open ends thereof communicating with the cavity, said tubes extending from the cavity in a direction away from the vibratile member, each of said tubes having sound permeable portions along the length thereof communicating with the sound field surrounding the microphone, the length of said sound permeable portion of each tube being less than one-half wavelength at the lowest frequency in the response range of the microphone.

2. A unidirectional pressure gradient microphone responsive throughout a continuous frequency range comprising the combination of claim It in combination with a layer of material covering the sound permeable portion of each of the tubes forming an acoustical impedance, all sound paths from the sound field exterior of the microphone to the other side of the vibratile member extending through one of the tubes and the layer of material covering the sound permeable portion thereof, the diameter of each of said tubes and the acoustical impedance of the material on each of said tubes attenuating sound waves generally proportional to frequency so that the effective distance between said other side of the vibratile member and the opening in each tube for frequencies at the upper limit of the response range of the microphone is approximately the shortest distance between said other side of the vibratile member and the closest region of the sound permeable portion of each tube, the length of said distances to the closest region of the sound permeable portion of each of said tubes being approximately the same, and said length to be closest region of each of said tubes and the effective acoustical impedance of the covering on each of said tubes producing a phase shift for sound waves having frequencies at the upper limit of the response range of the microphone approximately equal to the shift in phase of a sound wave of said frequency 5' travelling between said closest sound permeable regions of the tubes and the one side of the vibratile member, the length of the distances between the sound permeable region of each tube most remote from said other side of the vibratile member being approximately the same, and said length to the remotest sound permeable region of each tube and the effective acoustical impedance of the covering on each of said tubes producing a phase shift for sound waves at the lower limit of the frequency response range of the microphone approximately equal to the shift in phase of a sound wave of said lower limit frequency travelling between said remote sound permeable regions of the tubes and the one side of the vibratile member.

3. A unidirectional pressure gradient microphone comprising the combination of claim 2 wherein the sound permeable portions of each tube comprise solid wall tubes having an elongated slot extending longitudinally along each tube.

4. A unidirectional pressure gradient microphone comprising the combination of claim 2 in combination with means defining an air impermeable second cavity having a port communicating with the first cavity, and a layer of acoustical resistance material disposed in the port, the acoustical capacity of said second cavity and acoustical resistance of said layer forming a terminating impedance for the tubes.

5. A unidirectional pressure gradient microphone comprising the combination of claim 2 wherein the vibratile member comprises a diaphragm.

6. A unidirectional pressure gradient microphone comprising the combination of claim 5 wherein the electroacoustical transducer comprises a cylindrical voice coil mounted on the diaphragm for movement with the diaphragm, and means defining a cylindrical magnetic gap coaxially disposed about the voice coil.

7. A pressure gradient unidirectional microphone having a continuous frequency range comprising, in combination, a cylindrical casing, a front plate extending across the casing and having a central cylindrical opening therein, a U-shaped yoke having two spaced parallel legs mounted on the front plate at one end and a base portion interconnecting the other ends of the legs thereof, said base portion having an aperture extending therethrough, a cylindrical permanent magnet having a central channel extending therethrough mounted on the base of the yoke with the channel aligned with the aperture in the yoke, said magnet being coaxially disposed within the cylindrical opening of the front plate forming a magnetic gap with the front plate, a diaphragm mounted on the front plate carrying a cylindrical voice coil translatably disposed within the magnetic gap between the magnet and the front plate said front plate having a pair of orifices disposed on opposite sides of the opening in the front plate confronting the region between the voice coil and the perimeter of the diaphragm, said orifices being disposed on an axis normal to the plane of the U-shaped yoke, a tube support plate mounted on the side of the front plate remote from the diaphragm provided with an opening adjacent to each of the orifices, a back plate mounted on the casing parallel to the front plate on the side of the yoke opposite the diaphragm and spaced from the front plate, a pair of tubes sealed within the openings in the support plate extending parallel therefrom and engaging the back plate, each of said tubes having a slot confronting the casing, a layer of acoustical resistance material disposed on each of the tubes covering the slots thereof, the casing being provided with sound permeable means confronting each of the tubes.

8. A pressure gradient unidirectional microphone comprising the combination of claim 7 in combination with a tube mounted on the side of the base of the yoke remote from the diaphragm defining a sealed cavity, said tube having an aperture communicating with the channel of the magnet, and a layer of acoustical resistance material disposed over the end of the channel remote from the cavity.

9. A pressure gradient unidirectional microphone comprising the elements of claim 7 wherein the sound permeable means in the casing comprises a plurality of slots disposed along the longitudinal axis of the casing confronting each of the tubes.

10. A pressure gradient unidirectional microphone comprising the combination of claim 9 in combination with a screen mounted on the casing confronting each of the slots therein, said screen passing sound waves therethrough substantially unattenuated and without phase shift.

11. A pressure gradient microphone comprising an electroacoustical transducer having a vibratile member, a casing attached to the vibratile member and mounting the vibratile member with one side in communication with the sound field surrounding the microphone, said casing having a recess confronting the other side of the vibratile member forming a cavity, a straight tube open at one end mounted on the casing with the open end thereof communicating with the cavity, said tube extending from the vibratile member and being provided with a solid plug therein said plug having a channel of smaller diameter than the tube extending therethrough, said plug providing an acoustical inductance.

12. A pressure gradient microphone comprising the combination of claim 1 wherein each tube comprises an elongated tubular screen in combination with a tubular layer of porous material forming an acoustical resistance disposed on the exterior surface of the screen tube.

13. A pressure gradient microphone comprising the combination of claim 12 in combination with a plurality of spaced plugs disposed within the screen tube, each of said plugs being of solid material and having an elongated channel disposed parallel to the longitudinal axis of the screen tube.

14. A pressure gradient microphone comprising the combination of claim 12 wherein the layer of porous material disposed on the exterior surface of the screen tube comprises a continuous strand wound about the exterior surface of the screen tube in a plurality of adjacent turns spaced by a distance less than the diameter of the openings in the screen tube.

15. An acoustical transmission line comprising an elongated tubular member provided with a substantially continuous sound wave Window along the length of the tubular member, and a layer of acoustical resistance material disposed on a surface of the tube covering the perforations thereof comprising a strand of thread wound about the exterior surface of the tubular member and extending over the sound wave window.

16. -An acoustical transmission line comprising the combination of claim 15 wherein the tube comprises a cylindrical screen member.

17. An acoustical transmission line comprising the combination of claim 15 wherein the strand of thread is a single continuous winding with more turns per unit length of the tubular member in one region than in others, whereby said one region provides greater acoustical resistance than other regions.

18. An acoustical transmission line comprising the combination of claim 16 in combination with a plug disposed within the screen tube and constructed of solid material, said plug having a channel disposed parallel to the longitudinal axis of the tube forming an acoustical impedance.

References Cited UNITED STATES PATENTS 2,846,527 8/ 1958 Heintzelman 179-188 2,856,022 10/1958 Kurtze 179--1 3,095,484 6/1963 Beaverson 179115.5 3,115,207 12/1963 Wiggins 179121 OTHER REFERENCES Robertson, A. E: Microphones, New York, Hayden, 1963, pp. 162-164.

KATHLEEN H. CLAFFY, Primary Examiner.

A. A. MCGILL, Assistant Examiner.

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