Abstract:
This microphone has two opposed faces. The sound to be picked up may be directed toward either one of the two opposed faces with equivalent electrical output from the microphone in either case. Since background noise impinges equally on both of the two opposed faces, its effect on the microphone diaphragm is self-canceling, and causes substantially zero output from the transducer.

Description:
CROSS-REFERENCE 
     This microphone is intended for use in a communications headset of the type shown and claimed in the copending application of James P. Foley, Ser. No. 555,529, filed Mar. 5, 1975. 
     BRIEF SUMMARY 
     A microphone typically includes a diaphragm which is vibrated by sound waves impinging upon it. The diaphragm may carry an electrostatic element, e.g., an electret or a capacitor plate cooperating with a stationary electrostatic element. Alternatively, the diaphragm may carry a magnetic element cooperating with a stationary magnetic element. In either case, the cooperating elements vary an electrical condition as a function of the sound waves impinging on the diaphragm. The stationary element is coupled to a suitable electrical circuit. Acoustic waves impinging on the diaphragm are converted to electrical waves in the circuit by the action of the transducer. 
     Anti-noise microphones commonly have two unequal acoustic paths by which sound waves reach the opposite sides of the diaphragm. One path, directed toward the sound source, receives both the sound to be picked up and the background noise, and the other path, opening away from the sound source, receives primarily background noise. In most anti-noise microphones of the prior art, a leakage opening of adjustable dimensions is provided between the two acoustic paths. 
     A microphone constructed in accordance with the present invention has two opposite faces and two acoustic paths connecting the respective faces to the respective sides of the diaphragm. The speaker&#39;s voice, or other sound to be picked up, may be directed toward either face of the microphone, and the background noise is received at both faces. The two acoustic paths are completely separate. 
     Transducers are typically not physically symmetrical. The acoustic paths by which the sound waves reach the opposite sides of the diaphragm cannot be symmetrical because of the fundamental asymmetry of the transducer. Since the microphone is to respond equally to sound impinging upon it from either of the opposite faces, the two acoustic paths leading to the diaphragm from those opposite faces must be balanced. One path is acoustically longer than the other, and balance is secured by introducing into the paths acoustical impedances inversely related to the acoustic lengths of the paths. 
    
    
     DRAWINGS 
     FIG. 1 is a plan view, on an enlarged scale, of a microphone embodying the invention. 
     FIG. 2 is a sectional view taken along the line 2--2 of FIG. 1, on a further enlarged scale. 
     FIG. 3 is a view of the microphone of FIG. 2, partly in plan, with certain parts broken away to reveal the internal structure. 
     FIG. 4 is a plan view of a screen which serves as a first acoustic impedance in the microphone. 
     FIG. 5 is a plan view of another screen which serves as a second acoustic impedance in the microphone. 
     FIG. 6 is a plan view of a transducer employed in the microphone. 
     FIG. 7 is a cross-sectional view of the microphone of FIG. 1 taken on the line 7--7 of FIGS. 2, 6 and 8. 
     FIG. 8 is a sectional view of the transducer in the microphone of FIG. 7, taken along the line 8--8 of that figure. 
     FIG. 9 is a sectional view taken along the line 9--9 of FIG. 8. 
     FIG. 10 is a sectional view, on a smaller scale, taken along the line 10--10 of FIG. 3. 
    
    
     DETAILED DESCRIPTION 
     The microphone of the invention is enclosed in a housing 1 consisting of two identical parts, each indicated by the reference numeral 2. Each part, as best seen in FIGS. 2 and 3, consists of a face or transverse wall 2e and a peripheral wall projecting fron one side of the face wall. The peripheral wall includes a first section 2a, of part-cylindrical contour, having an inner face of constant radius and somewhat greater than a semicylinder. Second and third wall sections 2b and 2c have flat inner surfaces extending tangentially from the ends of the first section 2a to an end section 2d appearing at the left end of FIGS. 2 and 3. Face wall 2e is provided with sound transmitting openings 2f. 
     A septum 3 having a part-cylindrical surface 3a extends across the part 2 between the right-hand ends of the second and third wall sections 2b and 2c, as they appear in FIG. 3. Another septum 4 extends between the second and third wall sections 2b adjacent their left-hand ends as they appear in FIG. 3. The two housing parts 2 are placed with the end surfaces of their peripheral walls and of their septums in abutting contact (FIG. 2). 
     A conduit 5 extends through facing recesses in the end wall sections 2d and has its right-hand end attached to an anchor ring 6, as by welding. Anchor ring 6 is received in a recess defined by wall sections 2b and 2d and by septums 4. Two wires 7 extend through the conduit 5 and through aligned notches 4a in the surfaces of the transverse septums 4. The wires 7 separate in the recess defined by the septums 3 and 4 and the second wall section 2b. The septums 3 are provided in their end surfaces with two notches 3b through each of which one of the wires 7 passes into a recess defined by septums 3 and the arcuate wall sections 2a. A cylindrical transducer unit generally indicated at 10 is received in that recess, and fits snugly between the cylindrical surface of section 2a and the cylindrical surface 3a. The wires 7 are connected to the transducer. 
     A locating pin 11 projects from the wall section 2a, at a point spaced from the longitudinal axis of the housing part 2. A recess 2g for receiving a similar pin is provided in the wall section 2a. The locating pin 11 and the recess 2g are spaced from the longitudinal axis of the housing part by equal distances and on opposite sides thereof from one another, so that when one of the two housing parts is inverted and the end surfaces of their peripheral walls are placed in contact, the pin 11 on each housing part enters the recess 2g on the other housing part, as best seen in FIG. 10. After being so assembled, the two parts can be fastened together by sonic welding, bonding or the like. 
     The transducer 10 comprises a first cylindrical frame member 12, which is open at its right-hand end as viewed in FIG. 9. The opposite end of the frame member 12 is closed by a wall 12a having openings 12b (FIGS. 6 and 8) formed therein. 
     A second cylindrical frame member 13 is ring shaped and abuts the open end of the first frame member, as best seen in FIG. 9. The frame members 12 and 13 are attached to each other, as by bonding, at their abutting surfaces. A diaphragm 14 (FIG. 7) of magnetic material is seated at its periphery on a shoulder 13a on the seocond frame member 13. At its center, the diaphragm carries an armature 14a of magnetic material. 
     The diaphragm 14 and armature 14a are part of a magnetic circuit means which includes a central member 15 having a flange 15a attached as by bonding to the wall 12a of the frame member 12, and an integral sleeve 15b extending to the right as it appears in FIG. 9 from the flange 15a. A pole piece 16 is threaded into the sleeve 15b, and projects beyond the sleeve, integrally carrying at its projecting end a pole 16a which cooperates with the armature 14a, and is spaced therefrom by a distance which may be adjusted by rotating the threaded pole piece 16. After final adjustment, the pole piece is fixed in place, as by cement 17. An electromagnetic coil 21 is mounted on the outside of the sleeve 15b, and has its ends connected to the wires 7. A pair of permanent magnets 22 extend between the flange 15a and a peripheral pole piece 23 having a central aperture 23a concentric with the pole 16a. The pole 16a, diaphragm 14, the armature 14a, central member 15, the magnets 22 and the peripheral pole piece 23 together constitute the magnetic circuit means cooperating with the coil 21. After the diaphragm 14 is in place, as shown, it is held there by the attractive force of the permanent magnets 22 acting on the diaphragm and the armature 14a. 
     The acoustic path between the openings 2f in the lower housing part 2, as it appears in FIG. 7, and the under side of the diaphragm 14, is short and direct and extends through the openings 2f and thence through a screen 24 (FIGS. 2, 4 and 7) of relatively fine mesh and through the space immediately under the diaphragm 14. 
     The acoustic path between the openings 2f in the upper housing part 2, as it appears in FIG. 7, and the upper side of the diaphragm 14, is longer and more tortuous. It extends through the openings 2f, a screen 25 (FIGS. 2, 5 and 7) of relatively coarse mesh, openings 12b in the frame member 12, into the space within the frame members 12 and 13, and around the central member 15 and the coil 21 mounted thereon, to the upper side of diaphragm 14. 
     The lower end of the frame member 13, as viewed in FIGS. 2 and 7, extends downwardly beyond the shoulder 13a. A sealing washer 18, which may be a sheet of paper, Mylar, or other suitable material, coated on both sides with a pressure sensitive adhesive, is located between the lower end of the frame member 13 and the bottom of the inside of the lower housing part 2. The lower frame member 13, the sealing washer 18, and the diaphragm 14 cooperate to separate the two acoustic paths from each other. 
     It is desirable to match the frequency response curve of the transducer 10 with respect to sound impinging on it through the openings 2f in the upper housing part 2 with the frequency response curve of the transducer to sound impinging on it through the openings 2f in the lower housing part 2. The acoustic path between the openings 2f in the lower housing part 2 and the diaphragm 14 is shorter than the acoustic path between the openings 2f in the upper housing member and the diaphragm 14. In order to match the two frequency response curves, acoustic resistances are introduced into the two paths. The acoustic resistances are provided by the screens 24 and 25. The screen 24 is made with a finer mesh than the screen 25. Compare FIGS. 4 and 5. The screen 24 introduces greater loss into the acoustic path leading to the under side of the diaphragm than the screen 25 introduces into the path leading to the upper side of the diaphragm. The losses are inversely related to the length of the paths. In this way, the acoustic paths between either sets of openings 2f and the diaphragm are substantially balanced so that the voice or other sounds to be picked up may be directed into either side of the microphone. The openings on the other side of the microphone thereupon serve as antinoise openings to receive background noise which tends to cancel, since it appears simultaneously at both sides of the diaphragm. With this structure, the frequency response spectrum of the microphone is substantially the same, whether the sound source is on one side of the microphone or the other, and the directivity pattern is substantially a figure eight.