Title:
NOISE-CANCELLING MICROPHONE
United States Patent 3633705
Abstract:
A noise-cancelling microphone in which a differential diaphragm assembly is mounted on one end of a hollow, sound conducting acoustic tube that has an electroacoustic transducer mounted on its other end for translating sound conducted through the tube into corresponding electrical signals. The differential diaphragm assembly includes a small, inner diaphragm which seals off the end of the acoustic tube and a larger, outer diaphragm which is mounted in front of the inner diaphragm and is connected in its central region to the central region of the inner diaphragm. The outer diaphragm is exposed on both surface to the ambient atmosphere so as to receive sound waves on both its inner and outer surfaces, thereby acting as a noise-cancelling element in which background noise or distant sounds that impinge on one side of the diaphragm will be cancelled out by the same noise or sound impinging on the other side of the diaphragm. Nearby vocal sounds will, however, be picked up by the outer diaphragm since these sounds produce a substantial differential of pressure across the diaphragm. The vocal sounds picked up by the outer diaphragm are conducted by mechanical coupling to the inner diaphragm and from there down the acoustic tube to the electroacoustic transducer.
Application Number:
05/074029
Publication Date:
01/11/1972
Assignee:
The Telex Corporation (Tulsa, OK)
Other Classes:
181/163, 381/186, 381/357, 381/381
International Classes:
H04M1/05; H04R1/22; H04M1/04; H04R1/22; (IPC1-7): G10K13/00; H04R1/28
Field of Search:
181/31B,31A,31R,23,24,32R,33R 179
Primary Examiner:
Tomsky, Stephen J.
Claims:
I claim
1. A noise-cancelling microphone comprising a hollow acoustic tube for acoustically transmitting sound waves from one point to another, an electroacoustical transducer mounted on one end of said acoustic tube for translating said sound waves into corresponding electrical signals, a relatively small diaphragm mounted over the other end of said acoustic tube, a relatively large diaphragm mounted in front of said relatively small diaphragm, said relatively large diaphragm being exposed to the ambient atmosphere on both surfaces thereof, thereby acting as a differential pressure-responsive element which tends to cancel out acoustical background noise, and the central portion of said relatively large diaphragm being coupled to the central portion of said relatively small diaphragm for transmitting the vibrations thereof to the interior of said acoustic tube for transmission to said electroacoustic transducer at the other end thereof.
2. The noise-cancelling microphone of claim 1 wherein said relatively small diaphragm completely closes off the corresponding end of said hollow acoustic tube.
3. The noise-cancelling microphone of claim 1 wherein said relatively large diaphragm is mounted in a mouthpiece assembly attached to said acoustic tube adjacent to said relatively small diaphragm, said mouthpiece assembly having a front portion and a rear portion which cover opposite surfaces of said relatively large diaphragm, and both said front portion and said rear portion of said mouthpiece assembly being perforated so as to expose both sides of said relatively large diaphragm to the ambient atmosphere.
4. The noise-cancelling microphone of claim 3 wherein said front and rear portions of said mouthpiece assembly are substantially parallel to each other.
5. The noise-cancelling microphone of claim 3 wherein the rear portion of said mouthpiece assembly is substantially conically shaped and the front portion thereof is substantially flat.
6. The noise-cancelling microphone of claim 1 wherein said relatively large diaphragm is coupled to said relatively small diaphragm by means of a rigid drive pin attached between the central portions of the two diaphragms.
7. The noise-cancelling microphone of claim 1 wherein said relatively large diaphragm is cemented in its central portion to the central portion of said relatively small diaphragm.
8. The noise-cancelling microphone of claim 7 wherein the central portion of said relatively small diaphragm is outwardly convex and wherein the central portion of said relatively large diaphragm is inwardly convex and wherein the convex portions of the two diaphragms are joined together at the central point thereof.
Description:
BACKGROUND OF THE INVENTION
This invention relates to microphones of the type which utilize a hollow, sound conducting acoustic tube that extends between a mouthpiece which is mounted on one end of the tube and an electroacoustic transducer which is mounted on the other end of the tube. Microphones of this general type have been known and used in the past in connection with telephone or radio receiver headsets in which it is necessary to support the microphone mouthpiece on a small boom element which is attached at one end to the earphone and extends downwardly therefrom to support a mouthpiece near the user's lips. Because of weight considerations, it is preferable in this type of a microphone to mount the microphone's electroacoustic transducer element on one of the earpieces and to communicate the vocal acoustical vibrations from the mouthpiece to the transducer by means of a hollow acoustic tube or boom which also acts as a mechanical support for the mouthpiece. It will be readily appreciated by those skilled in the art that the use of such a hollow acoustical tube is preferable because the position of the mouthpiece with respect to the user's lips is relatively critical and the reduction of weight provided by an acoustic tube makes it easier to maintain the mouthpiece in its proper position. In addition, the acoustic tube mouthpiece structure is smaller and more attractive than one in which the entire microphone assembly is mounted on the end of the boom and connected to the earpiece by electrical conductors. Also, the elimination of the electrical conductors between the mouthpiece and earpiece of the assembly eliminates the possibility of damage to those conductors in use.
Microphones of the above-noted type have been known and used in the prior art, but such microphones have in the past had a serious drawback in that they tended to pick up and amplify background noise in addition to the vocal sounds that they were intended to pick up. This is particularly troublesome in telephone exchanges, where a large number of such microphones are used, and in which many simultaneous conversations occur in a small space, thereby generating a relatively high level of background noise. It has been proposed in the past to cancel out some of the background noise picked up by such microphones by using two hollow sound conducting tubes to support the mouthpiece, with the mouthpiece ends of the tubes being oriented in different directions and the other ends of the two tubes being coupled to opposing sides of a differential electroacoustic transducer that responds to the pressure differentials between the sounds transmitted down the two tubes. Such an arrangement is, however, relatively cumbersome and complex from the mechanical standpoint and it is highly desirable to provide an effective means of noise cancellation that can be used in connection with a single acoustic tube. Accordingly, the principal object of this invention is to provide a noise-cancelling mouthpiece assembly which can be utilized in the above noted type microphone to reduce the background noise picked up by the microphone without impeding its response to the nearby vocal sounds that it is designed to receive.
SUMMARY OF THE INVENTION
In accordance with this invention, it has been found that the above-noted object can be achieved by providing a differential diaphragm arrangement at the mouthpiece end of the acoustic tube in which a small, inner diaphragm is mounted over the end of the acoustic tube and a larger, outer diaphragm is mounted in front of the small diaphragm and attached thereto in its central portion. The outer diaphragm is open to the ambient atmosphere on both sides so as to receive acoustical inputs both on its inner and outer surface so that background noise impinging on one surface will be cancelled out by noise impinging on the other surface without impeding the response to vocal signals coming from a nearby source located on one side of the diaphragm. In the latter case, the nearby vocal signals produce a substantial pressure differential across the outer diaphragm whereas the distant background noise does not. The vocal signals picked up by the outer diaphragm are conducted by mechanical coupling to the inner diaphragm and from there down the acoustic tube to the electroacoustic transducer. In one embodiment of the invention, the inner and outer diaphragms are linked together by a rigid pin extending between the centers of the two diaphragms and in another embodiment of the invention the two diaphragms are cemented together in their central portion. The detailed structure and operation of the invention is described below in connection with three specific embodiments thereof which are illustrated in the attached drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a perspective view of one illustrative microphone assembly of this invention used in combination with an ear supported telephone operator's headset.
FIG. 2 is a cross-sectional view of the mouthpiece end of a first embodiment of the invention.
FIG. 3 is a cross-sectional view of the mouthpiece end of a second embodiment of the invention.
FIG. 4 is a cross-sectional view of the mouthpiece end of a third embodiment of the invention.
DETAILED DESCRIPTION OF THE INVENTION
Referring to FIG. 1, one illustrative embodiment of the invention comprises a hollow acoustic tube 10 which is attached at one end to a housing 12 which is adapted to be supported by the user's ear by means of an ear engaging hook 14 that terminates in an insertable speaker tube 16. Acoustic tube 10 acts as a supporting boom for a mouthpiece assembly 18 and conducts acoustical vibrations received thereby to an electroacoustic transducer 20 mounted within case 12. Electrical conductors 22 are connected to the housing 12 for attachment to the electroacoustic transducer 20 and to the insertable speaker tube 16. It will be understood by those skilled in the art that separate electrical conductors are provided for transducer 20 and speaker tube 16.
The novel aspects of the above-disclosed embodiment are limited to the mouthpiece thereof, and three different differential diaphragm assemblies for the mouthpiece are illustrated in FIGS. 2, 3 and 4. As described above, the problem encountered in this type of microphone assembly is to substantially reduce the level of background noise picked up without appreciably decreasing the response to the nearby vocal signals that the microphone is designed to receive. Referring to FIG. 2, this is done by providing a small inner diaphragm 24 which seals off the mouthpiece end of the acoustic tube 10 and a larger outer diaphragm 26 which is mounted in front of diaphragm 24 in such a way that diaphragm 26 will be open to the ambient atmosphere on both sides so as to be responsive to acoustic pressure differentials rather than to unilateral acoustic pressures. In this particular embodiment of the invention, the mounting for the differential diaphragm 26 includes a conical base member 28 which is attached to the end of acoustic tube 10 and a circular end plate 30 which is connected to the conical baseplate 28 at its periphery. In the disclosed embodiment, the acoustic tube 10, base member 28, and cover plate 30 are made of a plastic material and are joined together by a suitable plastic cement. Both the cover plate 30 and the base member 28 are perforated so as to expose both sides of outer diaphragm 26 to the ambient atmosphere, and the central portion of outer diaphragm 26 is coupled to the central portion of inner diaphragm 24 by means of a drive pin 32 which is attached between the two diaphragms by means of cement. The diaphragms 24 and 26 can be made of any suitable known material which is adapted to respond effectively to vocal signals.
In the operation of this embodiment of the invention, background noise vibrations impinge on both sides of the diaphragm 26 to a greater or lesser degree depending on their distance from the mouthpiece and also upon the arrangement of acoustically reflecting surfaces in the room where the microphone is being used. As a result, the background noise tends to be cancelled out to a significant degree. The vocal signals which come from the user, however, impinge with greater force upon the nearer surface of diaphragm 26 and thus produce a substantial differential pressure so as to induce corresponding vibrations in the diaphragm. These vibrations are coupled through the drive pin 32 to inner diaphragm 24 and from there down the acoustic tube 10 to the electroacoustical transducer 20. Since the inner diaphragm 24 is exposed to the ambient atmosphere on only one side, it will also respond to background noise, but the small area of diaphragm 24 with respect to that of diaphragm 26 minimizes the effect of such noise. In addition, the inertia of diaphragm 26 further reduces the response of diaphragm 24 to noise due to the mechanical linkage between the two diaphragms.
FIG. 3 shows a second embodiment of the invention in which the outer diaphragm 34 contains a conical depression in its central region and is cemented at the apex of this conical depression directly to the central portion of the inner diaphragm 24 to eliminate the drive pin 32 and provide a closer mechanical coupling between the two diaphragms. It will be noted by those skilled in the art that the central area where the two diaphragms 24 and 34 are joined will be subject to excitation by distant noise signals from the front of the mouthpiece while the peripheral area of diaphragm 24 is subject to excitation by distant noise signals from the rear of the mouthpiece, thereby providing a further reduction in noise pickup by diaphragm 24.
FIG. 4 shows a different diaphragm configuration in which both diaphragms are mounted in a relatively short, cylindrical mouthpiece comprising a cylindrical ring 36, an outer faceplate 38, and an inner faceplate 40 which is adapted in its central portion to be secured to the acoustic tube 10. In this embodiment of the invention, the inner diaphragm 42 contains an outward bulge in its central portion which matches an inward bulge in the outer diaphragm 44 so that the two diaphragms can be joined together at a central point. In this embodiment of the invention, the cylindrical ring 36 and the two end plates 38 and 40 are also made of a suitable plastic which is connected together by cement and the two diaphragms 42 and 44 are also connected within the case by means of suitable cement. The outward bulge of diaphragm 42 tends to further reduce the response to background noise.
From the foregoing description it will be apparent that this invention provides a noise-cancelling mouthpiece assembly which can be utilized in the above-noted type of microphone to reduce the background noise picked up by the microphone without impeding its response to the nearby vocal sounds that it is designed to receive. And although the invention has been described in connection with several illustrative embodiments thereof, it should be understood that the invention is by no means limited to the disclosed embodiments since many modifications can be made in the disclosed structures without altering their fundamental principle of operation. For example, although several illustrative shapes have been disclosed for the diaphragms, it will be apparent to those skilled in the art that other shapes could be employed if desired. In addition, it may be desirable to utilize a metallic acoustic tube in some applications instead of the plastic acoustic tube which is disclosed herein. These and many other modifications of the disclosed structure will be apparent to those skilled in the art, and this invention includes all modifications falling within the scope of the following claims.