US3004103A - Noise-reducing microphone assembly - Google Patents

Description

Oct. 10, 1961 H. A. HART 3,004,103

NOISE-REDUCING MICROPHONE ASSEMBLY 2 Sheets-Sheet 1 Filed Dec. 22, 1958 INVENTOR.

Ham/d A. Han

Oct. 10, 1961 H. A. HART 3,004,103

NOISE-REDUCING MICROPHONE ASSEMBLY Filed Dec. 22, 1958 2 Sheets-Sheet 2 5 l i BA IOC llC 00 0 4 o oo |5c A IN VENTOR. Ham/d A. Han

J K. $19 M AGENT States This invention relates to apparatus for the conversion of sound into electrical signals and more particularly to a noise-reducing microphone assembly.

The microphone assembly of the present invention permits the transmission of relatively low-intensity sounds, such as verbal communications, in areas of high noise level by nullifying a very high percentage of the background noise and transmitting practically nothing but the voice or other desired sound. Noise-reducing microphones have previously been placed in use, and where ambient noise levels are low to moderate, they have yielded worthwhile benefits. But where noise levels become quite high, for example in test facilities where, if a usable microphone were available, it would be desirable to employ it immediately adjacent an operating jet or rocket engine, previous microphones have been virtually useless in rejecting the great noise while clearly transmitting a voice or other desired sound of relatively low intensity. A need consequently has been felt for an improved noise-reducing microphone which not only would render a verbal communication, of a speaker located in a noisy area, more or less barely intelligible, but which would go further than this and by clear transmission of the voice, coupled with a virtually-complete nullification of the accompanying noise, make the communication clearly and easily understood. By way of example, such noisy areas include aircraft carriers, some locations at airports, wind tunnels, excessively noisy industrial areas, and many other applications, both industrial and military, from which it may be desired to make voice communications, remote broadcasts, etc.

Accordingly, it is a major object of this invention to provide a noise-reducing microphone assembly which will enable a speaker immersed in high-level noise to make clear verbal communication with another person.

A related object is to provide a noise-reducing microphone assembly which will distinctly transmit a desired sound while rejecting accompanying high-level noise.

Another object is to provide a noise-reducing microphone assembly which is small and light yet ruggedly constructed.

A further object is to provide a noise-reducing microphone assembly which is relatively immune to the production of electrical signals in response to considerable oscillations of the entire assembly which may be effected by high-intensity noise.

Yet another object is to provide, in such an assembly, a unit which can be inexpensively produced.

Other objects and advantages will be apparent from the specification and claims and from the accompanying drawing which illustrates an embodiment, together with several modifications, of the invention.

In the drawing,

FIGURE 1 is a perspective view of one form of microphone assembly embodying features of the invention;

FIGURE 2 is a longitudinal sectional view taken as along line IIII of FIGURE 1 and showing schematically a mode of electrical connect-ion of the microphones;

FIGURE 3 is a common previously employed mode of electrical connection of microphones;

FIGURE 4 is a plan view of a microphone assembly employing bosses of a shape varying from those of FIG- URE 1;

FIGURE 5 is a longitudinal sectional view taken as along line VV of FIGURE 4 and showing a microatent 3,004,103 Patented Oct. 10, 1961 2 phone assembly employing staggered microphones, the electrical connections being omitted for simplicity of representation of the other features;

FIGURE 6 is a longitudinal sectional view, similar to that shown in FIGURE 2, of a microphone assembly wherein the passages leading to the microphones are slanted relative to each other and wherein an alternate I two microphones 10, 11 are preferably well-balanced in impedance and sensitivity and, within the audible range, in frequency response in order that, in response to their excitation by identical sounds, their outputs may be as nearly identical in character and strength as possible.

The term identical sounds as employed herein refers to a given sound as felt at two or more points separated in space from each other and the sound source and so located relative to the sound source that the sound, as it reaches one of the points, is in phase with the sound as it reaches the one or more other points, nothing having affected the sound to change significantly its intensity and other characteristics, as felt at one point, from its characteristics as felt at the other point or points. Two identical sounds, then, refers to two difierently located parts of one same sound which are distinguished, for present purposes, only by their separation in space from each other. The type of microphone employed is not critical: condenser, dynamic, ribbon, and still other types of mic'ro'phones and other sound-to-electrical transducers lend themselves to the present purpose, as also do small magnetic transducers (for instance, those used in hearing aids) when used as n'licrophone'st All these contain vibratile elements with respective planes of zero response and, when arrangedi'n accordance with the principles set forth below, will provide satisfactory operation enabling voice and other desired sounds to be picked up by the microphones and transmitted clearly even though the noise level of the area in which the microphone assembly 50 is located is extremely high (for example, up to and including noise of energy levels so high as to be capable of elfecting physical injury to organs and parts of the body other than the ears).

While the invention is by no means limited to their use, a. pair of dynamic or moving-coil microphones 10, 11 are shown in FIGURE 2, and this type of microphone has yielded excellent results when incorporated into the noise-reducing microphone assembly. The individual case 2.4 of the microphone 10' is shown partially cut away to bring into view the vibratile element, in this case, a diaphragm 12, and to show the means for producing an electrical output in response to vibration of the diaphragm. This latter means, as is typical of this type of microphone, includes a coil '13 attached to the vibratile element 12 and moved by the latter in, and relative to, the field of a permanent magnet 14 mounted in the case 24. While not shown, a second means for producing an electrical output, in this case in response to vibration of the tive plane of zero response 17 or 18, which latter term is consistently employed herein to designate the plane in which sound waves could approach and reach the vibratile element 12 or 15 of the microphone 10 or 11 substantially without eliciting any electrical response from the microphone. In the dynamic microphones '10, 11 shown, the plane of zero response of each diaphragm 12 or 15 passes through the diaphragm and is parallel with the fiat upper and lower faces of the diaphragm.

In the specific embodiment shown in FIGURES 1 and 2, the two microphones 10, 11 are so arranged that the respective planes of zero response 17, 18 of their vibratile elements 12, 15 are actually or effectively parallel with each other and are the same effective distance from any sound source lying on a line 16 perpendicular to the planes of zero response and passing midway between the vibratile elements '12, 15. One way of securing this relationship is to locate the vibratile elements 12, 15, as shown in FIGURE 2, so that their planes of zero response 17, 18 lie in a common plane 19 and the microphones It 11 are side-by-side. For purposes of exposition, this common plane 19 will be designated as having first and second sides 20, 21; and the first side 20, in the particular example, is taken as lying on the side of the vibratile element 12 which is opposite that on which the coil 13 lies. Note that, as expressive of its relationship with the two microphones 10, 11, the line 16 is a centerline, and it will be so termed Wherever referred to below.

A means substantially impervious to sound is located near the diaphragms 12, 15 on at least the first side of the common plane 19, and this means is provided by the housing 22 whose front face 23 lies in shielding relationship between the diaphragms 12, 15 and sound Waves which may approach the latter from at least the first side 20 of the common plane 19. As explained more fully in a later paragraph, it is preferable, at least where very high noise levels must be coped with, to shield the vibratile elements not only on the first side 20 of the common plane 19, but also from sound approaching from all other directions. This is accomplished by so forming the housing 22 that it constitutes, as shown, a body completely encapsulating the vibratile elements 12, 15; and it of course may be made to support the vibratile elements 12, 15, magnets, such as 14, etc. directly, rather than individually through the microphone cases 24, 25 shown, by dispensing with the latter. As shown in FIGURE 2, the housing 22 may be a shell which is made of a metal, plastic, or other desired material and in which the microphones 10, 11 are mounted by any convenient and effective means. Other preferred constructions are discussed later. According to the use to which the microphone assembly 50 is put, it may be boom-mounted, incorporated into a hand-held device, etc.; or it may, as shown in FIGURE 1, be mounted on a conventional headset by a clamp 52 which grips the housing 22 and in cooperation with one or more rods or wires 53 holds the microphone assembly 50 directly in front of the users lips.

It is preferable that means be provided for partitioning the vibratile elements 12, 15 from each other, and where the microphones 1t), 11 are mounted in individual cases 24, 25, this is accomplished by those cases. A further a item of the partitioning means is shown at 26 and is optional where the microphones 10, 11, as shown, have individual cases 24, 25, but is more urgently necessary where the housing 22 itself directly mounts the vibratile elements 12, 15, magnets such as 14, and other microphone parts. 7 The item 26 is a partition dividing the interior of the housing 22 into two chambers, one of which chambers is occupied by one and the other by the other of the microphones 1t 11. Where the individual microphone cases 24, 25 are omitted, the partition 26 will of course be varied in form (for instance, as at 26 B in FIG- URE 8) to cooperate in providing suitable shaped indi- 4 vidual chambers for receiving the two sets of microphone components, but the principle remains the same since in either case a partition or partitions are provided which separate the vibratile elements 12, 15 from each other.

The vibratile elements 12, 15 have communication with the exterior of the housing 22 through a pair of housing openings 27 28 which preferably are symmetrically spaced to each side of the centerline 16 (FIGURE 2). The plane in which the openings 27, 28 lie preferably is parallel with the planes of zero response 17, 18 of the vibratile elements 12, 15. Their spacing from each other, as measured from their respective centers, and their diameters are treated in a later paragraph.

Extending from each opening 27, 28 to a respective microphone 10 or 11 is a passage which may be defined, as shown, by a tube 29 or 30 made of metal or other material. Through the passage 29 .or 30' and associated opening 27 or 28, the vibratile element 12 or 15 of each microphone 11) or 11 has communication through the front face 23 with locations outside the microphone assembly 50. Thus, the openings 27, 28 permit the passage of sound waves, originating on the first side 20 of the common plane 19, through the means 22 impervious to sound and, via the passages 29, 30, to the vibratile elements 12, 15. Further, the openings 27, 28 provide the only possible route of communication of the vibratile elements 12, 15 through the sound-impervious means 22 with locations lying on the first side 20 of the common plane 19 and separated from the vibratile elements 12, 15 by the sound-impervious means 22. Each opening 27, 28 and passage 29, 311 communicates with one (and only one) of the vibratile elements 12 or 15. Since it extends within the housing 22 and passes through the front face 23 of the latter, the wall bounding each passage 29, 30 is enclosed at least in part by the sound-impervious body 22. The passages 29, 32 may extend beyond the front face 23, however, as by extending the tubes containing the passages 29, 30 outwardly from the latter. In such a case, the projecting ends of the tubes may be left bare and themselves define the openings 27, 28. In some applications, however, it may be found that improved results are obtained if each of the outwardly extending ends of the tubes containing the passages 29, 31) is surrounded .by a raised boss which terminates substantially flush with the outer end of the respective passage 29, 30 and in which the opening 27 or 28 is thus located. In the specific example shown in FIGURES 1 and 2, one opening 27 is located in an elongated boss 32 which extends transversely of the front face 23, while the other opening 28 is located in a boss 33 which extends at an angle, preferably to the first.

The'walls of the passages 29, 30 may be smooth and straight. In some applications, however, it has been found that small internal ridges 34 extending generally circumferentially of the passages 29, 30 are beneficial to the operation of the device. A convenient way of forming these ridges 34 is by tapping the passages 29, 30 to pro duce female threads therein.

The passages 29, 30' are of substantially equal length in order that a sound originating at any given point on the centerline '16 may reach both vibratile elements 12, 15 simultaneously and hence in phase. While exactly simultaneous impingement on the vibratile elements 12, 15 of sounds approaching from points to one side or the other of the centerline 16 will not be obtained, the arrangement specified above is preferable since it ensures that the average difference in phase of sounds reaching the two vibratile elements 12, 15 will be at a minimum. The length of the passages 29, 30 is notcritical, and passages up to three feet in length have been employed with worthwhile results. The existence of the passages 29, 30 makes the openings 27, 28 more receptive to sounds approaching along or near the centerline 16 and less receptive to those approaching at a considerable angle to the centerline 16.

Consequently, the passages 29, 30*shonld be long enough to suflicientlyefiect this desirable result, and passages 29, 30 as short as inch in length have been employed to. excellent advantage. A change of diameter of the passages 29, 30 has been observed to be accompanied by a change in frequency response of the microphone assembly 50. For example, in a particular case, it was found that a decrease in passage diameter was accompanied by a downward shift in the band-pass characteristics of the frequency response. This efiect is not always altogether undesirable since it may profitably be employed to attest variations in the sensitivity of the microphone assembly for favoring a desired frequency range. For best communication, for example, the diameter of the passages 29, 30 may be chosen for showing some peaking within a range giving good voice intelligibility while exhibiting a relatively decreased sensitivity to frequencies below and above this range, thus facilitating the task of nullification of unwanted noise by narrowing the range within which nullification must be accomplished. As to the spacing of the openings 27', 28 from each other, the nullification of high-frequency noise is favored by bringing them close together since this minimizes the phase difference between sounds arriving at the vibratile elements 12, 15 from points displaced from the centerline. Bringing the openings 27, 28 too close together is undesirable, however, because it makes it difiicult for a speaker to direct his voice significantly more into, one opening 27 or 28 than into the other. The spacing of the openings 27, 28 thus must be a compromise. Neither can a best diameter of the passages 29, 30* be given which will hold true in all cases, for these values vary according to the specific sound-sensitive elements with which they are employed. In a representative usage, however, in which an. excellently superior but most inexpensive microphone assembly was constructed using, as microphones, a pair of small magnetic transducers marketed as hearing aids, the openings equivalent to 27, 28 were spaced 0.875 inch apart at their centers; and the passages '29, 30' and openings 27, 28, were 0.11 inch in diameter and approximately /6 inch in length. As a practical minimum, the opening and passage diameter preferably should not exceed inch; and it is found that the phase difference between a sound as it arrives at one microphone 10 or 11, as compared with the sound as it arrives at the other microphone, may become excessive if the openings 29, 3% are spaced much more widely, center-to-center, than two inches and if the sound approaches from an efiective source well to the side of the centerline 16.

Several modes of connecting the microphones 10, 11 are possible. It should be noted that, at any one instant at which the microphones 1t 11 produce an output in response to identical sounds, the output terminals of each individual microphone 12 or 15 have a potential across them and thus have a polarity. In microphone assemblies previously provided in the art, paired microphones, while not physically oriented relative to each other in the mannet of the microphones of FIGURE 2, have been employed in attempts to accomplish the cancellation of unwanted noise while responding to and transmitting a desired sound. The mode of connection (FIGURE which usually has been employed in these previous attempts has been to connect a terminal of one microphone 35 with a same-polarity terminal of the other microphone 36. One of the output leads 37 has then been connected to one and the other output lead 38 to the other of the remaining terminals of the microphones 35, 36. This mode of connection will operate With some benefit in the microphone assembly of the present invention; but according to a preferred mode, the microphones 10,, 11 are connected as shown in FIGURE 2. Thus, an electrical interconnection is made, as at 39, between a terminal of one microphone 10, a terminal of opposite polanity of the other microphone 11, and one of the output heads 40; The two. remaining terminals of the micro-. phones 1 0; 11 thus are of Opposite polarities, andv they areinterconnected with each other and with the otheroutput lead- 43. For reasons which. will become apparent, it is preferable that the last interconnection be made, as shown, by a potentiometer having a resistance element 41 (which connects the two remainingterminals of the microphones 10, 11 and a movable contact 42- to which the otheroutput lead 43 is connected. The free ends of the output leads 40, 43, shown as cut away in the drawing, should be electrically connected to a desired item of equipment making; use of the electrical output imposed upon them. Such art-item of equipment may, for example, be an amplifying device of any suitable form, such being well known, which will amplify the output received from the leads 40', 43 and supply it to a suitable transducer such as headphones or a speaker. In connection with the laterdescribedcircuit shown in FIGURE 6, it may be mentioned thatsimilar connection of the leads 40A, 43A shown therein is to be understood. In all the figures of the drawing, it will be understood that the electrical connections shown may be effected by elements located within the housnig 22 (FIGURE 1), 22A (FIGURES 4, 5), 22B" ("FIGURE 6), or 22C (FIGURES 7, 8), and that the schematic mode of representationemployed (wherein mostof the wiring is shown, diagrammatically, as outside the housings) is adopted only for facilitation of understanding of'the connections employed.

FIGURES 4 and 5 a modification of the microphone assembly which permits the use of microphones 10A, 11A whose diameter is greater than a desired spacing of the openings. 27A, 28A. According to this modification, the openings 27A, 28A are located at the desired spacing, and the microphones 10A, 11A are placed in overlapping relationship, one being located in front of the other and their planes of zero response 17A, 18A being spaced from each other and preferably kept parallel. The passage 30A from one of the microphones 11A to its opening 28A may be made straight, while the passage 29A of the other microphone 10A to its opening 27A is made tortuous, i.e., bent'or crooked, as required to make its effective acoustical length substantially the same as that of the straight passage 30A. Note that in this modification, the openings 27A, 28A do not lie evenly spaced to each side of the centerline 16A running between the microphones 10A, 103. In FIGURES 4 and 5, there is also shown a variant and optional form of the bosses 32A, 33A, which like those shown in FIGURES 1 and 2, are raised from the front face 23A of the housing, but in the present instance are round rather than elongated. Still further modifications are shown. The housing 22A is made of a resilient plastic material which closely conforms to and encloses the microphones 10A, 11A. The housing 22A may be preformed to fit closely about the microphones 10A, 11A or, according to a preferred mode of fabrication of the assembly, it may be cast in one piece about them. Suitable materials for the housing include a rub,- ber or an epoxy resin, etc. which retains an appreciable degree of resiliency when cured. While no wiring is shown in FIGURE 4, it will be understood that the microphones 10A, 11A may be electrically connected to each other and to a pair of output leads in the manner shown in FIGURE 2 or, as desired, as shown in FIG- URE 6.

openings 27B, 28B is solved by locating the openings 27B, 28B, symmetrically on. each side of the centerline 16B running between the microphones 10B, 11B with the intervalbetween them being as. close as. desired. up to a spacing so narrow as to render difiicult a speaking more directly into one opening 27B or 28B than into the other. At the openings 2'7B,'28B, the passages 29B, 30B have outer- end segments 54, 55 which extend toward the microphones B, 11B and are parallel with the centerline 16B. From the inner ends of the segments 54, 55, the passages 29B, 30B extend directly and symmetrically to the respective microphones 10B, 11B. The passages 29B, 30B hence are opposite-handed, but otherwise identical; any they diverge trom each other as they progress inwardly from the parallel segments 54, 55.

FIGURE 6 shows an alternate mode of connection of the microphones 10B, 11B. In this modification, each microphone 10B, 11B is connected across the primary winding 44 or 45 of a respective transformer. Thus, at any given instant when a current flows through the primary coils 44, 45 because of energization of the microphones ltlB, 11B by substantially identical sounds, one end of each secondary coil 46 or 47 will be of one polarity and the opposite end will be of another. An end of one secondary coil 46 is connected to one of the output leads 49A and with an end, of opposite polarity, of the other secondary coil 47. The resistance element 41A of a potentiometer having a movable contact 42A is connected across the remaining ends of the secondary coils 46, 47 and its movable contact 42A is connected to the other output lead 43A. This arrangement ensures that the polarities of the coils 46, 47, at their ends connected to the resistance element 41A, will be opposite to each other.

FIGURES 7 and 8 show a modification StlC of the noise-reducing microphone assembly wherein one opening 27C communicates with the front side of the vibratile element 12C of the first microphone 10C and the back side of the vibratile element 150 of the second microphone 11C, while the other opening 28C communicates with the back side of the vibratile element 12C of the first microphone 10C and the front side of the vibratile element C of the second microphone 110. Each respective side of each vibratile element 12C, 15C thus has communication through only one opening 27C or 28C with the exterior of the housing 22C. In the specific embodiment shown, the housing 22C serves as the soundimpervious means and is constructed in accordance with the principles set forth above. A magnet 14A, which may be of the familiar form shown, is mounted in one end of the housing 22C, and mounted on or adjacent the magnet 14A is the first diaphragm 12C bearing a coil 13A which, when the diaphragm 12C moves, is moved in the the field produced by the magnet 14A. In the other endof the housing 22C is similarly mounted another magnet 31, diaphragm 15C, and coil 51. Though other modes of construction are possible, it is convenient to orient the magnets 14A, 31 in back-to-back relation. A partition 263, which may be integral with the housing 220, acoustically isolates the two diaphragms 12C, 15C and divides the interior of the housing 22C into two chambers, one occupied by one diaphragm 12C and the other by the other diaphragm 15C. Each diaphragm 12C or 15C in turn subdivides its respective chamber. Two openings 27C, 28C are provided in the front face 23C of the housing 22C, and these, as in all the other modifications of the invention, lie in one same plane. The first opening 27C marks the end of the first passage 29C which, from the opening 27C, extends toward the vibratile elements 12C, 15C and, between the latter and the opening 27C, divides into one branch 58 which enters the end of the chamber bordered by the front side of the first diaphragm 12C (i.e., the side of the diaphragm 12C facing away which communicates with the front side of the other diaphragm 150. In microphones, such as the microphones 10C, 11C shown, where the magnet 14A or 31 would block communication between the passage arm 59 or 61 leading to the back side of a diaphragm 12C or 15C, suitable means, such as one or more openings 63 through the magnets 14A, 31 or the inner material of the housing 22C supporting them are provided to allow the necessary communication. It should be noted that the distance between the inner ends of the passage arms 59, 61 and the associated diaphragms 15C, 12C is, in the somewhat diagrammatic drawing, somewhat greater than that between the inner ends of the other arms 58, 62 and the front sides of the diaphragms 15C, 12C. As will be explained, such diiferences, if slight, do not defeat the purpose of the microphone assembly 50C; but, for the very best results, the length of the arms 58, 62 should be adjusted relative to that of the arms 59, 61 until the effective acoustical distance between the front side of the diaphragm 12C and the opening 27C is to all practical purposes the same as that between the back side of the same diaphragm 12C and the opening 28C and between the front and back sides of the diaphragm 15C and associated openings 28C, 27C. The front and back sides of the diaphragms 12C, 15C are specifically identified above for purposes of exposition; but in actuality, either side of either diaphragm 12C, 150 may be called a front side so long as its connection to a respective opening 27C or 28C, and the connection of the opposite side of the same diaphragm 12C or 15C to the other opening 270 or 28C, is as described above. One side of each of the diaphragms 12C, 15C is apt to be more responsive than the other, and where this is the case, it is preferable that the less responsive side he termed the back side and the more responsive side the front, and that connection of the front and back sides of the diaphragms 12C, 15C with the openings 29C, 30C then be made as specified above and as shown in the drawing. Although the passages 29C, 30C may be defined by tubes which extend into the housing 230 as necessary for opening, at their inner ends, to the diaphragms 12C, 150 as described, and throughout most of their lengths extend outside the housing 22C, it is preferred, Where the length of the passages 2C, 30C conveniently permits, to enclose the passages 29C, 30C, to their outer ends defining the openings 27C, 28C, in the housing 22C. A laterally extending portion 60 of the housing 22C fulfills this purpose by surrounding the passages 2C, 30C and is preferably integral with the main body portion 61 of the housing 22C. In the embodiment shown, the main body portion 61 is substantially circular in cross-section, and its hollow interior houses the microphones 10C, as described above. Where resonances within the chambers in which the diaphragms 12C, are located are excessive, it has been found that they may be damped by, for example, forming a large number of small holes which pierce the surfaces bounding the chambers housing the diaphragms 12C, 15C but which, of course, do not extend through the partition 26B or to the outside of the housing 22C.

In operation, a sound originating at any point on the centerline 16 shown in FIGURE 2 reaches both openings 27, 28 at the same time and thus in phase. Since the opening 27 and passage 29 are identical in dimensions with the opening 28 and passage 39, the sound-portions reaching the diaphragms 12, 15 are in phase and are virtually identical in intensity. As a consequence, each diaphragm 12 or 15 is moved by the same amount, at

the same rate, and in the same direction; and the output of the first microphone 10 is substantially identical to that of the second microphone 11 since the two microphones are Well matched in frequency response and other output values. The outputs of the two microphones 12, 15 :thus being the same, the movable contact 42 should be adjusted to a position in which it occupies the electrical center of the resistance element 41. Under these conditions, no voltage appears across the output leads 40, 43 because the lead 43 is connected through equal portions of the resistance element 41 to a terminal of one microphone and to a terminal, of opposite polarity, of the other microphone 11; and the other lead 40' is directly connected to the two remaining terminals (also of opposite polarities) of the microphones 10, 11, and the substantially equal microphone outputs, as impressed upon the leads 40, 43, substantially or entirely nullify each other. In practice, the theoretical ideal of perfectly matched microphones 10., 11 is never-completely realized, and there is always some, even if only a little, difference in output of the two microphones. This being the case, a portion of the stronger output from, for example, the output of the microphone 10, is consumed in overriding, or nullifying, the weaker output of the other microphone 11; and what is left over of the stronger output appears as a voltage across the output leads 40, 43. Thus, where the difference in outputs is quite small, they substantially entirely balance each other and, as a result, little or no voltage change appears across the output leads 40, 43 which appears as perceptible sounds in headphones,

a speaker, etc. driven by the suitably amplified output taken across the output leads 40, 43. To perfect the I degree of nullification eifected, the movable contact 42 may be positioned as necessary to make, the outputs of the microphones 10, 11, as imposed upon the output leads 40, 43, as nearly equal as possible. Thus, if the output of the first microphone 10 is the stronger, the movable contact 42 is shifted slightly downwardly to bring it suificiently nearer the second microphone 11 than the first microphone 10 to eliminate the voltage diiference appearing across the output leads 40, 43 when the microphones 1t}, 11 are excited identically by a sound coming from a point on the centerline 16. Since means for amplification of microphone-produced electrical signals, and speakers or other means for transforming the amplified signals into sound, are well known in the art, they have not been shown or described herein. The amplifying means, of course, should receive the output measurable across the two leads 40, 43.

A sound coming from the microphones from a point located away from the centerline 16 has farther to travel in reaching one of the microphones 10 or 11 than the other and consequently reaches it later. Where the point from which the sound travels is spaced by a substantial distance, for example several feet, from the microphones 10, 11, its intensity at the microphone 10' or 11 to which it must travel the farther will not be perceptibly lower than at the other microphone 10 or 11. For this reason, the respective outputs of the two microphones 10, 11 in response to such a sound are virtually the same in intensity; but they are to some extent out of phase with each other. Hence, the output of one microphone '10 or 11 appears across the output, leads 40, 43 before that of the other; and since the later output persists longer than the first, it also appears alone, at a later time, across the output leads 40, 43. Because they are somewhat out of phase, the time at which the output of one microphone 1 0 or 11 is at a maximum in response to the sound is, different from that at which the other microphone 10 or 11 produces its maximum output in response to the sound. As a result, during the times at which there is a difference in the strengths of the two outputs, the two outputs will fail to nullify each other to the extent of that difference. By placing the openings 27, 28 near or at, for example, A; inch apart, however, the difierences in output strength and phase, for most sounds within the audible range, become very small; and the accompanying differences between the outputs of the two microphones 10, 11 are so slight that a (theoretically) incomplete but in practice very excellent nullification of the noise occurs. As a definitive example of the efiiciency of cancellation, a microphone assembly well constructed according to the teachings herein and placed within an, enclosure containing a large turbojet aircraft engine operated at full r.p.m. and with the afterburner in operation will yield an output so. noisefree that, when suitably amplified and fed to a speaker or headphones, the engine noise is reduced to a small background sound detracting but little, if at all, from the intelligibility of sentences spoken by a voice directed, as will now be explained, into the microphone assembly. This clear, noise-free operation is. obtained even when the speaker is standing immediately beside the engine.

7 In using the microphone assembly to produce a useable output in, response to a desired sound, for example a voice, while rejecting accompanying noises, the sound to be transmitted is directed as much into one of the openings 27 or 28. as possible and as little as possible into the other. The output of one of the microphones 10. or 11 in response to a desired sound thus is much stronger than that of the other microphone 10 or 11, and this stronger output tends to appear across the output leads 30, 43. To the extent that the desired sound reaches the other microphone '10 or 11, it also produces an output responsive to the desiredso'und, and this tends to nullify the corresponding signal'placed on the output leads 4 0, 43 by the microphone 10 or 11 'with the stronger output. Consequently, it is preferable to locate the opening 27 or 28 into which the desired sound is. to be most strongly directed as close to the source of the. desired sound as is possible. For best reults, the interval separating the sound source and the desired opening 27. or 28 should be relatively small in comparison withv the interval separating the openings 27, 28.

Proper adjustment of the location of a desired opening 27 or 28 relative to the source of the desired sound is much facilitated, particularly Where the microphone is employed for receiving spoken sounds, by the bosses 32, 33. By holding or mounting the microphone assembly so that, for example, the boss 33 is substantially parallel with the speakers lips, the boss 33 may be lightly placed against one of the lips, or lightly between both lips, of the speaker for bringing it into good speaking position. The user of the microphone assembly is thus provided with a tactile clue which keeps him advised at. all times of whether the microphone assembly is in its position most useful to him. The other boss 32 is turned at an angle, say to the boss 33 containing the opening 28 spoken into and acts as a fence which deflects away from the other opening 27 much of the spoken sounds which approach it, thus minimizing the excitation by the spoken sounds of the microphone 10 connected to that other opening 27.

Throughout the above, the means substantially impervious to sound', namely, the housing 22, prevents any sounds, originating on the first side 26 of the common plane 19, from reaching the microphones 10, 11 except through the openings 27, 28 and passages 29, 30. This is. accomplished by the. front face 23 of the housing 22, through which the passages 29, 30 extend. With the rest of the housing 22 besides the front face 23 absent and with. the desired sounds selectively directed to one of the. openings 27 or 28 as described above, transmission of the desired sounds and nullification of noise reaching the microphones through the openings 27, 28 occurs as described. In cases where noises reaching the microphones from the second side of the common plane will ordinarily come only from sources close to the centerline 16, nullification of these noises will tend to ocour, and the microphone assembly, with only the front face 23 of the housing 22 present, will be found to be of some value.

At least two important reasons, however, dictate the complete enclosure of the microphones 10 11 by the housing 22. First, it will be found that, in most situations, noise will tend to come from points, on the second side 21 of the common plane 19, other than points on or near the centerline 16; and the phase differences in these sounds may be so great, as they strike the vibratile elements 12, 15, that excessively imperfect nullification will be effected. It is therefore preferable to shut the microphones 10, 11 substantially completely away from all sounds except as these are transmitted, as described, through the openings 27, 28. Since these latter and the passages 29, 30 into which they open are relatively small in diameter, they do not readily accept sounds approaching them from a sharp angle and more readily accept sounds approaching along the centerline 16; accordingly, they tend to discriminate against sounds apt to be considerably out of phase as they reach the microphones 10, 11. Secondly, since the nullification process is never perfect, there is no virtue in requiring the device to cope with, and nullify, a larger noiseinduced energy-level than necessary, and by cutting off substantially all noise except as admitted through the openings 27, 28, the magnitude of the task of nullification is in effect much reduced.

In the microphone assembly 50A shown in FIGURES 4 and 5, the overlapping relationship of the microphones A, 11A permits the use of microphones of larger diameter while keeping a relatively narrow spacing of the openings 27A, 28A. Because the crooked passage 29A is the same length as the straight passage 60A, a sound arriving at the two openings 27A, 28A at the same time arrives at the microphones 10A, 11A in phase. Somewhat larger attenuation of sound may be expected to occur in the crooked passage 29A, and this may be com pensated by making this passage slightly larger in diameter than the other passage 30A; or, more simply, the microphones 10A, 11A may be connected into a circuit such as shown in FIGURE 2, or as shown in FIGURE 6, and the sliding contact 42 (or 42A in FIGURE 6) adjusted until the outputs of the microphones 10A, 11A in response to random noise are substantially equal.

In both FIGURES 4 and 6, the housing 22A or 22B is preformed to fit closely about the microphones 10A, 11A or 10B, 11B or, as preferred, is molded about them, thus ensuring a surrounding of the microphones with an adequate thickness, on all sides, of the plastic material of which the housing 22A or 22B is made while keeping to a minimum the overall dimensions and weight of the entire microphone assembly. Low weight and small overall dimensions are of obvious advantage particularly where the microphone assembly is mounted, as shown in FIGURE 1, by means attaching it to a head-set. The provision of an adequate thickness of material enclosing the microphones 10A, 11A or 10B, 11B is of importance in efliciently shielding them from high ambient noise levels. An added advantage is gained in constructing the housing 22A or 22B, as mentioned previously, of a somewhat resilient material: whereas a rigid material, even though relatively thick, tends to transmit sound vibrations to the microphones, the resilient material tends to damp out such vibrations and transmits much less noise to the microphones.

It will be noted that, in all the versions 54 50A, and 50B of the microphone assembly shown in FIGURES l-6, the vibratile elements of the microphones are not only substantially parallel, but their sound-receiving sides face in the same direction. Very good results, however, may be obtained when the microphones 10, 11, etc. are oriented in mutual spatial relationships other than those shown in FIGURES 1-6, and the vibratile elements may be other than parallel to each other so long as the openings lie in one plane, the passages are of equal lengths, and at least the outer ends of the passages are parallel. Besides the advantages previously listed, however, a further and important advantage lies in a feature of operation of the microphones which is made possible by employing the construction shown in FIGURES l6, and

this advantage now will be described. Where noise levels .are very high, as immediately adjacent a large jet or rocket engine, all objects,,including theperson or mount ing device holding a noise-reducing microphone, tend to exihibit a quite significant degree of oscillation. As explained above, excitation of the microphones by identical (hence in-phase) sounds results in microphone outputs which cancel each other. Equal, simultaneous, samedirection movements of the vibratile elements such as 12, 15 thus produce outputs whose net result, across the output leads, for instance, the leads 40, 43, is effectively zero, and this is true not only where the vibratile elements are moved by noise admitted through the openings provided, for example the openings 27, 28, but also where their own inertia causes the vibratile elements, for instance the diaphragms 12, 15, to change position relative to the housing 22. and the magnet 14 firmly attached thereto when the housing 22 is shaken by high noise levels. Consequently, whereas such shaking produces added noise in the output of parallel but facing microphones (where a net output is produced when vibratile elements are moved in the same direction), or in a microphone with a single diaphragm bearing coils on its opposite faces, such oscillation or jarring of the microphone assembly 50, '50A or 50B of the present invention has no significant effect .on and does not appear in the signal delivered to the output leads 4%, 43 or 40A, 43A.

Where microphones are used which are of such great mass that oscillations of the entire noise-reducing assembly such as described above do not tend to present this problem, their vibratile elements need not be parallel, and they need not face in the same direction, as long as the openings 27, 28 or equivalent lie in one same plane and each of the passages 29, 30 into which they open are the same length. Thus, although microphones with vibratile elements whose planes of Zero response are parallel are shown by 'Way of specific example, it is not intended that the invention be understood as limited to a device in which the microphones are so related.

In FIGURE 6, location 'of the openings 27B, 28B symmetrically on each side of the centerline 16B between the two side-by-side microphones 10B, 1113, providing an initial, outer segment 54 or 55 of each passage 29B, 30B which is parallel to the centerline 16B, and forming the openings 27B, 28B in a plane parallel to the common plane of zero response19A of the microphones 10B, 11B ensures that the passages 29B, 303 will eifect substantially identical pick-up of noise approaching the microphone assembly 5013 from the side on which the front face 23B is located. Provision of outer segments 54, 55 of the passages 29B, 3013 which are parallel to the centerline 16B is desirable to prevent the greater receptivity to sounds coming from one side of the centerline 163 than from the other which would be found if the angled, divergent portions 29B, 30B of the passages continued, uninterrupted by the parallel portions 54, 55,

to the openings 27B, 28B. Meanwhile because of their divergence from each other as they approach the microphones 10B, 1113, the passages 29B, 30B allow larger side-by-side microphones 10B, 11B to be used without increasing the space between the openings 27B, 28B.

Connection of the microphones 10B, 11B into the circuit with which they are employed may be made otherwise than in the direct manner shown in FIGURE 2. Not only acceptable but even very excellent results may be obtained where'the connection is made through an indirect coupling device, an example of such device being the transformers 48, 49 shown in FIGURE 6. Operation of the circuit shown in FIGURE 6 is similar to that of FIGURE 2 except that the electrical energy developed at the microphones 10B, 11B is passed to the balance of the circuit through the matched pair of coupling transformers 4-8, 49. An advantage of this mode of connection is that, by choice of transformers 48, 49 whose frequency response is such as to favor a desired frequency range, frequencies above and/ or below this range may to its back side.

be discriminated against, thus narrowing the band within which the desired nullification of noise must occur. This is particularly advantageous when some of the noise is of quite high frequency, since it is in this area that significant phase differences may conceivably occur in sounds reaching the microphones .iitB, 11B despite as close a spacing of the openings 27B, 28B asis consistent with facility of speaking more into one opening 273 or 2813 than into the other. 7

Electrical connections of the microphones MC, 110 (FIGURE 8) is made in the manner described in connection with FIGURE 2 or according to that described in connection with FIGURE 6. More specifically, the polarities of the microphones 10C, 11C, as connected into the circuit of FIGURE 2 or FIGURE 6, must be such that their electrical outputs produced simultaneously in response to identical sounds imposed upon them through both the openings 27C, 28C tend to nullify each other, as will be more fully explained in later paragraphs. Where the two sides of the diaphragms 12C, 150 are not equally responsive, a preferred mode of connection is one in which nullification tends to occur most strongly when the output of each microphone 10C, 11C is in response to movement of its diaphragm 12C or C toward and away from its respective magnet 14A or 31 simultaneously with exactly equivalent movement of the other diaphragm 12C or 15C, as contrasted with a movement in which one diaphragm 12C is approaching its magnet 14A While the other diaphragm 15C moves away from its respective magnet 31.

Where noise levels are sufiiciently high, the amplitude of vibration of the diaphragms, such as 12 in FIGURE 2, is relatively very great, and this not only may be deleterious to the microphone in which the diaphragm is employed but requires that the circuit (FIGURE 2 or FIG- URE 6) employed nullify noise-induced outputs which may be relatively very large in comparison with the output responsive to the desired sound, which sound may be very small relative to the ambient noise. The modification shown in FIGURES 7 and 8 greatly reduces the diaphragm deflection and hence minimizes-the outputs of the microphones 16C, 11C in response to noise While multiplying by many times the relative strength oftheir output in response to a desired sound. Noise reaches and enters each opening 27C, 28C substantially equally since they lie in the same plane and since the passages 29C, 30C, at the openings 27C, 28C, are parallel. Since the effective distance from the opening 28C, through the passage 30C via the arm 61 to the back side of the diaphragm 12C of the first microphone NC is preferably exactly the same as that from the other opening 27C through the passage 30C and arm 58 to the front side of that same diaphragm 12C, noise is imposed substantially equally and in phase on both sides of the diaphragm 12C. Equal, in-phase imposition of noise on both sides of the other diaphragm 15C is similarly (and for the same reason) accomplished through the opening 28C and passage 30C with arm 62 leading to its front side and opening 270 and passage 29C with arm 59 leading Theoretically, then, no output (and actually, very much less output) is produced by the individual microphones 10C, 11C in response to noise. Since one side of each diaphragm 12C, 15C, for example,

its back side (nearer the magnet 14A or 31) may be less responsive than its opposite, front side, as well as conceivably for other reasons, perfect nullification of the noise at the diaphragm 12C or 15C is seldom if ever obtained, and some noise-induced signal may be produced by the microphones 10C, 11C. If the acoustical length of the distance separating the front face of the diaphragm 120 from the opening 27C is not exactly the same as that from the back face of the diaphragm 12C to the opening 28C, some phase difference will exist between a sound received at one face of the microphone and an otherwise identical sound received at the other face. If

the two sounds were in phase, they would cancel each other and the diaphragm would not move; to the extent that they are out of phase, they fail to do this and an electrical output is produced. Even a somewhat imperfect cancellation is obviously of great value in diminishing the output in response to noise,'however, and by keeping the distance from the diaphragm through the arm 58 to the opening 27C equal to that from the diaphragm through the arm 62 to the opening 28C, and the corresponding distance through the arm 59 equal to that through the arm 61, the same degree of cancellation of noise is attained at both diaphragms 12C, 15C. Thus, if individual outputs are produced by the microphones 10C, 11C in response to noise, the polarities of the noiseinduced signals, at any one instant, and as impressed upon the interconnection 39 or 39A (FIGURE 2 or 6) are opposite; and, as imposed across the resistor 41 or 41A (FIGURE 2 or 6), they are opposite. Hence, they tend to nullify each other. When, however, a desired sound is presented more strongly at, for example, the opening 27C than at the other opening 28C, it enters the passage 29C and progresses in equal amounts through the arms 58, 59 to the front side of one diaphragm 12C and to the back side of the other diaphragm 150. As a result, a sound which at any one instant causes the diaphragm 12C to move toward its magnet 14A at the same instant causes a motion of substantially equal mag.- nitude of the other diaphragm 15C away fromits magnet 31. At any one instant, the outputs of the microphones 10C, 11C in response to the desired sounds are consequently of one same polarity at their terminals connect'ed by the interconnection 30 or 39A (FIGURE 2 or 6), and this polarity is opposite that of their two remaining terminals, which are connected, according ;to whether the circuit of FIGURE 2 or that of FIGURE 6 is employed, across the resistance element 41 or 41A, and they consequently tend to add to and reinforce each other and to appear as a strong signal across the output lead s 40, 43 (FIGURE 2) or 40A, 43A (FIGURE 6). Consequently, the great diminution of the noise-induced outputs of the microphones 10C, 11C lessens the task of nullification of such outputs in the circuit (FIGURE 2 or 6) into which the microphones 10C, 11C are connected; and this is accompanied by a great relativeincrease in the signal supplied to the output leads 40, 43 (FIGURE 2) or 40A, 43A (FIGURE 6) in response to a desired sound.

Where, because of operating conditions or because of the particular type of microphones 10C, "11C employed in the assembly, the vibration of the diaphragjrns 12C,

15C as a result of noise-induced oscillation of the entire assembly 59C is serious, the nucrophones-1UC,-'11C shown in FIGURE 8 may be mounted in any spatial arrangement in which both microphones 10C, 11C face the same direction: for example, front-t'o-back or side-byside. The advantage of this arrangement is that oscillations of the assembly 50C tend to move both diaphragms 12C, 15C in the same direction relative to their respective magnets 14A, 31 and the electrical signals produced by these movements tend, as explained in a previous paragraph, to nullify each other. In all cases, however, the arms 58, 59,61, 62 must be connected to thermomphones as explained in connection with'FIGURE 8.

While only one embodiment of the invention has been described herein and shown in the accompanying drawing together with several modifications thereof, it will be evident that various further modifications are possible in the arrangement and construction of the noise-reducing microphone assembly components without departing from the scope of the invention.

I claim:

1. In a noise-reducing microphone assembly, the elements including, in combination: a pair of vibratile elements; means substantially impervious to sound completely enclosing said vibratile elements and lying in shielding relation between said vibratile elements and sound waves approaching said means substantially impervious to sound; means partitioning said vibratile elements one from the other Within said means substantially impervious to sound, said means substantially impervious to sound having provided therein first and second openings lying substantially in one same plane; a first passage enclosed by said means substantially impervious to sound and terminating at said first opening, said passage leading from said first opening to one of said vibratile elements; a second passage similarly enclosed and terminating at said second opening, leading from said second opening to the other of said vibratile elements, and of substantially the same length as the first passage, said first and second openings permitting the passage of sound via said first and second passages to said vibratile elements through said means substantially impervious to sound, said means substantially impervious to sound having an exterior surface which is continuous except where the means substantially impervious to sound is provided with said openings; a first means producing an electrical output in response to movement of one of said vibratile elements and a second means producing an electrical output in response to movement of the other of said vibratile elements; and means for electrically connecting together said first and second means in such polarity that their outputs tend to nullify each other when substantially identical sounds are imposed on both said vibratile elements through both said openings, whereby a sound admitted more strongly into one than the other of said openings appears as an electrical signal on said output leads.

2. The combination claimed in claim 1 in which said passages have outer end portions which terminate at said openings and which are parallel.

3. In a noise-reducing microphone assembly: a pair of vibratile elements each having a plane of zero response; a common plane in which said planes of zero response substantially lie, said plane having a first side and a sec ond side; first means producing an electrical output in response to vibration of one of said elements and second means producing an electrical output in response to vibr-ation of the other of said elements; means substantially impervious to sound and placed in shielding relation between said vibratile elements and sound waves approaching said elements from at least said first side of said common plane whereby the impingement on said elements of sound waves approaching said elements from at least said first side of said common plane is substantially prevented; means partitioning said vibratile elements one from the other; said means relativcly'impervious to sound having a first opening communicating with only one, as distinguished from the other, of said vibratile elements and a second opening similarly communicating exclusively with the other one of said vibratile elements, said openings permitting the passage of sound waves, through said means substantially impervious to sound, to said vibratile elements from said first side of said common plane, said first and second openings lying in one same plane and said means substantially impervious to sound, having a surface which is continuous except where provided with said openings; and means for electrically connecting together said first and second means in such polarity that their outputs tend to nullify each other when substantially identical sounds are imposed on said vibratile elements, whereby a sound admitted more strongly into one than into the other of said openings appears as an electrical signal on said output leads.

4. The combination claimed in claim 3, said means substantially impervious to sound lying in enclosing relation to said vibratile elements.

5. The combination claimed in claim 3, wherein said openings are of substantially equal diameters of less than 7 inch and are spaced apart by an interval of the order tion, each of said microphones having a vibratile element with a plane of zero response; a common plane in which said planes of zero response substantially jointly lie; a body highly impervious to sound completely enclosing and supporting said microphones; said body having therein a pair of openings communicating between the interior and exterior of said body and located in a plane substantially parallel to said common plane; a pair of passages of substantially equal length enclosed in said body and communicating between said openings and said vibratile elements, one of said passages leading exclusively from one of said openings to one of said microphones and the other of said passages leading exclusively from said other opening to said other microphone; a pair of output leads; and means for connecting said microphones to each other and to said leads in such a relation of polarities that, when substantially identical sounds are imposed on said microphones through said openings, the outputs of said microphones tend mutually to nullify each other as meas ured across said leads.

7. The assembly claimed in claim 6 wherein said body is made of a resilient plastic material.

8. The assembly claimed in claim 6 wherein said body is made of a resilient plastic material which closely encloses and conforms to said microphones.

9. The assembly claimed in claim 6 wherein each of said passages is bounded by a wall enclosed at least in part by said body and has ridges extending generally circumferentially of said passage.

10. The assembly claimed in claim 6 wherein said body is provided with a pair of raised bosses and one of said openings is provided in each of said bosses.

11..The assembly claimed in claim 10 wherein said bosses are elongated and lie in angled relation to each other.

12. A noise-reducing microphone assembly comprising: first and second microphones located in side-byside relation, each of said microphones having a vibratile element with a plane of zero response; a common plane in which said planes of Zero response substantially jointly lie; a body highly impervious to sound completely enclosing and supporting said microphones and made of a material lying in close contact with substantially all the exterior surface of each; a pair of raised bosses on said body; a pair of openings located in a plane substantially parallel to said common plane, spaced apart at their centers by an interval smaller than one inch, and each communicating between the interior and exterior of said body through a respective one of said bosses; a pair of passages of substantially equal length communicating between said openings and said vibratile elements, each of said passages leading from a respective opening to a respective microphone; a pair of output leads; and means for connecting said microphones to said leads in such a relation of polarities that, when substantially identical sounds are imposed on said microphones through said openings, the outputs of said microphones tend mutually to nullify each other as measured across said leads, whereby a sound imposed more on one of said microphones through said openings than on the other of said microphones tends to produce an electrical output measurable across said leads.

13. A noise-reducing microphone assembly comprising: first and second microphones each having a vibratile element with a respective plane of zero response and disposed in overlapping relation with each other with said planes of zero response of said vibratile elements spaced apart and substantially parallel; a body highly impervious to sound completely enclosing and supporting said microphones and having a pair of openings communicating between the interior and exterior thereof, and located in a plane substantially parallel to said planes of zero response; said body having formed therein a pair of passages communicating between said openings and said vibratile elements, each passage leading from a respective one of said openings to a respective one of said microphones and one of said passages having a source sulficiently more tortuous than that of the other for rendering said passages of substantially equal length; a pair of output leads; and means for connecting said microphones to said leads in such a relation of polarities that, when substantially identical sounds are imposed on said microphones through said openings, the outputs of said microphones tend mutually to nullify each other as measured across said leads.

14. A noise-reducing microphone assembly comprising: first and second microphones located in side-by-side relation, each of said microphones having a vibratile element with a plane of zero response; a common plane in which said planes of zero response substantially jointly lie; a body highly impervious to sound completely enclosing and supporting said microphones; said body having a pair of openings which communicate between the interior and exterior of said body and are located in a plane substantially parallel to said common plane; said body having formed therein a pair of passages of substantially equal length communicating between said openings and said vibratile elements and parallel with each other at said openings, each of said passages leading from a respective opening to a respective microphone and said passages diverging from each other between said openings and said microphones; a pair of output leads; and means for connecting said microphones to said leads in such a relation of polarities that, when substantially identical sounds are imposed on said microphones through said openings, the outputs of said microphones tend mutually to nullify each other as measured across said leads.

15. A noise-reducing microphone assembly comprising: first and second microphones located in side-by-side relation, each of said microphones having a vibratile element with a plane of zero response; a common plane in which said planes of zero response substantially jointly lie; a body highly impervious to sound completely enclosing and supporting said microphones and having a pair of openings communicating between the interior and eX- terior thereof and located in a plane substantially parallel to said common plane; said body having located therein a pair of passages of substantially equal length communicating between said openings and said vibratile elements, each of said passages leading from a respective opening to a respective microphone; a pair of output leads; a pair of output terminals for each of said microphones; means connecting one of said output terminals of one of said microphones with one of said output terminals of the other of said microphones and with one of said output leads, said terminals so connected being of opposite polarities when said microphones are energized by substantially identical sounds; and a potentiometer having a said resistance element which is connected between the remaining one of said output terminals of said microphones and a movable contact which is connected to the other of said output leads.

16. A noise-reducing microphone assembly comprising: first and second microphones located in side-by-side relation, each of said microphones having a vibratile element with a plane of zero response; a common plane in which said planes of zero response substantially jointly lie; a body highly impervious to sound completely enclosing and supporting said microphones and having a pair of openings communicating between the interior and exterior thereof and located in a plane substantially parallel to said common plane; said body having therein a pair of passages of substantially equal length communicating between said openings and said vibratile elements, each of said passages leading from a respective opening to a respective microphone; a pair of output leads; a pair of coupling transformers each having a. primary coil and a secondary coil; a potentiometer having a resistance element connected between one end of each of said secondary coils and a movable contact connected to one of said output leads; means connecting the other end of each of said secondary coils to the other of said output leads; and means connecting one of said primary coils across one of said microphones and the other of said primary coils across the other of said microphones in such manner that voltages tending to be induced in said secondary coils upon energization of said microphones by identical sounds reaching said microphones are of opposite polarities at said ends of said secondary coils connected to said other of said output leads.

17. In a noise-reducing microphone assembly, in combination: a pair of vibratile elements, each of said elements having a front and a back side; means substantially impervious to sound enclosing said vibratile elements and lying in shielding relation between said elements and sounds originating outside said means substantially impervious to sound; means partitioning said vibratile elements one from the other; said means substantially impervious to sound having first and second openings lying substantially in one same plane; a first passage leading from said first opening and divided into two arms of substantially equal length, one of said arms leading to said front side of said first vibratile element, and the other of said arms leading to the back side of said second vibratile element; a second passage leading from said second opening and divided into two arms of substantially equal length, and of a total length substantially equal to that of said first passage, one of said arms of said second passage leading to said back side of said first vibratile element and the other of said arms of said second passage leading to the front side of said second vibratile element; a first means producing an electrical output in response to movement of one of said vibratile elements and a second means producing an electrical output in response to movement of the other of said vibratile elements; and means for electrically connecting together said first and second means in such polarity that their outputs tend to nullify each other when substantially identical sounds are imposed on both said vibratile elements through both said openings, whereby a sound admitted more strongly into one than the other of said openings appears as an electrical signal on said output leads.

18. The combination claimed in claim 17, said vibratile elements having each a plane of zero response, the plane of zero response of one of said elements lying parallel to that of the other, said first and second means tending to produce, in response to simultaneous movement of said vibratile elements in the same direction and amount, electrical outputs which nullify each other as measured across said output leads.

19. A noise-reducing microphone assembly comprising: first and second microphones, each having a vibratileelement; a body substantially impervious to sound completely enclosing and supporting said microphones, said body having an outer surface; a pair of openings, in the outer surface of the body, located substantially in one same plane; a pair of passages of substantially equal length communicating between said openings and said vibratile elements, each of said passages leading from a respective one of said openings to a respective one of said vibratile elements; and means electrically connecting said microphones in such polarity that their outputs tend to nullify each other when substantially identical sounds are imposed on both of said vibratile elements through said openings, said body having an outer surface which is continuous except where said body is provided with said openings.

References Cited in the file of this patent UNITED STATES PATENTS 2,393,318 Fraser Ian. 22, 1946 2,485,405 Olney et al Oct. 18, 1949 2,835,735 Moen May 20, 1958 UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No 3,004, 103 October 10', 1961 Harold A. Hart It is hereby certified that error appears in the above numbered patent requiring correction and that the said Letters Patent should read as corrected below.

Column 3, line "75, for "suitable" read suitably column 5, line 41, for "5/6" read 5/8 column 6, line 23, for "housnig" read housing line 29, before "a"- modification" insert show line 70, for "plate" read plane column 7, line 9, for "any" read and column 10, line 28, for "reults" read results line 71, after "10" insert a comma; column 12, line 56; after "Meanwhile" insert a comma; column 15, line 58, after "sound" strike out the comma; column 17, line 2, for "source" read course line 53, strike out "said".

Signed and'sealed this 6th day of February 1962.

(SEAL) Attest:

ERNEST W. SWIDER DAVID L. LADD Attesting Officer Commissioner of Patents

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