US2520714A - Electrodynamic hand microphone of the pressure gradient type - Google Patents

Description

Aug. 29, 1950 2,520,714

R. K. DUNCAN ELECTRODYNAMIC HAND MICROPHONE OF THE PRESSURE GRADIENT TYPE Filed Aug. 25, 1949 FI7. 4. 4i

Zhwentor REBERT KDENEAN Gttorneg Patented Aug. 29, 1950 ELECTRODYNAMIC HAND MICROPHONE OF THE PRESSURE GRADIENT TYPE Robert K. Duncan, Mount Ephraim, N. J., assignor to Radio Corporation of America, a corporation of Delaware Application August 25, 194-9, Serial No. 112,227

9 Claims. (Cl. 179-4155) This invention relates to electro-acoustical transducers, and more particularly to noise cancelling, pressure gradient responsive microphones of the close talking type.

My present invention has been made especially with the idea of providing a close talking microphone for use in locations where there is considerable noise, such, for example, as those involving military operations. In many cases, microphones are used in the proximity of navigation equipment, or in the presence of other instruments which are particularly affected by a magnetic field. In these cases, it is essential that the microphone have a minimum external stray magnetic field, the requirements often being that the instrument should not deflect a compass needle more than two degrees at a distance of eight inches from the microphone. Another factor to be considered is that involving humidity which, in many cases, creates spurious or irregular response.

Many prior art close talking microphones have been proposed to solve problems involving the presence of external stray magnetic flux, among the more common being those using a soft iron cap or shield which will create a low reluctance path for the external leakage flux lines. While this method is satisfactory for many applica-- tions, it has a disadvantage in one respect in that it tends to reduce the density of magnetic flux in the air gap. Another disadvantage is the difficulty of providing sufficient shielding to meet rigid specification requirements.

The primary object of my present invention is to provide an improved, noise cancelling, pressure gradient responsive microphone of the close talking type which will overcome the above, as well as other, disadvantages of prior art close talking microphones.

It is also an object of my present invention to provide a close talking microphone which is highly discriminatory against ambient noise.

Another object of my invention is to provide a noise cancelling, close talking microphone having a balanced magnetic circuit with substantially no external stray magnetic field.

Still another object of my present invention is to provide a close talking microphone which will readily adapt itself to all changes in humidity.

A further object of my present invention is to provide a pressure gradient responsive, close talking microphone which is compact in construction, easily portable, and highly efficient in use.

In accordance with my present invention, 1

provide a pressure gradient responsive microphone having a symmetrical magnetic structure disposed on opposite sides of a disc diaphragm. The magnetic structure comprises a pair of cylindrical, magnetic units each having an annular air gap disposed in an end thereof, the air gap ends being recessed from the peripheral edges of the cylindrical outer casing. The two magnetic units are mounted end to end along a common axis with their adjacent ends abutting to provide magnetic coupling between the two units thereby to obtain a balance of magnetic flux in the two air gaps supplied from suitable magnetic cores which are disposed within each of the units in conventional manner. A disc diaphragm responsive to differences in sound wave pressure on opposite sides thereof is mounted intermediate the recessed ends in spaced relation thereto and having separate voice coils mounted on opposite sides thereof and arranged for disposition within the annular air gaps. Openings are provided in the cylindrical outer casings, as well as in the recessed ends, to permit sound waves to impinge on opposite sides of the diaphragm. The two magnetic units are mounted for independent rotation with respect to each other about their common axis so that the casing apertures may be spaced apart circumferentially, thereby to provide an adjustable sound wave path length to opposite sides of the diaphragm thus enabling a user of the microphone to adjust it to eliminate undesirable response.

The novel features of my invention, as well as additional objects and advantages thereof, will be better understood from the following detailed description of one embodiment thereof when read in connection with the accompanying drawing, in which:

Figure 1 is a front elevation of a hand held microphone in accordance with my present invention,

Figure 2 is a side elevation of the microphone shown in Figure 1,

Figure 3 is an enlarged end view of the internal microphone structure, one of the magnetic units being removed to show the diaphragm, and,

Figure 4 is an enlarged, central sectional view of the microphone structure shown in Figure 1, taken on the line 4-4 of Figure 1.

Referring more particularly to the drawing, wherein similar reference characters desi corresponding parts throughout, there is shown a noise cancelling, pressure gradient responsive, hand held microphone l comprising, generally, a cyl ndrical. m crophone structure- 3 and a hand 3 support member 5. The cylindrical microphone structure 3 has separate cylindrical magnetic units or structures 1 symmetrically disposed on opposite sides of a vibratile member or disc diaphragm 9 to provide balanced magnetic circuits on opposite sides thereof, the magnetic structures being mounted end'tosend, alonggacommon axis. Each magnetic structure comprises an annular receptacle or cup-shaped member 5 i of magnetic material with a magnetic core I3 concentrically l mounted on the interior ofrthe -base...or outer A end 85. An annular recess I! is provided around.- the inner peripheral edge at the open end of each cup shaped member ll. A--disc-like'-polep1ate member i9 is disposed within each annularrecessl ll, each plate 19 having a diameter substantially equal to that of the recess H1; Eachci-rcular' pole plate member I9 is provided with a central aperture 2| and a plurality of smaller apertures 23 surrounding the central aperture and constitutesan. outer -.poleE- plate 'ofr-fth'eg magnetic structure. A cylindricalipol zca 25 'bfij'nagnefig material is mounted on thefi'ee end-of mag 'netic core l3, extendingewithin the associated1 the center thereof which -are arranged in the;

form of a clover leaf,thepurposeofgthisbeing to 1 add rigidity to the-central port-ion; lnyorder" to effect a well balanced acouSticAstructura-the portions 29 are symmetricallyarranged-around the centerof the diaphragm. and with, adjacent portions extending outwardly 'in opposite-dime tions, Radial corrugations 3l*'are,- provided adjacent the margin of the discdiaphragm 9 "for; the purpose of flexibly supportingthe diaphragm central portion for freedom 0f;jmovement;in directions perpendicular to'the plane in which'the diaphragm lies. As seen inFig'ure4 ofthe draw.- ing; the radial corrugations} Pare symmetrically v disposed on-oppos ite sides .ofthe plane of (the diaphragm to effect ;an -a coustically balanced;

structure. Separate voice coils 33 are concentrically mounted on opposite sides ofjthe discu. diaphragm ,9 for disposition .within the..air gaps 21 ,their electrical outputs-being conncctedjto produce an additive effect;

In order. to facilitate assembly of the a component parts of the mocrophone.3,fand in.,order= o insure, that, the voice coils 33;will'be ceh e l within the air gaps .273it has gbeen found advantageous to mount the ,outerrpole; plate meme bers. l 9, andthe disc diaphragm ,9 1 as a unit. As shown inFigure 4,;thedisc diaphragm} is dis"-.v posed between the outenpole plate members l$1.,' being supported at its periphery between a pair of annular spacing rings 35 of non-magnetic material, such as brass, which are mounted between the peripheral portions of the pole plate members l9, thereby to 'space the diaphragm?! equally from each'of the pole plate members 19 i so 'that the diaphragm center-porti n il free to move without touching the pole plates. The pole plates l9, spacing rings 35 and the disc diaphragm 9 are substantially all of the same outer diameter as the diameter of the annular recess H, and their combined thicknesses are substantially equal to the combined depths of bothjannular recesses ll whenthewtwo magnetic structures 7 are mountedend to end. Inorder to maintain the diaphragm and pole plate unit in assembled relation, a plurality of screws 31 are provided adjacentv the. outer edge, being so disposedandarranged as to securely hold the parts together.

Theassembledmicrophone 3 comprises a hollow cylindrical casing consisting of a pair of magnetic units or structures I mounted end to end with their adjacent ends 39 abutting, and with the diaphragm 9 dividing the interior of the microphone 3 into separate, substantially acoustically balanced chambers or compartments 4|. Imorden :thatv soundswavesmay. :be transmitted to;v opposite -sides aof: the. disc diaphragm 9 sound-openings '43 are. provided: in the easing-' Walls of the magnetic structures ifibeinglocated adjacent to theplate members' l9 on the side thereof remote from the: diaphragm :9. For the purpose of providing damping,=as well'as to pre vent dirt-particles vfrom entering the structure,

suitable damping material 45,such asfelt'orhairg is disposed within the openings 43, or, in the-event a screen such as silkyor the like; is found preferable, the same may bemounted onthe interior-= casing. wall'anduispos ed over the openings-e In order to achieve a more desirable frequency re"- sponse, the acoustical circuitprovided by the relatively large cavities or: chambers 4| within the casings is modified fillingthe -cavities-or'= chambers a desired amount-with wax,-:Bake1ite,-

or other suitable material '41-? While: itv is.-intendedthat both magnetic struc air gaps.

will not be as perfect a contact between theabut-- ting surfaces as would bedesired 'because'of" minute variations which are encountered in theprocess of manufacture and assembly. In order to eifect a more'perfect coupling between the two magnetic structures and hence-effect an' effi cient, well balanced magnetic structure on both sides of the diaphragm, a ring 480f magnetic* material is closely -fittedaround the magnetic structures 1 and connecting their abutted ends:

Furthermore, the ring 43 also assists in maintaining alignment of the two magnetic struc tures i. In addition thereto, sincethe respective elements of both magnetic structures Tare-of like polarity, there willexist a bucking? or repellent action between the two magnetic struc;-- tures when mounted end to end.- As' a result of this bucking action; part of the normalexternal leakage flux in the vicinity of the pole plate ends will be forced back into the air;gap and this will increase the magnetic flux densityin each air gap. One-microphone, constructed in accordance withmy invention, showed as much as 12% increase infmagnetic flux density: in-eachj air gap. Hence-the total output-of-s'uch" a microphone with' tlie -.voice coils connected to produce an additive effect is considerably increased and may be found to be approximately 25% greater than the output of two separate microphone structures, or that obtained from a single structure proportionately larger I Inasmuch as this type of microphone is responsive to sound waves only when there exists a difierence'in pressure on opposite sides of the diaphragm, it becomes necessary to provide a difference in sound wave path length to opposite sides of the diaphragm from the desirable sound source. For this purpose, one magnetic structure ;1 is made independently rotatable with respect to the other so that the, spacing between the sound openings 43 may be varied. In order that this may be done, and further in order to prevent separation of the two magnetic structures in axial directions, the hand held support'5 is arranged to engage opposite ends of the microphone. For the purpose of illustration, the hand support 5 isshown herein as comprising a bifurcated member 49 disposed on one end of a handle '50. A pair of set screws 5| are threadedly mounted within'and adjacent to the bifurcated ends of the support S, each screw 5| having a pointed end 53 which provides a bearingsurface for engaging a complementary detent disposed in the adjacent endof thev microphone 3. With this arrangement, either unit I of the entire microphone 3 may be rotated with respect to the member 49. Thus, when the microphone is held in front of the mouth, the user may rotate either unit relative to'the otherto orient or dispose its opening 43 directly in front of his mouth and sound waves may be transmitted directly into that opening along the shortest path thereto. This mounting arrangement will also permit the two magnetic structures to be rotated with respect to each other thereby to separate the openings 43 circumferentially about thecylindrical casing any desired amount. In order'to limit rotation of either magnetic structure 1,thereby to prevent breaking the leads 55 from the voice coils, stops 51' are provided on the magnetic structures I as well as on the bifurcated ends of the hand support 5.

Providing an adjustable sound wave path length between the two sides of the diaphragm is a particularly desirable feature of my improved noise cancelling, pressure gradient responsive microphone. It is well known to those persons skilled in the art that this type of microphone ceases to operate correctly as a pressure gradient responsive device as the frequency of the sound wave approaches such a value that the path length from one side of the diaphragm to the other side of the diaphragm is an appreciable portion of the wavelength. The generally accepted limit for correct operation is taken to be that point at which the path length front to back of the diaphragm is one quarter wave length. For example, a microphone as herein described, with a path length of one-half inch, would efficiently cancel ambient noise up to about 7000 cycles per second where the ambient or random noise originates at a relatively remote point from the microphone, whereas one having a path length of one-quarter inch would efficiently cancel ambient noise up to about 14,000 cycles per second, under the same conditions. It is readily seen that the shorter the path length between the two surfaces of the diaphragm, the higher will be the limit of high frequency cancellation for sounds originating at a remote point. Therefore, it would be desirable to keep this patlr length as short as possible.

On the other hand, one must also be able to direct the desired sound more'strongly to one side of the diaphragm than to the other in order that the microphone will reproduce the desired sounds. It is obvious, therefore, that the shorter the front to back sound wave path length, the more cancellation of desirable sounds takes place and there results a lower electrical output. Thus, to pick up desirable sound waves, the front to back path length should be as long as possible.

Considering these two contradictory factors, it is readily seen that a microphone with a variable sound Wave path length is desirable. Under extreme conditions of high frequency noise, a short path length would be desirable, even though a somewhat decreased electrical output would be tolerated. Under different conditions, where the noise is predominately of low frequency, one could increase the path length and obtain a gain in desired electrical output without appreciably decreasing the signal to noise ratio.

Thus, it will be evident to those persons skilled in the art, that I have provided an efficient, noise cancelling, pressure gradient responsive microphone which has substantially no external stray magnetic field. and one which has an increased output over other pressure gradient responsive microphones comparable in size. It will also be recognized that the microphone is simple and easy to construct, and, \with suitable choice of materials, it may be made relatively small in size.

While I have shown and described but a single modification of a pressure gradient responsive microphone in accordance with m present invention, it will become obvious to those persons skilled in the art that other modifications and changes are possible within the spirit of my invention. Therefore, I desire that the particular form of my invention described herein be considered as illustrative and not as limiting.

What is claimed. is:

l. A pressure gradient responsive microphone of the moving coil type comprising a casing of magnetic material, a diaphragm mounted within said casing and dividing the interior thereof into separate chambers on opposite sides of said diaphragm, magnetic structures including said casing symmetrically disposed on opposite sides of said diaphragm and providing balanced magnetic circuits on opposite sides of said diaphragm, said magnetic structures including a pair of air gaps, one in each of said chambers, and coil means carried by said diaphragm on opposite sides thereof, said coil means being disposed each in a separate one of said air gaps, and said casing being provided with spaced apertures communicating separately with each of said chambers.

2. A pressure gradient responsive microphone of the moving coil type comprising a casing of magnetic material, a diaphragm mounted within said casing and dividing the interior thereof into substantially acoustically balanced chambers on opposite sides of said diaphragm, magnetic structures including said casing symmetricall disposed on opposite sides of said diaphragm and providing balanced magnetic circuits on opposite sides of said diaphragm, said magnetic structures including a pair of air gaps, one in each of said chambers, and coil means carried by said diaphragm substantially centrally thereof and in coaxial relation equally on oposite sides thereof, said coil means being disposed each in a separate 7 one of said air gapsjsaid casing :being provided with spaced apertures c'ommunicating separately :with each of'said chambers, and said magnetic structures {being rotatablewith respect to each other for varying the spacing betweensaid aper- 'tures:

-53. A pressure gradient :responsive microphone according to claim 2 characterizedin that said "magnetic structures are of corresponding polar vity'. 4. A pressure gradient responsive microphone of the moving 'coil type comprising a casing of *magnetic material, adi-aphra-g-m mounted within saidcasing and dividing the interior thereof into substantially acoustically balanced chambers on opposite sidesof said'diaphragm, separate magneticstructures including said casing'symmetrically disposed'on opposite sides of said diaphragm and providing balanced magnetic circuits on opposite sides thereof, said magnetic structures in- ;:.c1uding two substantially equal air gaps disposed -,c'entrally and coaxiall onoppositesides of said diaphragm, andcoil means carried by said diaphragmpentrally on opposite sides thereof and disposed withinsaid air gaps, said coil *means on opposite sidesof said diaphragm being electri- -cally connected to produce an additive effect, said casingbeing providedwvith spaced apertures communicatingseparately with each of said chambers, ancl-said magnetic structures-being rotatable with respect to each other for varying the spacing between said apertures. a

'5 A pressure gradient responsive microphone according to claim' i' characterized in that said casingcomprises a pair of cup-shaped members each having a recess fdisposed around'the inner peripheral edge-at the open-endthereof, said cupshaped-members being c'oaxiall mounted with the openends thereof abutting, characterized further by the addition'of plate members eachhaving a'plura'lity of apertures thereinfsaidplate members constituting outer pcleplates disposed respectively within said recesses, one 'of said apertures in each plate member being centrally dis- ;posed, and characterized further in that said' magnetic structures include magnetic core means attached to each of said cup-shaped members,

, each core means having one end thereof disposed 8 withinthe centralaperture of-i-tszasseciatcd plate member in spaced relation to itssaid associated plate member to provide ,one of said air gaps therebetween. V

6. A pressure gradient responsive microphone according to claim 5 characterized in that each 7 spaced'relation thereto. 7 1

8. A pressure gradient responsive micro-phone according to claim :7. characterized in that said casing apertures'are disposed in the Walls of said cup-shaped'members adjacent to said outer pole plates on the sides thereof remote from said diaphragm, and: further characterized in that the remainder of said outerpole plate apertures comprise passages for sound waves entering said chambers through said casin aperturesto opposite sides of said diaphragm.-

-9. A pressure g-radienteresponsive microphone comprising a diaphragm responsive to difierences in sound wave pressure on opposite sides thereof, 7

separate magnetic structuresdisposed in abutting relation to .providean enclosed casingfor said diaphragm';said magnetic structures being symmetrically disposed on opposite sides of said diaphragm andlsupporting said diaphragm at its periphery, said magnetic structures including a pair of 'air gaps, one on each side of said dia phragm, and coil means attached to said diaphragm on opposite sides thereof, said coil means being disposed each in a separate one of said air gaps, said casinghaving apertures therein for transmitting sound Waves to opposite sides of said diaphragm, and-said magnetic structures being rotatable with respect to each other for Varying the spacing between saidapertures.

, ROBERT K. DUNCAN.

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