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Publication numberUS2773933 A
Publication typeGrant
Publication dateDec 11, 1956
Filing dateFeb 1, 1954
Priority dateFeb 1, 1954
Publication numberUS 2773933 A, US 2773933A, US-A-2773933, US2773933 A, US2773933A
InventorsHawley Mones E
Original AssigneeHawley Mones E
Export CitationBiBTeX, EndNote, RefMan
External Links: USPTO, USPTO Assignment, Espacenet
Third order pressure gradient responsive microphone
US 2773933 A
Abstract  available in
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Claims  available in
Description  (OCR text may contain errors)

Dec. 11, 1956 M. E. HAWLEY THIRD ORDER PRESSURE GRADIENT RESPONSIVE MICROPHONE Filed Feb. 1, 1954 INVENTOR. Mzzwflfiazwiey A TTOR NE Y THERE ORDER PRESURE GRADIENT REPUNSVE MICROPHQNE Mones E. Hawley, Collingswood, N. 5., assignor, by mesne assignments, to the United dtates of America as represented by the Secretary of the Navy Application February 1, 1954, Serial No. 407,529

9 Claims. (Cl. 179-1) This invention relates to sound translating apparatus, and, more particularly, to a third order, pressure gradient responsive microphone.

Third order pressure gradient responsive microphones are particularly useful in providing microphones with highly directional characteristics, and an ability to discriminate against noise. Such microphones are useful in numerous commercial fields, especially in television broadcasting where it is desirable to pick up sound usually from a single direction while excluding all other noises, such as those from camera or other sources.

A pressure gradient responsive microphone is one in which the output is substantially proportional to a derivative order sound pressure with respect to distance from the source. Microphones of this type are classified according to the order of the pressure derivative. Thus, a first order microphone has an output proportional to the first derivative. A second order microphone has an output proportional to the second derivative. An nth order microphone has an output proportional to the nth derivative. First order pressure gradient responsive microphones may comprise either two elements responsive to the pressure of a sound wave, or a single element responsive to the pressure gradient of a sound wave. A second order, pressure gradient, responsive microphone would require either two first order microphones or four pressure microphones. A third order microphone would require either two second order microphones or four first order microphones, or eight pressure microphones. An nth order microphone would require 2 pressure microphones.

Pressure gradient responsive microphones having third order operation have been effected with but a single pressure sensitive element. This has been accomplished by picking up acoustic energy from different points and transmitting the energy through acoustic chambers on opposite sides of the pressure sensitive element. A number of tubes or conduits are used to connect the ambient to the pressure sensitive elements. Considerable difficulty is encountered in constructing acoustically symmetrical systems when a single pressure sensitive element is used. This is due to the fact that a slight variation in construction will result in a vast difference in the acoustical impedance within the system.

It is an object of the present invention to provide an improved microphone structure having a third order, pressure gradient response and having improved frequency response.

It is a further object of this invention to provide an improved microphone structure having a third order response which utilizes a minimum number of parts and which requires but a single pressure sensitive element.

It is a further object of this invention to provide an improved microphone structure having a third order response which is relatively simple and easy to construct.

It is a still further object of this invention to provide an improved microphone structure in which the acoustical impedances within the system are more easily balanced.

Patented Dec. 11, 1956 In accordance with the present invention, the third order, pressure gradient responsive microphone comprises a housing member having a single pressure sensitive element or diaphragm. A pressure plate is disposed on each side of the diaphragm within the housing. Each of the pressure plates has an annular groove. A plurality of spaced apart openings or indentations connects the annular grooves with the ambient. The sizes of the openings in the pressure plate and the acoustical impedances of the annular grooves are controlled to bear ratios to each other in a manner to provide third order operation.

Other objects and advantages of the present invention will become apparent and suggest themselves to those skilled in the art to which the present invention is directed from a reading of the following specification in connection with the accompanying drawing in which:

Figure 1 is a view in side elevation of a preferred embodiment of a third order pressure gradient responsive microphone in accordance with the present invention;

Figure 2 is an exploded perspective view partly in section of the microphone shown in Figure 1; and

Figure 3 is a schematic diagram of a simple electrical circuit equivalent to the acoustical network of the vibrating system of the microphone shown in Figure 1.

Before referring to the accompanying drawing in greater detail, it is desired to point out the background theory which prompted the development of the present invention. In accordance with this theory, if it is desired to build a third order, pressure gradient responsive microphone, instead of using eight pressure microphones, only five are necessary since one can utilize pressures P2, P3 and P4 twice. Thus, for the first pressure difference one obtains (Pi-P2), for the second pressure difference one has (P2-P3), for the third pressure difference one has (Pa-P4) and for the fourth pressure difference (Pr-P5). These may be combined and expressed as follows to obtain third order operation:

Expression 1 may be simplified and written as follows:

It will be noted that the third pressure P cancels out.

Therefore, it is not necessary to utilize the microphone providing pressure P at all. A third order, pressure gradient responsive microphone may be constructed using a single pressure-sensitive element. According to Expression 1 above, five pressure-sensitive elements must be considered in the case of a third order microphone. These elements may be denoted as the first, second, third, fourth and fifth elements respectively. Although it is necessary to take into account all five elements, Expression 2 shows that only four elements are actually necessary. The output of the third element is not used but is taken into account in the spacing of the other four elements. In other words, a third order microphone may be provided by admitting acoustic energy to a single vibratory element or diaphragm through four apertures (denoted herein as first, second, third and fourth elements corresponding, respectively, to the pressure at the first, second, fourth and fifth positions in space referred to in Equation 2 above), directing the energy to one side of the diaphragm through the first and third apertures and to the other side of the diaphragm through the second and fourth apertures and making the diaphragm movement twice as sensitive to a unit quantity of energy entering either the second or third apertures as to a unit quantity of energy entering either the first or fourth apertures.

Referring particularly to Figures 1 and 2 of the drawing, there is shown an end view of a preferred embodiment of the invention. An electro-mechanical inverter 2, is mounted on the exterior of a casing or housing member 4 by a suitable attaching means and is connected to a diaphragm 6 by a drive rod 8. The drive rod extends through an opening 9 in a pressure plate it The casing member has four apertures l2, l4, l6 and lit to permit the acoustic chambers on the two sides of the diaphragm to be connected to the ambient, as will be subsequently described.

The pressure plate It) consists of a circular disc having an annular groove 26). A pair of slots or indentations 22 and 24 are out along the peripheral edge of the disc and serve to connect an acoustic chamber formed by the annular groove 2% within the pressure plate to the ambient when the pressure plate is assembled and placed against the peripheral edge of a diaphragm 6. The central portion 26 of the pressure plate is disc shaped and adapted to be the same shape as the diaphragm. A washer St? is adapted to provide proper spacing between the pressure plate lit and the diaphragm 6 when the unit is assembled. The diaphragm 6 comprises a central dished portion 32 having an upraised portion 33 and an integral, annular, peripheral portion 34. stiffening flutes or ribs 36 are provided between the central portion and the periphery of the diaphragm. This type of diaphragm permits enhanced vibration to be obtained. A washer or spacing member 33 is used to engage the peripheral portion of the diaphragm and to provide proper spacing between the diaphragm and a second pressure plate 4h. The second pressure plate also has an annular groove 42 to provide a second acoustic chamber and a pair of slot openings or indentations 44 and as to permit the acoustic chamber of the diaphragm to be connected to the ambient. The central part of the second pressure plate is provided with a raised portion 43 of substantially the same shape as the corresponding portion of the diaphragm. A central opening St) is provided to permit an adjustable spacing member "52 to be inserted therein. A pair of set screws 54 and 56 are provided to hold the spacing member in place. A holdoif screw 52; is provided to maintain the operating member at the desired distance. This permits a certain amount of balancing between the acoustic chambers on the two sides of diaphragms to be attained.

The assembled microphone unit comprising the two pressure plates with the diaphragm and the washers or spacing members mounted therebetween is designed to fit into the cylindrical casing The circular ring so is de signed to threadedly engage the inner portion of the easing and to exert a pressure against the upper pressure plate iii. The bottom pressure plate 50 rests against a flange 51 which is integral with the case. A compact unit, as seen in Figure l, is thus provided when microphone is assembled and the ring so is screwed into position.

In considering the operation of this microphone after assembly, it is seen that the diaphragm or pressure sensitive element is mounted in a manner to respond to sound pressure impinging on oppostie surfaces thereof.

The pressure plates it? and at) are arranged with the slot openings 22, 24, M and 46 disposed such that they open at spaced intervals on the cylindrical casing d. The four openings that are provided correspond, respectively, to the first, second, third and fourth elements referred to above. The openings are arranged in series relation. Two of the openings 22 and as occupy extreme positions and the two openings 24 and 4.4 occupy mean positions. As shown in Figure l, the openings are arranged alternatively on opposite sides of the plane of the diaphragm 6 so that the successive openings in the series are connected to alternative acoustic chambers which include the annular grooves 20 and 42.

in order that the second and third openings 24 and 44- provide twice the sensitivity to the diaphragms as the first and fourth openings the acoustical impedances of the secend and third openings are made half as large as the acoustical impedances than the first and fourth openings.

The diaphragm is mounted between the two pressure plates and is spaced therefrom at the peripheral edge by the thin annular washers. Each of the slot Openings is connected to the acoustic chamber.

The acoustical circuit for the third order microphone, such as described, is illustrated by a simplified electrical circuit shown in Figure 3, wherein:

R1=the lumped acoustical resistance of the slot opening 12 or pressure point;

R2=the lumped acoustical resistance of the second opening 24 or pressure sensitive point;

Ra the lumped acoustical resistance of the third opening 44 or pressure sensitive point;

R4=the lumped acoustical resistance of the fourth opening 46 or pressure sensitive point;

Rn =the lumped acoustical resistance of the diaphragm;

L1=the lumped acoustical inertance of the first opena;

L2=the lumped acoustical inertance of the second opena;

L3=the lumped acoustical inertance of the third opens;

L4=the lumped acoustical inertance of the fourth opens;

Ln=the lumped acoustical inertance of the diaphragm;

C1=the lumped acoustical compliance of the cavity on the left hand side of the diaphragm as viewed in Figure 2;

Cz=the lumped acoustical compliance of the cavity on the other side of the diaphragm;

Cn=the lumped acoustical compliance of the diaphragm;

V1=the sound pressure at the first opening leading to one surface of the diaphragm;

V2=tl1e sound pressure at the third opening leading to the same surface of the diaphragm;

V =the sound pressure at the second opening leading to the other surface of the diaphragm;

V4=the sound pressure at the foruth opening leading to the same surface of the diaphragm as Vs;

in=tl1e acoustical volume current through the diaphragm.

If the sound pressure at all the apertures is identical at the same time, the voltage output of the microphone will be 0. If the sound pressure p is incident upon the second aperture and the pressure at all the other apertures is 0, the displacement of the diaphragm will be twice that obtaining when the same sound pressure is incident upon the fourth aperture and the pressure at all the other apertures is zero. The same analogy may be drawn with respect to apertures 1 and 3.

Referring particularly to Figure 3, in must be Zero when V1=V2=V3=V4. When V1=V and V2=Vs=V4=0, in must be half the value obtaining when V2=V and where V is any arbitrary voltage. Similarly, when V4=V and V1=V2=Va=0, in must be half the value obtaining when V3=V and V1=V2=V4=0.

These conditions are satisfied by the present invention by designing the microphone so that Ci cz, and

The voltage output will be zero when this condition is met and the sound pressure at all the apertures is identical at the same time.

Other impedance ratios and other hole spacings are possible and will suggest themselves to those normally skilled in the art. For example if the separation between the second and third apertures is made equal to the separations between the first and second and the third and fourth apertures, and if the impedances of the two diaphragm cavities are equal, the impedances of the first and fourth apertures must be three times the impedances of the second and third apertures.

This invention provides improved frequency response characteristics possible by eliminating the long acoustic conduits which cause irregularity in the response frequency characteristics at the conduit resonant frequency. The end corrections for the impedance of the tubes as they lead into cavities are unnecessary in the present invention where the length of these tubes is extremely short. It is also much easier to damp the microphone by the insertion of suitable acoustical resistance material and thus improve the overall response frequency characteristic. An additional advantage is that the symmetry of the diaphragm pressure plates is easy to obtain and proper balancing of the microphone is dependent upon fewer precision parts.

The annular grooves within the pressure plates provide acoustical chambers which are relatively easily matched during the manufacture of the pressure plates. Proper design of the pressure plates Will assure a minimum amount of adjustment after manufacture.

It is seen that this microphone requires a minimum number of parts with only the two pressure plates requiring any great degree of care in the manufacturing process. This, of course, permits easy assembly and lower costs of construction.

The fluted diaphragm used in combination with the other parts of the microphone aids in providing a microphone which responds accurately and reproduces truly and without rattle substantially all the frequencies within the normal audible range.

What is claimed is:

l. A third order, pressure gradient responsive microphone comprising a pressure sensitive element, a pressure plate disposed on one side of said pressure sensitive element, said pressure plate having an annular groove to form an acoustic chamber, said pressure plate also having a plurality of spaced apart indentations connecting said annular groove with the ambient, a second pressure plate disposed on the opposite side of said pressure sensitive element, said second pressure plate having a second annular groove to form a second acoustic chamber, and a plurality of spaced apart indentations connecting said second annular groove to the ambient.

2. A third order, pressure gradient responsive microphone comprising a housing, a pressure sensitive element mounted within said housing, a pressure plate disposed on one side of said pressure sensitive element, said pressure plate having an annular groove to form an acoustic chamber, said pressure plate also having a plurality of spaced apart indentations connecting said annular groove with the ambient, one of said indentations being so dimensioned as to provide a larger acoustical impedance than one of the other indentations, a second pressure plate disposed on the opposite side of said pressure sensitive element, said second pressure plate having a second annular groove to form a second acoustic chamber, and a plurality of spaced apart indentations connecting said second annular groove to the ambient, one of said indentations of said second pressure plate being so dimensioned as to provide a lar er acoustical impedance than one of said other indentations.

3. A third order, pressure gradient responsive micro phone comprising a housing, a diaphragm having a fluted portion, a pressure plate disposed on one side of said diaphragm, said pressure plate having an annular groove to form an acoustic chamber, said pressure plate also having a plurality of spaced apart indentations connecting said annular groove with the ambient, said annular groove being in substantial alignment with said fluted portion of said diaphragm, a second pressure plate disposed on the opposite side of said diaphragm, said second pressure plate having a second annular groove to form a second acoustic chamber and a plurality of spaced apart indentations connecting said second annular groove to the ambient, said second annular groove being in substantial alignment with said fluted portion of said diaphragm.

4. A third order, pressure gradient responsive microphone comprising a housing, a diaphragm having a fluted portion mounted within said housing, a pressure plate disposed on one side of said diaphragm, said pressure plate having an annular groove to form an acoustic chamber, said pressure plate also having a plurality of spaced apart indentations connecting said annular groove with the ambient, one of said indentations being so dimensioned as to provide a larger acoustical impedance than one of the other indentations, said annular groove being in substantial alignment with said fluted portion of said diaphragm, a second pressure plate disposed on the opposite side of said diaphragm, said second pressure plate having a second annular groove to form a second acoustic chamber, and a plurality of spaced apart indentations connecting said second annular groove to the ambient, one of said indentations of said second pressure plate being so dimensioned as to provide a larger acoustical impedance than one of said other indentations, said second annular groove being in substantial alignment with said fluted portions of said diaphragm.

5. A third order, pressure gradient responsivemicrophone comprising a pressure sensitive element, a pressure plate disposed on one side of said pressure sensitive element, said pressure plate having an annular groove to form an acoustic chamber, said pressure sensitive element also having a pair of spaced apart indentations connecting said annular groove with the ambient, one of said openings being so dimensioned as to provide twice the acoustical impedance as the other, a second pressure plate disposed on the opposite side of said pressure sensitive element, said second pressure plate having a second annular groove to form a second acoustic chamher, and a pair of spaced apart indentations connecting said second annular groove to the ambient, one of said openings of said second pressure plate being so dimensioned as to provide twice the acoustical impedance as said other opening.

6. A microphone as defined in claim 5 wherein said indentations are disposed in series in spaced apart relation alternately on opposite sides of said pressure sensitive element to provide a pair of extreme openings and a pair of mean openings, said extreme openings each being dimensioned to provide substantially twice the acoustic impedance of other of said mean openings, successive ones of said series of openings being connected to alternate sides of said acoustic chambers.

7. A third order, pressure gradient responsive microphone comprising a pressure sensitive element, means connected with said pressure-sensitive element for converting vibrations thereof into corresponding electrical signals, a pressure plate disposed on one side of said pressure sensitive element, said pressure plate having an annular groove to form an acoustic chamber, said pressure plate also having a pair of spaced apart indentations connecting said annular groove with the ambient, one of said indentations being so dimensioned as to provide substantially twice the acoustic impedance as the other indentation, said pressure plate further having a central aperture, a spacing member adapted to fit into said central aperture, means to attach said spacing member to said pressure plate, adjustable means to vary the spacing between said pressure plate and said pressure responsive element, a second pressure plate disposed on the opposite side of said pressure sentitive element, said second pressure plate having a second annular groove to form a second acoustic chamber and a pair of spaced apart indentations connecting said second annular groove to the ambient, one of said indentations of said second pressure plate being so dimensioned as to provide substantially twice the acoustical impedance as said other indentation.

8. A third order, pressure gradient responsive microphone comprising a housing member, a pressure sensitive element disposed within said housing member, a pres- 7 sure plate disposed on one side of said pressure sensitive element, said pressure plate having an annular groove to form an acoustic chamber, said pressure plate also having a plurality of spaced apart openings connecting said annular groove with the ambient, a second pressure plate disposed on the opposite side of said pressure sensitive element, said second pressure plate having a second annular groove to form a second acoustic chamber and a plurality of spaced apart openings connecting said second annular groove to the ambient, said openings being disposed in series in spaced apart relation alternately on opposite sides of said pressure sensitive element to provide a pair of extreme openings and a pair of mean openings, said extreme openings each being dimensioned to provide substantially twice the acoustical impedance as either of said mean openings, successive ones of said series of openings being connected to alternate sides of said acoustic chambers.

9. A microphone as defined in claim 8 wherein said housing member is provided with a series of apertures in alignment with said openings, and a retaining ring is provided to threadedly engage said housing member to hold said second pressure plate in an operative position.

References Cited in the file of this patent UNITED STATES PATENTS 1,844,487 Tyrrell Feb. 9, 1932 2,475,782 Giannini July 12, 1949

Patent Citations
Cited PatentFiling datePublication dateApplicantTitle
US1844487 *Jun 14, 1928Feb 9, 1932Rca CorpDiaphragm
US2475782 *Dec 3, 1942Jul 12, 1949Automatic Elect LabCellular support for loudspeakers, including acoustic chambers
Referenced by
Citing PatentFiling datePublication dateApplicantTitle
US2892290 *Jan 22, 1957Jun 30, 1959Ryan John WToy control system
US3258543 *Oct 1, 1962Jun 28, 1966Electro VoiceDynamic microphone
US3414675 *Jul 30, 1965Dec 3, 1968Akg Akustische Kino GeraeteTelephone handset with pressure gradient microphone substituted for carbon microphone
US4768614 *Nov 28, 1986Sep 6, 1988Case Eliot MUnidirectional enhancer for microphones
Classifications
U.S. Classification381/357, 181/158
International ClassificationH04R1/40
Cooperative ClassificationH04R1/406
European ClassificationH04R1/40C