US 3130275 A
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A ril 21, 1964 P. HAGEY MICROPHONE Filed Oct. 15, 1962 Q5: iii w m 4 I: LE
United States Patent Ofiice 3,130,275 Patented Apr. 21, 1964 3,130,275 MICROPHONE Lee P. Hagey, Buchanan, Mich., assignor to Electro-Voice, Incorporated, Buchanan, Mich., a corporation of Indiana Filed Oct. 15, 1962, Ser. No. 230,578 3 Claims. (Cl. 179-110) The present invention relates generally to microphones, and more particularly to microphones which are adapted for use in electrical stethoscopes.
Electrical stethoscopes have long been known to the art, Patent No. 1,791,932 to Miller disclosing such a stethoscope in 1931. An electrical stethoscope employs an electromechanical transducer which is mechanically coupled to the body of a person under investigation and which generates an electrical impulse. The electrical impulse is utilized to excite an amplifier circuit, and the output of the amplifier circuit is observed either visually or audibly.
A stethoscope should reproduce the sound generated by the body of the person under investigation without alteration, and should be relatively insensitive to gross body movements. This requires the microphone of the stethoscope to have a frequency response range adequate to reproduce the sounds generated by the human body and it further requires coupling of the microphone sensitive element to the body of the person under investigation. Microphones for use in electrical stethoscopes prior to the present invention have produced relatively small outputs, thus requiring expensive and generally bulky amplifiers. Further, the small output from prior microphones for use with electrical stethoscopes has tended to introduce distortion and other types of untrue sound reproduction. It is one of the objects of the present invention to provide a microphone for use in electrical stethoscopes which will produce high electrical output relative to the microphones that have existed for this purpose heretofore.
One of the ditficulties in obtaining substantial output from a microphone for use in an electrical stethoscope is in obtaining adequate coupling between the sensitive element of the microphone and the skin of the person under investigation. Prior to the present invention, microphones were not provided with coupling mechanisms between the body of the person under investigation and the sensitive element of the microphone which would achieve adequate impedance matching. It is therefore a further object of the present invention to provide a microphone for use in an electrical stethoscope which is provided with a coupling element adapted to be placed in contact with the skin of the person under investigation for the purpose of matching the impedance of this persons skin to the mechanical impedance of the sensitive element of the microphone.
Prior to the present invention, there was a tendency to utilize magnetic structures in microphones for use with electrical stethoscopes. Such structures are mechanically heavier than desirable, more fragile than desirable, and relatively costly. It is a further object of the present invention to provide a microphone for use in electrical stethoscopes which is durable, simple in construction, and relatively inexpensive.
These and further objects of the present invention will become readily apparent to those skilled in the art from a further consideration of this specification, particularly when viewed in the light of the drawings, in which:
FIGURE 1 is a plan view of a microphone constructed according to the teachings of the present invention;
FIGURE 2 is a side elevational view of the microphone illustrated in FIGURE 1; and
FIGURE 3 is an enlarged sectional view taken along the line 33 of FIGURE 1.
As indicated in the figures, the microphone has a cylindrical shell 10 with a fiat circular base 12 and a fiat circular face 14. The face 14 has a circular opening 16 with a conical surface 17 which is closed by a circular membrane 18 which protrudes from the face 14. A two wire conductor cable 20 extends from the shell 10 for connecting the microphone to an amplifier in the conventional manner.
As illustrated in FIGURE 3, the fiat base 12 of the microphone is formed by a circular disc which is cemented within a circular groove 22 at one end of the cylindrical shell 10. The shell 10 and base 12 are constructed of electrically insulating material, such as Lexan plastic, nylon plastic, hard rubber, or some other plastic material. A cylindrical sleeve assembly 24 is disposed within the cylindrical shell 10 and in abutment therewith. The sleeve assembly 24 has an upper portion 26 and a lower portion 28, both constructed of electrically conducting material such as brass. The lower portion 28 has a circular recess 30 confronting the upper portion 26 and the upper portion 26 has a shallow circular ridge 31 at its inner surface confronting the lower portion 28. A circular piezoelectric ceramic element 32 is disposed between the ridge 31 of the upper portion 26 and the lower portion 28. A gasket 33 of electrically insulating material is disposed between the upper portion 26 and the piezoelectric element 32, and an identical gasket 35 is disposed between the lower portion 28 and the piezoelectric element 32. The piezoelectric element 32 has a pair of ceramic discs 34 and 36 which are provided with a common electrically conducting coating 38 which secures the discs 34 and 36 together. In addition, the disc 34 has an electrically conducting coating 40 on the surface thereof opposite the coating 38, and the disc 36 has an electrically conducting coating 42 on the surface thereof opposite the coating 38. The two discs 34 and 36 are electrically polarized in opposite directions relative to the common coating 38, so that displacement of the disc in a direction normal to the coating 38 results in the generation of electrical potentials of opposite sign on the coatings 40 and 42. The coating 42 is connected to a leaf strip 43 of electrically conducting material, such as copper, which extends between the gasket 35 and the lower portion 28 and between the gasket 33 and the upper portion 26 of the assembly 24, thereby forming an electrical connection between the coating 42 and the upper and lower portions of the assembly 24.
A disc 44 of electrically conducting material has its periphery in electrical contact with the surface of the upper portion 26 of the sleeve 24 in abutment with the base 12. The electrically conducting disc 44 and the upper and lower portions of the sleeve assembly 24 form an electrostatic shield about the ceramic element 32. Also, the electrically insulating gasket 35 is cemented on the piezoelectric element 32 and on the lower portion 28 of the assembly 24 and forms a fluid tight seal therebetween. The end of the sleeve assembly 24 opposite the base 12 terminates in a circular inwardly tapering ridge 50, and the membrane 18 is cemented at its perimeter on the exterior surface of this ridge 50. The membrane 18 is thereby disposed between the ridge and the con fronting surface of the opening 16 in the shell 10. The membrane 18 is constructed of flexible material to transmit vibrations, and further is constructed of material with compliance, such as rubber.
The region between the membrane 18 and the disc 44 is a cavity, designated 52, and this cavity is filled with a liquid designated 54. It has been found that a light oil, such as silicone oil, is preferable for the liquid 54 in that it provides positive coupling for the transmission of sound waves from the membrane 18 to the piezoelectric element 32. The membrane 18 must be flexible to transmit sound waves to the body membrane 18 must have compliance in order to'permit l of silicone oil or other liquid. 2
The shell [10 is provided with an "ings 40 and 42 of the element, 7
54 ofliquid, and the the cavity 5210' be thoroughly filled with the body 54 aperture 56 which is aligned with a threaded orifice 58in the lower portion: 28 of the sleeve assembly 24 to permit the cavity 52 to be filled with the body 54 of liquid, The orifice -8 is sealed by a screw 60 having a washer 61 after the cavity 52*has been filled. n i I, I I
The shell is provided withfasecond aperture 62 which confronts a second orifice 64 inthe upper portion I 26 of thesleeve assembly 24. The electrical conductor extends through the aperture 62 and orifice 64, and
has a grounding shield 66 which is" wedged between the cable 20 and the surface of theupper portion 26 of the sleeve assembly 24 to provide electrical connection to the coating 42 of the piezoelectric element 32. The con- I When it is desired to use the microphone illustrated in FIGURES 1 through 3, the membrane 18 of the micro phone is placed in contactjwith the surface of the skin of the person under investigation; Generally, a stethoscope'is employed to listen to theheart'andi lung action of'a' person. Sound Waves transmitted'from the body/of? the personunder investigation pass ome a the membrane 18 into the body 54fof liquid,yand are thereafter] transmitted tothe piezoelectric element' 32; The piezo for mounting the stethoonly by the appended claims, I claim: l
l. A'microphone comprising a cylindrical shell of elecideally insulating material having a circular opening at perforated tape (not shown) to strap the microphone place on a patienh one end, a cylindrical sleeve of electrically conducting materialdisposcdcoaxially within'the shell having an the upper and lower portions and extending across the sleeve, said element having two parallel adjacent fiat discs and a common electrically conducting layer disposed 'therebetween, said element having a first and a second electrically conducting coating disposed on the fiat surfaces thereof opposite the electrically conductinglayer and said element being electrically polarized in opposite directions relative to the electrically conducting layer, the t first of said coatingsbeing electrically connected to the electric element 32 is free to vibrate, thereby generating I periodic electrical potentials ofopposite signon the coat- The shell 10 and sleeve assembly 24 have a combined mass far greater than the mass of the piezoelectric element 32, so that movements of the patient result in movements of the entire microphone, but the piezoelectric element 32' is not placed under strain relative to theshell 10 and sleeve assembly 24. Thus, operation analogous to an accelerometer is avoided.
The microphone will operate properly with the body 54 of liquid disposed on both sides of the piezoelectric element 32. However, it is preferable to cement the perimeter of the piezoelectric element 32 on the groove of the lower portion 28 of the sleeve assembly 24 to prevent the body 54 of silicone oil from passing to the other side'of the piezoelectric element 32.
From the foregoing specification, those skilled in the 7 art will readily devise many embodiments of this invention withinithe intended scope of the invention Further, a microphone constructed in the manner set forth above,
may be utilized for purposes other than and in addition electrically conducting sleeve, means for sealing the portion of the interior of the sleeve between the openings thereof and the elernent' against leakage, of fluid including; 1 i
" afleadble membrane extcndingfovc'r the openingther i 1 a and seated; thereto, a body; of liquid; disposed Wlijhlll i ifsleeve' andffilling the portion thereofi between 'the membrane and the piezoelectric; element, said: body of liquid inechanieallycoupling the piezoelectric element and the membrane, and an electrical cable extending I j through the sleeve andshell having a first conductor elecitricallyconncted :tolthe sleeve and a second electrical conductor electrically connected to the second coating or the piezoelectric'element.
2. A microphone comprising the elements of claim 1 wherein the means for sealing the interior of the sleeve against fluid leakage includes an electrically insulating gasket cemented between the piezoelectric element and the confronting surface of the sleeve.
3. A microphone comprising the combination of claim 1 wherein the sleeve is provided with openings at both ends of the cylindrical portion thereof and the opening remote from the diaphragm is covered with an electrically conducting disc.
References Cited in the fileof this patent UNITED STATES, PATENTS 2,564,562" Chess n-2,. Aug. 14, 1951 2,571,899 Kroft et a1. -QOct. 16', 1951' n Parssinen et all Sept, 11-, 1962 l