Microphone for the transmission of body sounds

Claim 2

A microphone for the reception and conversion of low frequency sound, especially body sound signals, which can be used advantageously in conjunction with blood pressure measuring devices and the like, the microphone comprising in combination: a cup-shaped microphone housing having a bottom and a generally tubular side wall with a rim defining an open upper end of the housing; a circular membrane covering the open end of the housing and having a narrow peripheral edge portion clamped against the rim of the side wall; a substantially flat flexural piezoelectric transducing wafer arranged inside the housing; means for mounting the transducing wafer above the housing bottom, a distance from the membrane, and in substantially parallel alignment with the latter, the mounting means holding the transducing wafer in a cantilever fashion so as to allowing a portion thereof to flex freely in displacement alignment with the center of the membrane; and a resiliently compressible transmitter body interposed between and engaging both the membrane and said flexing portion of the wafer, at least a portion of the transmitter body, including the extremity with which it is engaged against the wafer, being of a cellular, foamed material suited for the transmission of low frequency sound vibrations.

Claim 3

A low frequency microphone as defined in claim 1 wherein the transducing wafer is a Seignette salt crystal wafer, having a generally quadrangular outline; and the wafer mounting means engages the wafer on three of its four corners, leaving the fourth corner free to flex with the membrane.

Claim 4

A low frequency microphone as defined in claim 2, wherein the Seignette salt crystal wafer is square in outline, having a side length of approximately 12.5 mm and a thickness of approximately 0.6 mm.

Claim 5

A low frequency microphone as defined in claim 2, wherein the transmitter body is a substantially cylindrical body of foamed synthetic plastic, having a diameter of approximately 6 mm and a height of approximately 4 mm.

Claim 6

A low frequency microphone as defined in claim 2, wherein the material of the transmitter body is a foamed polyester of the type MB, having the following physical characteristics: ______________________________________ designation: polyester foam; type: MB; density: 22 kg/m.sup.3 ; indentation hardness: 40% - 20 kp, per DIN 53576; compression hardness per cm.sup.2 : 40% - 40 kp, per DIN 53576; tensile strength: 1.5 kp per cm.sup.2, per DIN 53571; failure elongation in %: > 200%, per DIN 53571; residual compressive 50% over 20, per DIN 53571; deformation and micropores approx. 0.09 mm dia. ______________________________________

Claim 7

A low frequency microphone as defined in claim 2, wherein the transmitter body comprises, in addition to a first body portion of said cellular foamed material, a second body portion which includes the extremity with which it is engaged against the membrane and which serves as a vibrating mass; and the vibrating mass is attached to the inner side of the membrane, near its center.

Claim 8

A low frequency microphone as defined in claim 6, wherein the vibrating mass is of solid plastic material, generally cylindrical in shape, and has a diameter of approximately 6 mm and a height of approximately 3.5 mm.

Claim 9

A low frequency microphone as defined in claim 6, wherein the material of the vibrating mass is a hard PVC per DIN Standard No. 4077, having the following physical characteristics: ______________________________________ impact resistance: unbroken at 0° C, per DIN 52433; notched bar impact response: 4 cm kp/cm.sup.2 (with bars of 20 to 70 mm dia.); failure elongation: > 100%; tensile strength: > 500 kp/cm.sup.2 ; modulus of elasticity: 300000 kp/cm.sup.2 ; and compressive strength: 800 kp/cm.sup.2. ______________________________________ -9. A low frequency microphone as defined in claim 6, wherein the vibrating mass is attached to the membrane by means of an adhesive bond; and the opposite end of the vibrating mass is attached to the formed portion of

Claim 10

the transmitter body by means of an adhesive bond. 10. A low frequency microphone as defined in claim 1, werein the microphone membrane is a circular disc of approximately 0.3 mm thickness, having a diameter in the vicinity of 36.6 mm, and being made of an epoxy glass fiber fabric, known as fiber glass fabric with epoxy resins, per DIN Standard No. 7735, having the following physical characteristics: ______________________________________ flexural strength: > 3500 kp/cm.sup.2, per DIN 53452; impact resistance: > 100 cm kp/cm.sup.2 ; tensile strength: 2200 kp/cm.sup.2 ; compressive strength parallel > 4000 kp/cm.sup.2 ; to grain: compressive strength > 2000 kp; across grain: splitting resistance: > 300 kp; modulus of elasticity: 180000 kp/cm.sup.2 ; continuous heat resistance: 130° C; raw density: 1.7 to 1.9 gr/cm.sup.3 ; and surface resistance: > 10.sup.12 ohms. ______________________________________ -11. A low frequency microphone as defined in claim 1, further comprising: membrane abutting means for limiting the inward deflection of the membrane

Claim 11

under a large mechanical load inadvertantly applied to the membrane. 12. A low frequency microphone as defined in claim 11, wherein the membrane abutting means is a substantially rigid, centrally apertured safety plate mounted underneath the membrane, at such a distance therefrom that the membrane does not contact the safety plate during normal

Claim 12

operation of the microphone. 13. A low frequency microphone as defined in claim 12, wherein the vertical distance between the safety plate and the membrane is such that the maximum inward deflection of the latter is limited to between 0.7

Claim 13

and 0.8 mm. 14. A low frequency microphone as defined in claim 12, wherein the microphone housing includes a shoulder on the inner side of the housing side wall; and the safety plate rests on said shoulder and is held in place by the housing

Claim 14

side wall. 15. A low frequency microphone as defined in claim 1, wherein the membrane includes means for diverting from the transmitter body and transducing wafer any electrostatic changes received from an extraneous

Claim 15

source. 16. A low frequency microphone as defined in claim 15, wherein the charge diverting means is constituted by electrical conductivity of at

Claim 16

least a portion of the membrane. 17. A low frequency microphone as defined in claim 16, wherein said electrically conductive portion of the membrane is a thin metal strip attached to the inner side of the membrane and extending across the center

Claim 17

area of the latter. 18. A low frequency microphone as defined in claim 17, wherein the membrane has a diameter of approximately 36 mm; and the metal stip is a strip of semi-hard brass sheet of 0.06 mm thickness,

Claim 18

being 25 mm long and 10 mm wide. 19. A low frequency microphone as defined in claim 1, further comprising: means for peripherally clamping the membrane against the rim of the housing, said clamping means including a clamping collar engaging the membrane from the outside and cooperating with the housing rim to exert opposing substantially parallel oriented clamping forces against said

Claim 19

peripheral edge portion of the membrane. 20. A low frequency microphone as defined in claim 19, wherein the clamping collar and the housing rim have substantially parallel spaced opposing clamping faces with which they bear against said edge portion of the membrane; and said opposing clamping faces are geometrically defined by two shallow frustums having an included angle of approximately 170°, the clamping faces being accordingly oriented in such a way that they impose on a normally flat membrane an upwardly bulging convex

Claim 20

outline, when clamped therebetween. 21. A low frequency microphone as defined in claim 19, wherein the clamping collar and the housing rim have aligned oppositely facing circular knife edge profiles with which they bear against the peripheral edge portion of the membrane, thereby creating said clamping engagement.

Claim 21

. A low frequency microphone as defined in claim 21, wherein the clamping collar is an assembly of parts, including a collar element cooperating with the housing rim, an edge ring defining one of said knife edge profiles, and means for anchoring the edge ring against the collar

Claim 22

element. 23. A low frequency microphone as defined in claim 22, wherein the anchoring means of the clamping collar assembly includes: an annular inwardly facing groove in the collar element, adapted to receive therein the edge ring; an undercut cross-sectional contour on said ring; and an anchoring mass disposed in the space between said groove and said undercut edge ring contour; and said anchoring mass is a self-curing hard mass, having been applied in a

Claim 23

soft, uncured state during assembly of the microphone. 24. A low frequency microphone as defined in claim 1, further comprising an outwardly protruding touch plate arranged on the outer side of the membrane, the touch plate covering an area that is smaller than that of the membrane, thereby presenting a reduced contact area to a limb against which the microphone may be held and thus focusing the receptivity of its membrane onto said reduced area; and wherein the touch plate is of a material suited for the transmission of low

Claim 24

frequency sound signals. 25. A low frequency microphone as defined in claim 24, wherein the touch plate is adhesively bonded to the membrane; and the material of the touch plate is a silicone rubber having physical characteristics approximating those of a two-component silicone rubber mass which is commercially available from Wacker-Chemie, Munich, Germany, and which consists of the components designated as silicon rubber filler

Claim 25

RTV, Part No. KTR-2264 028 143 and curing agent TK, Part No. KTR-801. 26. A low frequency microphone as defined in claim 24 wherein the shape of the touch plate approximates that of a sector of a body of

Claim 26

rotation having an arched generatrix. 27. A low frequency microphone as defined in claim 26, wherein the shape of the touch plate approximates that of a sector of a minor-axis

Claim 27

ellipsoid, cut along a plane parallel to its rotational axis. 28. A low frequency microphone as defined in claim 1, wherein the microphone housing is metallic and includes a sound-insulating jacket enveloping at least a major portion of the outer surface of the housing; the housing and jacket define between them an intermediate space; and

Claim 28

said intermediate space contains a resilient filler material. 29. A low frequency microphone as defined in claim 28, wherein the jacket is a cup-shaped body enveloping the bottom and side wall of the microphone housing; and the material of the jacket is a silicone rubber mass having physical characteristics approximating those of a silicone rubber filler which is commercially available from Wacker-Chemie, Munich Germany, under the trade

Claim 29

name Elastosil 59. 30. A low frequency microphone as defined in claim 28, wherein the jacket is a cup-shaped body of polyamide, enveloping the bottom and side wall of the microphone housing; and

Claim 30

the wall thickness of the jacket is at least 1 mm. 31. A low frequency microphone as defined in claim 30, wherein the intermediate space between the jacket and the housing is in all places

Claim 31

at least 1 mm. 32. A low frequency microphone as defined in claim 28, wherein the jacket portion enveloping the bottom of the microphone housing has a rounded outline with a curvature approximating that of a sector of a body

Claim 32

of rotation having an arched generatrix. 33. A low frequency microphone as defined in claim 32, wherein the outline of said jacket portion approximates the curvature of a sector of a minor-axis ellipsoid, cut along a plane parallel to its rotational axis.