US 3573397 A
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Description (OCR text may contain errors)
United States Patent 1,072,477 9/1913 Lumiere Inventors Joseph A. Sawyer;
George C. Tibbetts, Camden, Maine Appl. No. 638,926 Filed May 16, 1967 Patented Apr. 6, 1971 Assignee Tibbetts Industries, Inc.
Camden, Maine ACOUSTIC DIAPHRAGM AND TRANSLATING 1lO,6, 114 (R), 115, 115 (R), 115.5 (ES), 138, 181; l8l/3l, 32, (inquired) References Cited UNITED STATES PATENTS 2,927,977 3/1960 Knauert 3,251,954 5/1966 Carlson 179/1 14X l79/ll5 ABSTRACT: An acoustic diaphragm means for a translating device comprises a diaphragm portion and a surround therefor, a flexural pivot pin that extends substantially perpendicular to the diaphragm portion and attaches the diaphragm portion to the translating device near one edge of the diaphragm portion, and means for connecting another portion of the diaphragm portion to the vibratable element of the translating device, away from the center of pressure of the diaphragm means. The diaphragm utilized with a translating device, such as a magnetic tape having a vibratable armature, provides a very high degree of acoustic compliance which enables the armature to be made thicker, and also permits the diaphragm to be connected to the armature other than at the PATENTEU APR 5 I97! SHEET 2 [IF 2 ACOUSTIC DIAPHRAGM AND TRANSLATING DEVICE UTILIZING SAME BACKGROUND OF THE INVENTION The diaphragm of this invention is particularly useful with a miniaturized trari'slatingdevice. More specifically, it may be employed in an electroacoustic transducer utilized in hearing aids. Such transducers, which are bilateral in operation, may be used as microphones to convert acoustic energy to an electrical signal or as receivers to convert electrical energy to acoustic energy.
Electroacoustic transducers are being miniaturized for use with electronic amplifiers of very small size, in hearing aids that may be small enough to fit well within the pinna of the ear of the user and to extend into a substantial portion of the external auditory meatus. As the size of the transducer is reduced, new structures must be created to obtain optimum performance in reduced size.
One element of particular concern is the diaphragm of the electroacoustic transducer. Such diaphragm must permit the associated transducer to be fabricated easily in very small size, and it must utilize the shape and size of such transducer to best advantage. For example, the diaphragm must be able to be attached to the freely vibrating portion of the active element of the transducer at an optimum position near the point of maximum excursion of the vibrating portion, with no detrimental effect on the size or performance of the transducer. I
The prior art has partially achieved such features in various ways. Thus in the series magnetic circuit configuration of translating device disclosed in the copending application of George C. Tibbetts, Ser. No. 168,]83 filed Jan. 23, 1962, and in the diaphragm construction thereof disclosed more fully in US. Pat. No. 3,166, I48 of George C. Tibbetts, the diaphragm means may comprise a-laminate sheet of plastic and metal, with part of the sheet defining a diaphragm portion, another part defining a compliant surround etched free of metal, and a third part defining a support area exterior of the surround. The support area is secured to a frame attached to the housing of the translating device, and the center of pressure of the diaphragm portion is connected by a drive pin to a portion of the armature near its free end. The center of acoustic pressure must coincide with the central drive point and thus it is not possible to have the diaphragm portion utilize the entire geometrical area available. As can be seen in FIG. 5 of either the patent or the copending application, the diaphragm is square (symmetrical about the drive point) and located near one end of the rectangular frame, with a portion of the frame occupying an area that could otherwise be utilized by the diaphragm portion if the center of acoustic pressure did not have to coincide with the drive point. By pivoting or hinging one end of the diaphragm and connecting the drive pin to the opposite end thereof as in the present invention, this restriction is eliminated.
SUMMARY OF THE INVENTION We have devised an improved diaphragm for use in electroacoustic translating devices. While the invention will be described in connection with a magnetic electromechanical transducer utilized in miniaturized hearing aids, the diaphragm of this invention is not of such limited application and may be equally useful in electroacoustic translating devices of other types and for other applications.
Accordingly we provide an acoustic diaphragm pivotally attached by a substantially perpendicular flexural pivot pin to a support member near one edge of the diaphragm, with a drive pin connected near the opposite edge of the diaphragm on the other side of and away from the center of pressure of the diaphragm. The drive pin leads to and is connected to the vibratable element of a translating device at one edge portion thereof, with the vibratable element extending substantially parallel to the diaphragm from the drive pin toward the pivotal attachment.
Such diaphragm permits the associated translating elements and overall electroacoustic translating device to be fabricated more easily and in very small sizes, including fabrication in elongate shapes of very small cross section. The diaphragm is useful with translating devices that utilize to best advantage the shape and size constraints on devices applicable in practice. The pivoted diaphragm eliminates a spurious mode of vibration, and enables greater utilization of the available geometrical area since the center of acoustic pressure does not have to coincide with the drive point. Such diaphragm when utilized with an end-driven translating device provides high acoustic compliance and, in receiver embodiments, an acoustic volume displacement equal to or better than that obtained with a conventional translating device with a diaphragm driven at its geometric center. Due to the nature of the present system, its resistance to damage by shock is better than that of the more conventional type of electroacoustic translating devices, and its accelerometer sensitivity is less.
These and further objects will become apparent from the accompanying specification and drawings.
BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a side sectional view of a general representation of the diaphragm of this invention;
FIG. 2 is another side sectional view similar to FIG. 1 with a drive pin in a different position;
CFIG. 3 is an exploded perspective view of a magnetic-type electroacoustic transducer utilizing a diaphragm according to the invention; and
FIG. 4 is a side view of a general representation of prior diaphragms showing a mode of vibration that may be undesirable when not constrained.
SPECIFIC DESCRIPTION OF THE PREFERRED EMBODIMENTS Referring to the drawings, FIG. I shows a general representation of a diaphragm means I comprising a diaphragm portion or reinforcement 2 and a flexible surround 3 connected to the periphery of the diaphragm reinforcement and to a supporting means shown generally at 4. Unlike diaphragms of the prior art, diaphragm portion 2 is pivotally attached near one edge to a supporting means 5 by any suitable means such as a hinge, but preferably is connected by a flexural pivot 6 to means 5. A drive pin or connecting pin 7 is attached to the diaphragm reinforcement on the other side of and away from the center of pressure of the diaphragm, which may be near one edge of the diaphragm reinforcement 2 as shown. In FIG. 2 the connecting pin is not as near to the edge of the diaphragm reinforcement. Connecting pin 7 leads to and is connected to the vibratable element 8 of a translating device which operates in flexure or torsion, preferably at or near one edge 'such as the end thereof. For best utilization of available space, the vibratable element extends substantially parallel to the diaphragm from the connecting pin 7 toward the pivotal attachment 6, and may be attached to a portion of supporting means 5 such as in the vicinity of flexural pivot 6.
Pivoting the diaphragm near one edge to a supporting means enables the diaphragm reinforcement to function substantially as a lever. In a microphone embodiment, sound waves drive the diaphragm portion over its entire area, and can be treated as a force b applied at the center of pressure of the diaphragm reinforcement at a distance a from the flexural pivot 6. Driving force Y can be taken off the diaphragm reinforcement at any desired distance x from the pivot 6, with the diaphragm reinforcement functioning substantially as a lever. The driving force available and the amplitude of movement vary linearly with change of distance from the pivot point. As the distance x increases the amplitude of movement increases and the driving force y decreases.
However, referring to FIG. 2, the stiffness s of the vibratable element 8 of the translating device is inversely proportionalgto the cube of t, where I is the distance from the point of attachment of element 8 at supporting means 5 along element 8 to connecting pin 7. As the drive point is shifted away from the point of attachment, the stiffness decreases at an ever-increasing nonlinear rate, much faster than the linear decrease of driving force y. At some point a net gain in acoustic compliance first appears. The greatest-gain results when the vibratable element 8 is driven at its extreme end. The above discussion is merely a generalized qualitative description of the effects of the construction shown in FIGS. 1 and 2, with losses ignored for purposes of simplicity.
lf a conventional acoustic transducer of magnetic type with drive at the midpoint of the armature (at a distance a from the fixed end of the armature) is compared to the present enddriven armature (with drive at a distance 20 from the fixed end of the armature), the mechanical compliance at the drive point of the end-driven armature would be approximately eight times the compliance at the drive point of the midpointdriven armature, for a given thickness of the armature, and in combination with the pivoted diaphragm would provide an acoustic compliance approximately twice that of the conventional configuration.
That acoustic compliance can be made equal to the acoustic compliance of the assumed conventional case by increasing the thickness of the armature by the cube root of 2, and therefore the diaphragm of this invention allows an armature thicker by this factor when designing for a specified acoustical compliance of the combination. In a magnetic transducer, the thicker armature provides increased flux-carrying capability. The end-driven configuration does not require an aperture through the armaturefor the drive pin, and thus the flux-carrying capability of the armature is also increased for that reason. The thicker armature has an increased stiffness, and a deflection that would be reduced except that the change of location of forces compensates for the otherwise reduced deflection and provides an amplitude of deflection at the end of the end-driven armature slightly more than twice that of the deflection at the midpoint of the conventional case. The pivoted diaphragm divides out the factor 2, leaving an estimate that the end-driven armature configuration produces, in a receiver embodiment, a slightly increased volume displacement over that provided by the conventional configuration. Because of the many variables indicated above, it is difficult to estimate the exact amount of increase of volume displacement.
The end-driven configuration has a clear advantage in its increased resistance to damage by mechanical shock, because the armature can be thicker as noted above. Furthermore, as stated above the end-driven configuration does not need an open aperture in the annature which acts as a stress concentrator, and the armature without an an aperture is thus more resistant to mechanical shock.
Because of its greater thickness, the mass of the armature in the end-driven configuration is greater than that in the midpoint-driven configuration. However, the increased mechanical stiffness more than compensates for this added mass, and therefore the accelerometer sensitivity will be less than that of the conventional configuration.
In the embodiment shown in FIG. 3, a diaphragm according to the invention may be utilized with a magnetic-type transducer such as that disclosed in copending application Ser. No. 638,878 of George C. Tibbetts et al. filed of even date. Preferably a diaphragm means of the type disclosed in US. Pat. 3,166,148 of George C. Tibbetts is employed, and may comprise a diaphragm portion 9, a diaphragm surround 10, and a self-frame 11 having a peripheral rim 12. The diaphragm portion 9 may have flats l3 and 14, and between the flats the diaphragm portion may be formed so as to increase greatly the flexural stiffness of the diaphragm portion. For example the form may correspond to a continuous trough generated by a portion of a sphere the projection of whose center follows a contour between flats 13 and 14. A frame 15 has a peripheral rim 16, an aperture defined by edge 17, and a formed boundary 18 adjacent the aperture. The diaphragm peripheral rim 12 fits within rim 16 and boundary 18 of the frame and is bonded and sealed together therewith by adhesive. Flexural pivot 19 connected to the magnetic translating device 24 extends up through aperture 20 in flat l3 and is attached to the diaphragm portion by an adhesive mass, not shown for the sake of clarity. Connecting pin 21 connected to the end 27 of the armature 26 extends up through aperture 22 in flat l3 and is similarly attached thereto by an adhesive mass not shown. The diaphragm is protected and a space above the diaphragm is provided by a cover 23 which is located by rim 16 of the frame 15. After adhesive is applied about the junction of the rim 16 with the sleeve 25 of the magnetic translating device, the cover is fitted in place, and the adhesive bonds the cover to the frame and to the sleeve and serves also to seal the frame to the sleeve. The frame 15 and cover 23 may be fabricated from magnetic material to provide magnetic shielding against stray flux traversing the apertures in the sleeve containing the flexural pivot 19 and connecting pin 21.
The remainder of the structure shown in FIG. 3 comprises the magnetic transducer 24 disclosed in said copending application of George C. Tibbetts et al., and includes a sleeve 25 of magnetic material, an armature 26 having an end 27 to which is attached connecting pin 21, a coil 28, a magnet stack 29, a
U-shaped closure member 30 at one end of the sleeve, a terv minal board 31 having terminals 32 thereon, and other components more fully disclosed in said copending application. The electroacoustic transducer of FIG. 3 may function as either a microphone or a receiver. In operation as a microphone, sound pressure may have access to the space between the diaphragm and cover by an aperture through the cover or by passage from the end of the transducer through appropriate conduits and chambers to said space. The diaphragm will be actuated by such sound pressure and will pivot about the end due to attachment to flexural pivot 19. The other end of the diaphragm will move connecting pin 21 which in turn actuates the armature 26. The armature can be thicker than in conventional transducers as noted above, and will have flux induced along its length by movement in the working gap of magnet stack 29,7 which flux will induce in E.M.F. in coil 28 led to the output terminals 32.
In prior structures such as the translating device shown in FIG. 2 of US. Pat. No. 3,154,172 of George C. Tibbetts, the connecting pin is attached to a portion of the armature inwardly of the end and extends up in a space between the coil and the magnet stack. That space is in addition to the space between the magnet stack and the end of the casing, necessary to prevent shorting the magnet stack. The connecting pin could not be of yoke form as in prior structure such as shown in FIG. 5 of US. Pat. No. 2,994,016 of Raymond W. Tibbetts et al., but due to the nature of the assembly had to be a straight pin extending through a hole in the armature. Such hole, acting as a stress concentrator, weakens the armature even though the armature is widened at that point. With the present invention the connecting pin may be attached to the end of the armature at the point of greatest compliance, with no hole through the armature, and no extra space is provided for the pin since it utilizes the space necessary to prevent shorting the magnet stack. Thus the coil and magnet stack can be compacted together. Furthermore, in manufacture this may be made a subassembly before insertion into the sleeve, simplifying the overall assembly process.
FIG. 4 shows a generalized form of the prior art diaphragm having a central drive pin. A flexible surround (not shown) substantially prevents translation of the diaphragm reinforcement 33 to the right or left of the figure, but provides very little restraint against rotation about the point of attachment to the connecting pin 34, as shown in dotted lines. During such rotation the connecting pin flexes, as also shown in dotted lines. In FIG. 4 the rotation is shown relative to an undeflected armature, illustrating that in prior art devices a hitherto unrecognized mode of vibration, i.e. an extra degree of freedom, exists in general. A mode of this type can absorb or reflect mechanical energy, and may and commonly does cause frequency response distortion within the passband of the transducer. The rotation of the diaphragm reinforcement is not undesirable in itself; in this invention the diaphragm operates in rotation about the pivotal point. What may be undesirable is the situation in which the diaphragm is free to rotate irrespective of the motion of the armature. In this invention, the rotation of the diaphragm is in one-to-one correspondence with the vertical deflection of the end of the armature, insofar as the diaphragm can be considered to be a rigid body. It follows'that the extra degree of freedom has been substantially removed.
Many modifications will be apparent to the artisan, in choice of materials or shape and location of elements, and the invention is applicable to circular, oval, elliptical or other shapes of diaphragms as well as to the square, rectangular, and racetrack shapes discussed above. For example a flat hinge in the plane of the diaphragm may be utilized instead of the flexural pivot pin, with the hinge taking the place of or comprising part of the surround. lf so-called mechanical advantage is desired, the pivot maybe near the edge portion neighboring the connecting pin, with the connecting pin spaced between the center of pressure of the diaphragm and the pivotal point, but still connected to the end of the translating element. Receiver embodiments may utilize configurations different from those used in corresponding microphone embodiments.
The invention is to be limited only by the scope of the following claims.
I. An acoustic diaphragm means for a translating device having a vibratable element, comprising a diaphragm portion and a surround therefor, means attached to the diaphragm portion away from the center of pressure thereof for connection to said vibratable element, and a flexural pivot pin near one edge of the diaphragm portion and extending substantially perpendicular therefrom for pivotal support thereof, said diaphragm portion being thereby restricted to rotary movement about said pivot pin.
2. An acoustic diaphragm means as in claim 1 wherein said connection means is on the other side of the center of pressure of the diaphragm portion away from the flexural pivot pin.
3. An acoustic diaphragm means as in claim 1 wherein the diaphragm portion is elongate, the flexural pivot pin is near one end of the diaphragm portion, and the connection means is near the other end of the diaphragm portion.
4. An acoustic transducer comprising a translating device having a vibratable element, diaphragm means comprising a diaphragm reinforcement and a surround therefor, means connecting said diaphragm reinforcement to said vibratable element away from the center of pressure of the diaphragm reinforcement, and a flexural pivot pin extending substantially perpendicular from, and pivotally attaching one edge of, said diaphragm reinforcement to said translating device, said diaphragm reinforcement being thereby restricted to rotary movement about said pivot pin.
5. An acoustic transducer as in claim 4 wherein said means connecting to the vibratable element is on the other side of the center of pressure of the diaphragm reinforcement away from the flexural pivot pin.
6. An acoustic transducer as in claim 5 wherein said vibratable element and said diaphragm reinforcement are each elongate in the same direction, said flexural pivot pin is near one end of said diaphragm reinforcement, and said connecting means is near the other end of the diaphragm reinforcement.
7. An acoustic transducer as in claim 6 wherein said translating device is electromagnetic and its vibratable element is an armature having a free end, and said connecting means is attached to the free end of the armature.
$22 3? UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No. Dated April 6,
Inventor(s) Joseph A. Sawyer and George C. Tibbetts It is certified that error appears in the above-identified patent and that said Letters Patent are hereby corrected as shown below:
In the Abstract, line 10, cancel "tape" and substitute --type-- 7 Column 4, line 40, after "29," cancel "7" Column 4, line 40, Cancel "in" and substitute --an-- Signed and sealed this 6th day of July 1 971 (SEAL) Attest:
LEE 1- .FLETCHER JR. WILLIAM E. SCHUY i ing Officer Commissioner of Pate