US3454912A - Transducer drive rod - Google Patents

Description

July 8, 1969 A. MORRISON TRANSDUCER DRIVE ROD Sheet Filed April 28, 1967 Tic}. 1. 22

N 0 mm W V 0 WM 4 J w July 8, 1969 A. MORRISON 1 TRANSDUCER DRIVE ROD Filed April 28, 1967 Sheet 5 of s I N VE NTOR. 100/3 4, Name/w y 1959 L. A. MORRISON 3,454,912

TRANSDUCER DRIVE ROD Filed April 28, 1967 Sheet 3 of s h M -H. H W V v\ I l FFfQUE/VCV //V CYCLES PEI? J'ECO/VD i E INVENTOR.

a v 100/: A Mam/J0 BY 4 m K 4 v ,4770XA/ 3,454,912 TRANSDUCER DRIVE ROD Louis A. Morrison, Madison, N.J., assignor to Roanwell Corporation, New York, N.Y., a corporation of New York Filed Apr. 28, 1967, Ser. No. 634,571 Int. Cl. H01f 7/08 US. Cl. 335231 2 Claims ABSTRACT OF THE DISCLOSURE BACKGROUND, OBJECTS AND SUMMARY OF THE INVENTION This invention relates to an electro-acoustic transducer and more particularly to a type of transducer known as the balanced armature type. Such a transducer is capable of serving as the active element in either a transmitter or a receiver.

The present invention is particularly concerned with the dynamic elements, that is, the armature-drive roddiaphragm combination in a balanced armature type of transducer and the invention constitutes an improvement in the drive rod design so as to obtain operating efficiency for the diaphragm in such a transducer.

Within the compass of the designation, balanced armature there happen to be a number of slightly different transducers. One of these is termed a reed armature and basically consists of an armature which is clamped at one end and is free to vibrate at the other end in a magnetic field. Typically this armature is produced in an E-shape although a U-shape may also be used. Considering the E-shape armature, the central or middle arm of the E is the vibratable arm. This reed armature type of transducer may be appreciated by reference to the patent to Carlson 3,111,563. In the transducer described in that patent the clamped end of the armature forms the spring which supplies the restoring force to the central vibratable arm when it is deflected from its rest position.

The other basic type of balanced armature transducer is one which is designated rocking armature, in which the armature is supported by a spring located, for example, at the middle of the armature. It is this type of balanced armature transducer which forms the context or ambience in which the concept of the present invention is most judiciously exploited. In order to furnish background material for a complete appreciation of the present invention, reference may be made to the patent to Blount 2,267,808 in which there is described a rocking armature type of electromagnetic transducer.

In the rocking armature transducer, both ends of the armature are free to vibrate in the magnetic fields and the spring which supplies the restoring force is not necessarily of the same material as the armature. Thus, comparing the two types, the rocking armature type (or 4-air gap structure) as described in the Blount patent, has the stiffness of its dynamic elements supplied by a spring material which has good elastic properties, while in the reed armature type as described in the Carlson patent, the spring stiffness is supplied by the armature itself, and

United States Patent hence is usually supplied by a material having poor elastic properties. In addition, the output of the rocking armature type is at least theoretically, 6 db greater than for the reed armature type.

It will be apparent from the preceding discussion that the rocking armature type of electromagnetic transducer is almost ideally suited for application to, for example, hearing aid receivers or other tiny head phone devices where extremely small sized elements having large acoustical outputs are required. Of course, this is not to say that balanced armature electromagnetic transducers are not also suited for micrpohone applications, that is, in the reversed sense of translating from an acoustical input to an electrical output. However, it can be appreciated that in a hearing aid application it is necessary to supply acoutical outputs which are equal to, or greater than, a threshhold of feeling, and this must be accomplished with a diaphragm having a very small area. In order to give some idea of the small sizes involved, one of the most Widely used, present day, miniature transducers fits into a case whose outside dimensions are X X .160".

In the miniaturization of balanced armature transducers, careful consideration is required of the basic parameters affecting the design. As is well-known, the output of an electro-acoustic transducer is a function of the force factor, the effective mass and stiffness of the system and effective area of the diaphragm. In general, stiffness is the controlling factor affecting output over the low frequency range up to the resonant frequency of the system.

An inherent advantage of the balanced armature electro-acoustic transducer is the fact that it is possible to maintain mechanical stability with lower system stiffness than is possible with other variable reluctance transducers. This decreased system stiffness permits the use of a diaphragm With low effective mass and low edge stiffness which, when incorporated with a stiff central portion increases the effective area of the diaphragm to a high percentage of its total area. All of these characteristics contribute to the high output, as noted previously, of the balanced armature transducer.

In order that the diaphragm in a balanced armature transducer will operate efficiently, With maximum effective area, it is necessary for it to move like a piston. Also, by virtue of its configuration, a balanced armature transducer requires a mechanical linkage, usually in the form of a drive rod, between the armature and the diaphragm. If a stiff drive rod is solidly attached at its ends to these elements the diaphragm will oscillate about its edges. To remedy this condition of oscillation, many manufacturers of balanced armature transducers use a stirrup drive rod. The stirrup drive rod is made from flat stock and one end is firmly attached to the diaphragm and the other end, which is made in the shape of a stirrup, engages the armature with a friction fit, thereby giving somewhat of a hinged effect. Such a stirrup rod arrangement can be appreciated by reference to the aforenoted Carlson patent 3,111,563.

The stirrup type of drive rod reduces the stiffness at the point of attachment to the armature and allows the diaphragm to move as a piston. However, this is an arrangement which introduces difficulties with respect to performance stability. These difficulties come about because of the somewhat critical adjustment of the armature in the air gaps, which is necessary to maintain maximum output. If the transducer is dropped in a direction which will result in a force which is large enough to overcome the friction fit, the drive rod can move along the armature and thus cause the armature to move out of its adjusted position. Also, from a production point of view this is a condition which is hard to control. Tools may be made which will insure the friction fit so as to minimize the unwanted movements of the drive rod after assembly. This is a condition, however, which will change with time as the tools become worn.

It has been recognized by me, and this constitutes the genesis of the present inventive concept, that the drive rod is the Achilles heel for the balanced armature transducer. Implementation of my inventive concept is directed to overcoming the severe problems attendant the driving of the diaphragm in a balanced armature transducer. My solution to these problems, in essence, provides a means of obtaining a high output without sacrificing the inherent advantages in a balanced armature transducer.

Accordingly, it is a primary object of the present invention to provide a superior drive rod arrangement in a balanced armature transducer so as to retain great mechanical stability for the system.

Mechanical stability could be improved by firmly attaching the drive rod to the armature rather than using the stirrup connection. This, however, as previously mentioned, will cause in the case of a thick or rigid drive rod, an unwarranted lateral motion of the diaphragm. With a thin drive rod which is quite short, the stiffness at the point of firm attachment to the armature is reduced and, of course, the armature will drive the diaphragm with the desired motion. However, for any reasonable length i.e. when the length of the drive rod is increased so as to be greater than approximately ten times its diameter, it will break up into a transverse mode of operation at certain frequencies which will cause disturbances in re sponse at these frequencies.

It is, therefore, another basic object of the present invention to provide a design which permits reasonable drive rod lengths while preventing disturbances due to breakup of the drive rod.

In fulfillment of the objects previously recited, it is a feature of the invention, broadly stated, that the drive rod is so designed as to be more substantial, to be greater in dimension, in its central portion and to be tapered down at the ends. In one form, the rod is quite thick and therefore rigid, at its center but is thin, and therefore flexible, at its ends, thereby to provide a hinge action.

More specifically, the low stiffness of the thin drive rod at its attachment points and the beneficial effect of the thick drive rod which reduces or eliminates unwanted resonances, are combined by flattening the ends of the drive rod and orienting its assembly in the direction of low stiffness. This calls for having the smaller dimension of the flattened end in line with the longitudinal axis of the armature. In effect, then, a more specific feature of the present invention resides in this precise configuration for the drive rod so as to gain, in a simple and efficacious manner, the previously recited advantages. However, this does not constitute the only approach to the desired end.

An ancillary feature, closely related to the design of the drive rod, is the design of the diaphragm so as to provide very low edge stiffness but to have an overall shape that will avoid certain undesirable effects. Such effects, for example, result from hand slap pressures to which hand set receivers are normally subjected. A spherical shape is provided for the diaphragm so as to result in an improvement in the stability to withstand these hand slap pressures.

When, however, the edge stiffness of the diaphragm is made quite low by the aforesaid provision for the diaphragm another problem is introduced into the design. This problem is caused by introducing the possibility of the diaphragm having two modes of vibration. One will be the desired piston-like, vertical motion, and the other will be a sideways transverse motion. At certain frequencies, the combination of these motions will cause disturbances in response. It is necessary, therefore, to

design the drive rod having the flattened ends in a way which will minimize any disturbances of this type which might occur Within the response-frequency pass band.

As has been pointed out hereinabove, the drive rod of the stirrup variety, which is now used in many balanced armature transducers, eliminates certain unwanted modes of diaphragm vibration. However, the price paid for the elimination in this manner results in a reduction in mechanical ruggedness which will appreciably reduce the effective life of the transducers.

Accordingly, it is a further object of the present invention to provide a drive rod design which substantially eliminates undesired diaphragm vibration but which enables an increase in ruggedness for the transducers.

The previously recited objectives are attained in a specific form by an arrangement of a tubular drive rod having its ends flattened.

It should be noted at this point that the design for the drive rod in accordance with the present invention will be referred to in the specific context of a receiver or earphone application, in which context the drive rod design has special advantages. However, it will be understood that, in general, the drive rod can be applied to a wide variety of transducers in which the recited advantages are desired.

The foregoing and other objects, features and advantages of the invention will be apparent from the following more particular description of preferred embodiments of the invention as illustrated in the accompanying drawings.

FIG. 1 is a side view in section of a balanced armature transducer illustrative of one embodiment of the present invention.

FIG. 2 is a sectional view taken on the line 2-2 in FIG. 1.

FIG. 3 is a fragmentary view from one end of the drive rod-armature-support assembly.

FIGS. 4a, 4b and 4c depict the several conditions that obtain when different drive rods are connected to a diaphragm. FIG. 40 in particular depicts a drive rod of the present invention.

FIG. 5 shows a number of response curves for balanced armature transducers using drive rods, according to the present invention, but varied in thickness.

Referring now to the drawings, and more particularly, to FIGS. 1-3 thereof, an electro-acoustic transducer is illustrated in its application as a receiver.

The transducer 10 comprises a basic frame member 12 to which the electromagnetic system 14 is affixed and supported. The electromagnetic system 14 is secured to the inner ring portion 12a, of the frame member 12 and the outer ring portion 12b of the member is supported by a cup-shaped plastic case 16. The diaphragm 18 is clamped at its periphery by means of a sealing ring 20 which abuts the periphery of the diaphragm to form a moisture-proof joint with the diaphragm. A cap or cover 22 fits over the top of the assembly and engages the peripheral edges of the diaphragm 18 and the ring 20', and also engages with a flange on the case 16. The cap 22 is in the form of a ferrule grid and is provided with suitable openings, one of which 22a is shown in FIG. 1.

The diaphragm 18 is a laminated diaphragm which consists of two layers 18a and 18b. The layer 18a is of aluminum and forms the diaphragm proper. The layer 18b is of Mylar and as seen in FIG. 1 this layer has a greater diameter so that at the periphery of the laminated diaphragm the support thereof is provided by clamping the layer 18b.

In order to prevent damage to the diaphragm by any sudden burst of energy, a blast stop 24 is provided adjacent the periphery of the diaphragm and is formed as part of the frame member 12. This, of course, limits the excursion of the diaphragm 18 within reasonable limits.

As is typical with the so-called balanced armature type of transducer, movement between the diaphragm and the armature is transmitted by way of a linkage in the form of a connecting pin or drive rod 26 which is afl'ixed at one end to the center of the diaphragm 18, being attached thereto by means of an epoxy cement or similar material. At its other end the drive rod 26 is soldered to the armature 28.

In the application illustrated in FIG. 1 of a receiver or earphone, the diaphragm 18 is driven by the electromagnetic system 14 which is contained within the cup 16. The electromagnetic system 14 includes a magnetic member in the form of a pair of substantially identical pole pieces 32 and 34 and a permanent magnet 36. Each of the pole pieces 32 and 34 includes a pair of pole tips or flanges 32a and 32b, and 34a, 3412 respectively. The pole pieces 32 and 34 are, of course, in contact with the permanent magnet means 36 so that a complete magnetic circuit is established. The pole pieces 32 and 34 are spaced in such a way that the pole tips 32a and 34a, and also 32b and 34b, are aligned and confront each other so as to form identical air gaps. The pole pieces 32 and 34 are securely clamped by suitable means against the permanent magnet 36. One means of so clamping would be by means of bolts fitted in the permanent magnet.

Extending between the pole tips is the magnetic armature 28 having its ends located in the air gaps between respective pairs of confronting pole tips. Thus, one end of the armature 28 is disposed between the pole tips 32a and 34a, while the other end is disposed between the pole tips 32b and 34b. In effect, then, four separate air gaps are defined. The armature 28 is pivotally mounted at its center in a spring support 40. Preferably the armature 28 passes through a slot 42 formed in the support 40 and is secured therein by means of solder. The support 40 is of a material which has good plastic properties and typically, is constituted of bronze material. The ends of the support 40 are fitted within slots 43 in brackets 44. The ends of the support 40 are clamped in the slots 43 by being embedded in an epoxy cement 45 contained within these slots. This technique of epoxy clamping of the ends forms-no part of the present invention. However, this feature is described in detail and is claimed in co-pending application Ser. No. 634,572 filed April 2 8, 1967, assigned to the assignee of the present application.

The brackets 44 are aflixed to the pole piece 32 by any suitable means. Preferably, these brackets are formed in integral fashion as part of the pole piece 32.

The drive rod 26 which extends down into the magnetic member is shown having its lower end secured to the armature 28 at the very end of the armature. However, it

will be appreciated that the location of the drive rod can be varied somewhat and that it is not essential to the concept of the present invention that the position illustrated in FIG. 1 be selected. Alternatively, for example, the end of the drive rod could be fastened somewhere along the length of the armature 28 and the same essential operation would be obtainable.

The armature 28 is entirely surrounded at one point by a coil 46 carried by an insulating form or bobbin. This coil may be held in place by the use of cement or the like. Here, again, although a single coil is depicted in the drawing, it will be understood that a plurality of coils can be provided to surround the armature. Conductors (not shown) establish electrical connection to the coil 46 and these conductors are taken out through a suitable opening such as the opening 50 provided at the bottom of the casing.

The design of the drive rod 26 is the fundamental feature of the present invention. This drive rod, in the embodiment depicted, is so designed that its center is relatively thick and therefore rigid while its ends are flattened to provide the requisite advantages. Thus, referring to the drawing, it will be noted that the drive rod 26 has a central portion of considerable diameter which may vary, for example, between a diameter of .020 and .010 inch.

6 The ends are flattened to have a thickness in the range of .010 to .004 inch.

Referring now to FIGS. 4a, 4b and 4c, the deficiencies that are present with drive rods of previous designs are depicted in the upper two FIGURES 4a and 4b, and the superior results obtainable with the drive rod configuration accor'dingto the present invention are shown in the lowermost FIGURE 40. The same reference numerals are used in connection with these figures as were used heretofore in explaining the layout of the receiver in FIGS. 1-3.

In --FIG. 4a a uniformly thick drive rod 26 is shown. The stiffness at the points of attachment of the drive rod to th e'armature '28 and the diaphragm 18 is very high. As a consequence, the diaphragm will oscillate as shown around the portion of its edge marked A. On the other half of the cycle the diaphragm will oscillate around the portion of its-edge marked B.

In contrast with the situation depicted in FLIG. 4a, an entirely different condition obtains in FIG. 4b in which a very thin drive rod is shown connected to the armature and diaphragm. The stiffness at the points of attachment to the armature and the diaphragm is low. As a consequence, were a short drive rod to be used, the diaphragm would move as a piston. However, when a reasonably long drive rod is actually used as shown in FIG. 4b, the long drive rod breaks up into other modes of vibration as depicted and causes the diaphragm to swing from side to side.

The extremely beneficial results that are obtained by the drive rod design of the present invention are shown in FIG. 40. Here, a thick drive rod is shown having its ends flattened. The drive rod is assembled to the attachment point with the flattened ends oriented as shown. The stiffness at these points will be low. The high stiffness of the drive rod between these attachment points will reduce the diaphragm break up in the frequency-response pass band and the diaphragm will move as a piston.

A number of drive rods having the same general configuration as depicted in FIG. 40 were tested. Data showing the effects of these drive rods with flattened ends are shown in RIG. 5. These data were taken with the receiver already described and shown in FIG. 1. The curves that are plotted in FIG. 5 show the relationship between the response in db and the frequency in cycles per second. Three sets of curves are shown in FIG. 5. The curve in each set which is shown dotted represents the earphone response without benefit of acoustic circuit, whereas the solid curve represents the earphone response with benefit of acoustic circuit. The upper set of curves were taken for a drive rod having an end thickness of approximately .009 inch. The middle and lower set of curves are for end thicknesses of .007 inch and .006 inch respectively. The particular drive rod for which the curves were taken was one made from seamless brass tubing having an outside diameter of 0.015 inch and an inside diameter of .009 inch. Thus, the wall thickness of the tubular rod is 0.003 inch.

The response of the earphone without acoustic circuits represented by the dotted lines shows the effects of the transverse motion of the diaphragm in the region of 2500 to 3000 c.p.s. for all of these drive rods. However, these data show that the magnitude of the disturbance decreases as the end thickness is decreased and that when the response with acoustic circuits is measured the disturbance is completely damped out in the case of the thinnest sample, i.e., .006 inch end thickness. Thus, in this particular sample the tubular rod ends were flattened until the opposed side walls were abutting against each other, that is, until the thickness of the rod was equal to twice the thickness of the tube side wall.

While there have been shown and described and pointed out the fundamental novel features of the invention as applied to the preferred embodiments, it will be understood that various omissions and substitutions and changes in the form and details of the device illustrated and in its operation may be made by those skilled in the art without departing from the spirit of the invention. It is the intention, therefore, to be limited only as indicated by the scope of the following claims.

What is claimed is: 1. A balanced armature transducer, comprising: (a) a circular diaphragm connected at its periphery to a fixed support having a central portion movable as a piston with respect to said support; (b) electromagnetic means for actuating the diaphragm (1) a magnetic circuit including a pair of air gaps; (2) electrical winding means linked to said magnetic circuits; (3) an armature supported in said air gaps and movable therein; and (c) means connecting one end of the armature to the center of the diaphragm for concurrent movement therewith; wherein the improvement comprises: (d)said connecting means consisting of an integral drive rod formed of tubular metal having both end portions only flattened with their short dimensions parallel to the longitudinal axis of said armaturew 2. A balanced armature transducer as defined in claim 1, wherein said drive rod has a tubular central portion, and the end portions at which the walls of the tube are 5 flattened against each other.

References Cited UNITED STATES PATENTS G. HARRIS, Primary Examiner.

US. Cl. X.R.

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