US2966558A

US2966558A - Transducer and vibratory diaphragm

Info

Publication number: US2966558A
Application number: US615406A
Authority: US
Inventors: Hugh S Knowles
Original assignee: Individual
Current assignee: Individual
Priority date: 1956-10-11
Filing date: 1956-10-11
Publication date: 1960-12-27
Anticipated expiration: 1977-12-27
Also published as: NL111663C; GB869234A; DK103946C; DE1081050B

Description

Dec. 27, 1960 H. s. KNOWLES 2,966,558

TRANSDUCER AND VIBRATORY DIAPHRAGM Filed Oct. 11, 1956 3 Sheets-Sheet 1 x/ i3 flLCii P1 18 6 g 10 4 u 14 19 i 2 22 20 E o -2 8 we 16' 3 i8 -s Q a k 28 F'RE'QUENCY (/f/LOCyCl-ES PER 5ECOND Dec. 27, 1960 H. s. KNOWLES 2,966,558

TRANSDUCER AND VIBRATORY DIAPHRAGM Filed Oct. 11, 1956 3 Sheets-Sheet 2 INVENTOR.

BYTA/U/Q Dec. 27, 1960 H. s. KNOWLES 2,966,558

TRANSDUCER AND VIBRATORY DIAPHRAGM Filed Oct. 11, 1956 5 Sheets-Sheet 3 .120 ;2Z I E r fi a I M INVENTOR. W fi zawzs United States Patent bud TRANSDUCER AND VIBRATORY DIAPHRAGM Hugh S. Knowles, Glen Ellyn, Ill. (9400 Belmont Ave., Franklin Park, Ill.)

Filed Oct. 11, 1956, Ser. No. 615,406

21 Claims. (Cl. 179-114) This invention relates toelectro-acoustic transducers, and particularly to miniature transducers for use as microphones in hearing aids. It also relates to the combination of the transducer and a hearing aid housing and to a new vibratory diaphragm which contains in its structure a Thuras or inertance tube. In this application, the term electro-acoustic transducer identifies a case providing a motor or back cavity, a motor in the cavity, a diaphragm, a drive connection between the motor and the diaphragm and a lid over the diaphragm to form a front cavity. A transducer is shown in Figures 4, and 6. In Figure 1 a transducer is schematically shown in a hearing aid housing 12 which contains besides the transducer 10, batteries, transistors, etc.

Applicants general object is to provide a miniature electro-accoustic transducer having the improved response in the lower frequencies provided by a Thuras tube while limiting the functioning of that tube to the transducer unit itself. Thuras taught in United States Letters Patent No. 1,869,178, dated July 26, 1932, that by venting the back cavity of an electro-accoustic transducer to the open air at a point adjacent to the diaphragm, the response of the diaphragm to low frequency sound, i.e., 50200 cycles, would be improved.

Hearing aid manufacturers, most of whom do not manufacture the delicate transducers, have followed Thuras teaching only in part, and this because Thuras taught that his tube must be vented to open air or at least to a large volume of air. While this teaching could be used in ordinary communication equipment, hearing aid manufacturers were obliged to place a transducer inside an attractive housing with the diaphragm concealed, and thereby they created a substantially closed outer cavity into which Thuras taught his tube could not be vented.

Thus, in Figure 1, applicant schematically shows a transducer unit 10 inside a sealed hearing aid housing 12, with a Thuras tube 14 venting the back cavity 16 into the chamber 18. The numeral 19 identifies the diaphragm and 20 is a rubber gasket positioned between the transducer 10 and the inside wall of the housing 12 having a small opening 21 for the entrance of sound. This assembly creates a front cavity 22, almost closed, into which the Thuras tube is not to be vented if the teaching of Thuras is to be followed. Hence the tube is located as shown, and the chamber 18 bears no relation to the sound receiving side of the diaphragm, i.e., to the cavity 22 or even in the outside space in front of the opening 21.

This construction produces, referring to Figure 2, the response indicated by the dot-dash line 26. While the response is god in the lower frequencies, it is followed by a low valley 15. This peak and valley result from resonance characteristics of the chamber 18, and from the fact that there is no air path between the motor cavity 16 and the sound side of the diaphragm. In this closed chamber 18 design, the hearing aid manufacturers can make the microphones uniform, but the poor response in the 450900 cycle range is unsatisfactory. 1

Still other manufacturers have provided ports or open- 2,966,558 Patented Dec. 27, 1960 ings such as 28, see Figure 3, in the chamber 18 which will allow the sound vibrations to escape from the chamber 18 and reach the front side of the diaphragm. Commonly, this passageway 28 is adjacent or around the volume control of the hearing aid. This construction may produce a response such as that indicated by the dotted graph 30 in Figure 2. This design is superior to that of Figure 1. It has, however, several drawbacks. Firstly, tests on a run of these hearing aid microphones show a lack of uniformity in response, indicating that it is difiicult to assemble uniformly the cases inside the hearing aid housings. Also, the air through the opening 28 has such a long and uncertain path in order to reach the face of the diaphragm that the benefit of the Thuras tube is largely lost. Again, the ports 28 provide a path for acoustic feedback at other frequencies and permit dust to reach the back cavity of the transducer.

It should be mentioned that leaks between improperly assembled mating parts of a hearing aid housing produce an effect similar to that obtained by the deliberately positioned port 28.

Also, in order to protect the diaphragm, transducer manufacturers have placed a cover with small center hole over the diaphragm, thereby creating a substantially closed front cavity, as illustrated in Figure 4 where the cover bears numeral 36. This entire assembly is sealed and the hearing aid manufacturer simply assembles it in a housing such as 12 in Figure 1 with the sound opening 58 of the transducer in alignment with or close to the opening 21 in the housing 12.

The first specific object of this invention is to connect a Thuras tube directly between the closed back cavity 16 and the semi-closed front cavity 22, see Figure l, of a hearing aid microphone, and particularly between the back cavity 44 and the front cavity 54 of Figure 4. This would make the electro-acoustic transducer a completely self-sufficient unit, affected by air only within its walls and capable of final testing at the transducer plant Where there are equipment and workmen capable of adjusting all transducers to a uniform response. Applicant set out to re-examine Thuras fundamental principle that has tube should be vented into an open space on the sound receiving side of the diaphragm, and found that a Thuras tube would function between a closed back cavity and a substantially closed front cavity, i.e., having an opening or openings whose total area is much less than the area of the diaphragm.

The next specific object of this invention is to incorporate the Thuras tube inside a standard hearing aid microphone case. Manufacturers of these electro-acoustic transducers house the internal working parts in small metallic cases that may be a cylinder or a right parallelpiped. These cases may have dimensions of the order of 5 x A x /2 inch. Two dimensions of the case are determined by the two major dimensions of the diaphragm and the other dimension of the case is determined by the thickness of the transducer motor. Applicant wishes to incorporate a Thuras tube of requisite volume within the planes of the outer surfaces of the diaphragm and motor as projected to form a regular case such as a closed cylinder or right parallelpiped.

In attaining this object, the first feature is the positioning of the Thuras tube through the vibrating portion of the diaphragm itself, i.e., inside the clamped configuration, with the tube in the vacant space usually available in the back cavity. In attaining this feature, applicant positions the tube at a point through the diaphragm which does not participate appreciably in the vibratory movements.

.The second general object of this invention is to mount a Thuras tube or a pluralityof Thuras tubes on the vibratory portion of the diaphragm itself. In all applications, space limitations are critical, and the volume of the Thuras tube and the relationship of its diameter to its length are dependent upon the relative sizes of the front and back cavities. There are several embodiments of these Thuras tube diaphragms. In each, it is essential that the existence of the Thuras tube on the diaphragm not interfere with the proper functioning of the diaphragm, and it is therefore dmirable that the Thuras tube or tubes be balanced with respect to the center of vibration of the diaphragm.

In a preferred embodiment, the Thuras tube is incorporated in the drive rod that connects the center of the diaphragm to the armature of the motor. in another embodiment, four short Thuras tubes are mounted 90 apart around the center of the diaphragm and substantially at right angles to its surface. In this embodiment, each Thuras tube projects above one or both surfaces of the diaphragm, and this requires cavities of suitable depth.

Several embodiments are based upon the Thuras tube lying in the plane of the diaphragm. This makes possible arcuate and spiral tubes of great length and small cross section. In one of these embodiments, tubes are mounted on the face of the diaphragm. A preferred construction employs a double layer diaphragm wherein the Thuras tubes are formed byfacing grooves in the two components. In one embodiment, the grooves are radial. In another embodiment, the grooves are spiral. The two spiral Thuras tubes are of great length and small cross section, thereby permitting the use of extremely shallow cavities on both sides of the diaphragm.

These, and such other objects and features as may hereinafter appear, are attained in the embodiments of the invention shown in the accompanying drawings, wherein: The relative depth of the grooves in the diaphragm components is so related to the convex side of the diaphragm that that portion of the Thuras tube near the diaphragms center lies predominantly on the concave side of the diaphragm, whereas that portion near clamped periphery lies predominantly on the convex side, thereby utilizing the available space most advantageously.

There are applications where it is not practicable to locate the Thuras tube on the diaphragm so that the diaphragm is balanced around its vibrational center. It is a feature of this invention.

Figure l is a schematic cross section of an electroacoustic transducer mounted within a sealed chamber;

Figure 2 is a graph showing the response of applicants directly connected Thuras tube compared with the response of Thuras tubes opening into a sealed chamber and into an unsealed chamber;

Figure 3 is a cross-sectional, schematic illustration of an electro-acoustic transducer mounted in an unsealed chamber of a hearing aid;

Figure 4 is a longitudinal section through an electroacoustic transducer disposed in its case with a fixed 'Iburas tube;

Figure 5 is a View taken on the iine 5-5 of Figure 4;

but partly cut away;

Figure 7 is a perspective view of a diaphragm and a tubular drive rod which constitutes the Thuras tube, a preferred form of applicants invention; a

Figure 8 is a sectional view taken on the line S@ of Figure 7;

Figure 9 is a sectional view of a modified form of the embodiment of Figures 7 and 8, wherein the opening through the cover is out of alignment with the tubular drive rod;

Figure 10 is a top view of. the portion of the case and diaphragm of Figure 9;

Figure 11 is a perspective view of a circular diaphra .n with four Thuras tubes mounted therein;

Figure 12 is a view taken on the line 32ll2 of Figure l1;

Figure 13 is a perspective view of a rectangular diaphragm, such as that in the embodiment shown in Figures 46, but composed of two sheets of vibratable material with the Thuras tubes in arcuate curves between them;

Figure 14 is a View taken on the line 14-14 of Figure 13;

Figure 15 is a plan view of a circular diaphragm showing two balanced spiral Thuras tubes on a circular diaphragm;

Figure 16 is a perspective view of a rectangular diaphragm having four radiating Thuras tubes; and

Figure 17 is a sectional view taken on the line 1717 of Figure 16.

Continuing to refer to the drawings, particularly Figures 4, 5 and 6, the numeral 32 identifies the case of a microphone over the open face of which is first positioned a bulkhead 34 carrying a diaphragm 46, and then a lid 36. The bulkhead 34 serves as the base for all operating parts of the transducer, and supports on its iower side a motor 38 and a Thuras tube 41 The bulkhead 34 has a reverse flange 42 around its perimeter and forms a cavity 44. Sealed to the outer face of the peripheral flange 42 is the diaphragm 46, t0 the vibratory center of which is anchored the armature drive rod 48. The drive rod 43 passes through an opening St) in the bulkhead 3d and drives an armature t? This opening constitutes the air communication between the back cavity 52 and that portion of the back cavity between the bulkhead 34 and the diaphragm 46. The recess 44 is the important part of the back cavity, but the back cavity includes the large space 52 which houses the motor because of the passageway 50 in the bulkhead 34. The front cavity is formed by the peripheral reverse flange 55 on the lid 36. The sound opening is centrally positioned at 58.

Referring to Figure 5, the diaphragm 46 carries a lozenge-shaped channel 47 which substantially confines vibrations within its boundaries, leaving the corner areas such as 43 comparatively rigid and therefore usable for the port to the edges of which the Thuras tube 40 may be sealed. The channel 47 is formed by two reverse bends. A single bend or parallel grooves may be used. In the embodiment shown in these figures, the corner area 55 of the bulkhead 34 is raised to the plane of the outer surface of the flange and the diaphragm lies flushly upon it.

For purposes of this invention, the motor need not be described. It has been shown in sufiicient detail to present clearly a vibratable armature and drive rod because, as will appear hereinafter, a hollow drive rod may be used as the Thuras tube.

Means for computing the diameter and length of the T huras tube 40 will not be set forth as these formulas are well known to those in the art and diflicult to apply to a particular structure. They do not affect the concept of positioning a Thuras tube between a closed cavity and one that is almost closed, particularly in a microphone. Moreover, the diameter and length of the tube, its position in the back cavity, and the position of its outlet in that cavity, are usually determined as a result of thoughful experimenting. To determine the Thuras tube specifications by formula, every obstacle presented by the motor to air movement and resonance of the cavities, as well as the weight and physical characteristics of the drive rod and armature, must be given some value. The selection of tube specifications by experimenting with tubes of different lengths and diameters, and testing to produce graphs such as those in Figure 2, is the practical procedure.

Applicants microphone operates as follows: The sound enters the port 58 in the lid 36 and vibrates the diaphragm 46. The diaphragm drives the armature 49 which varies a sustained magnetic field in which is positioned a coil and thereby generates a current in the conductors 51 and 53.

At frequencies below point a in Figure 2 the inertia of the air in the Thuras tube is so low that the difference in pressure on the two sides of the diaphragm is less than it would be in the absence of the tube because relatively rapid pressure equalization occurs through the tube. At frequencies above b the inertia of the air in the tube is so high that there is negligible displacement of the air through the tube and the pressure difference between the diaphragm sides is substantially what would exist without the tube. Between frequencies a and b the inertia of the tube air operating in combination with the stiflness of the air in the rear cavity (and to a very small extent the front cavity, if any) results in a time lag in the tube air displacement such that the pressure on the rear of the diaphragm is less and on the front of the diaphragm is greater than it would be without the tube.

Thuras tubes mounted on the vibratory portion of the diaphragm While applicants basic idea is shown in

Figrues

4, 5 and 6, where the Thuras tube connects a substantially closed front cavity with a closed rear cavity, most of the Thuras tubes shown herein are in fact mounted on the vibratory portion of the diaphragm. These are set forth in Figures 7 through 17. In these embodiments of the invention, only the diaphragm with the mounted Thuras tube is shown in the drawings and described indetail. In each instance, however, one is to consider the specific diaphragm being described as substituted for the diaphragm 46 in Figures 4, 5 and 6 so that its operation will be considered with the semi-closed front cavity 54. Indeed, all of the embodiments about to be described should be considered where the lid 36 is not used but an equivalent result is obtained by mounting the transducer case against the inside wall of the hearing aid assembly so as to produce a substantially closed front cavity which is the equivalent of cavity 54 in Figure 4.

A Thuras tube as the drive rod Referring for the moment to Figure 4, the drive rod 48 connects rigidly the center of vibration of the diaphragm 46 and a vibratable portion of the armature 49. The armature-drive rod-diaphragm assembly may be widely varied and does not affect the Thuras tube construction. In Figures 7 and 8, applicant utilizes a Thuras tube as a drive rod. The circular diaphragm is of the type that would be used in association with the electro-acoustic transducer shown in copending application Serial No. 436,416, filed June 14, 1954.

Continuing to refer to Figures 7 and 8, 70 is a circular diaphragm, intended to be fastened and sealed at its peripheral edge to a fixed member which will form a closed cavity similar to 44 and 52 in Figure 4, and with a lid a semi-closed cavity, such as 54 in Figure 4. A tube 72, having a flared end 75, is fastened either as shown in Figure 8, or directly to the under side of the diaphragm 78. To the bottom of the tube is affixed a yoke 74 having a transverse channel into which may be slipped the armature 62.

Returning to Figure 4, the point where the yoke 74 of Figures 7 and 8 would be attached to the drive rod 72 is at about 76. This determines the length of the Thuras tube. Approximately one-half of the entire thickness of the transducer can be utilized as a Thuras tube. The Thuras tube of Figures 7 and 8 vents directly into the opening 58 from outside the transducer. Alternatively this opening may be at a point remote from the drive rod, or the opening 58 of Figure 4 may be moved to a point remote from the center of the diameter. Where the applicant wishes to avoid venting the Thuras tube into the sound opening 58, a construction such as that shown in Figures 9 and 10 may be used. Here, the upper end of the tube '72 has, been greatly expanded and fastened to the lower surface of the diaphragm 76. A circle of holes 78 has been cut through an otherwise solid diaphragm. In either case, applicant is able to utilize space for the Thuras tube which presently exists and can easily be enlarged without enlarging the outer dimensions of the case.

Thuras tubes at right angles to the general plane of the diaphragm Another embodiment of the invention showing T huras tubes at right angles to the surface of the diaphragm is that presented in Figures 11 and 12. Here, the diaphragm with depressed channel 82, carries four

Thuras tubes

84, 86, 88 and 90, exactly spaced and centered around the diaphragm, the entire assembly being carefully balanced. In this construction, the Thuras tubes require deepening of the front and back cavities. A Thuras tube must contain a definite amount of air with respect to its diameter. A mere opening through the diaphragm will not function as a Thuras tube. The structures shown in Figures 11 and 12, therefore, are useful in those applications where the thickness of the case is not important.

Thuras tubes lying in the plane of the diaphragm In the remaining five figures, Figures l3l7, applicant shows three diaphragms wherein the Thuras tubes lie in the plane of the diaphragms. A preferred embodiment is shown in Figures 13 and 14. Here, the diaphragm is composed of two

metallic sheets

91 and 92. A lozenge-shaped channel 94 in sheet 91 tends to limit vibrations of the diaphragm to points within itself. The sheet 91 alone is clamped around its periphery to the wall forming one or both of the cavities. The sheet 92 is substantially stiffer than sheet 91.

Sheets

91 and 92 have formed in them complementary,

arcuate grooves

96, 97, 98 and 99. These grooves are exactly related to each other about the vibratory center 180. The

grooves

96 and 97 open into the upper side of the diaphragm at 181 and to the lower side at 102; and the

grooves

98 and 99 open into the upper side of the diaphragm at 103 and to the lower side at 104. These two grooves can be lengthened so as to have a length of substantially of arc. The passageway instead of being oval shaped as shown in Figure 14 may be substantially rectangular in shape. Importantly, the grooves may be in one sheet only, preferably the heavier sheet 92.

Referring to Figure 15, applicant shows the plan view of a circular diaphragm in which there are two spiral Thuras tubes 186 and 108. The tube 106 opens to the upper part of the diaphragm 105 at 118 and to the lower side at 112, while tube 108 opens to the upper side at 114 and to the lower side at 116. These tubes could be lengthened by reducing their width.

In Figures 16 and 17, applicant shows radially directed

Thuras tubes

120, 122, 124 and 126. These tubes may open directly into the channel 128. They may be useful in applications where it is desirable to introduce the air into the front cavity at points near its periphery, while introducing the air to the other cavity at points near its center, as for example, near the drive rod which passes through a small opening in the bulkhead between the shallow cavity behind the diaphragm and the motor cavity.

While the Thuras tubes lying parallel to the diaphragm have been shown as formed between a two-wall diaphragm, separate tubes mounted on top or on the bottom of the diaphragm may be employed.

With respect to the diaphragms, their peripheral configuration is not important sofar as this invention is concerned. Ordinarily, the diaphragm is clamped at its periphery between two holding members. In applicants construction of Figures 4-6, it is clamped between the reverse flange 56 of the lid 36 and the reverse flange 42 of the bulkhead 34, although in assembling, it is sealed by adhesive to the reverse flange 42 before the lid 36 is placed on the unit. Additionally, the lids are sealed on the units so that the hearing aid manufacturer does not open the unit.

The configuration of the vibratory portion of the diaphragm is not important. Thus, a diaphragm having a general rectangular area such as that shown in Figure 13 may have a circular bend to limit vibration to the area within the circle, and the Thuras tubes could be mounted as shown in Figure or Figure 7. The area outside the bend consitutes a sort of fixed partition and is useful for mounting purposes.

Referring to Figure 4, the sound opening 58 of the microphone is concentric with the axis 59 which is normal to the vibratable center of the diaphragm and is the axis of the drive rod 48. Better response is obtained if the sound vibrations received from the outside are evenly distributed around the vibratable center of the diaphragm, but hearing aid manufacturers do not always directly present the opening 58 to the outside air and hearing aid users often position the microphone under a coat or lapel so that the sound vibrations reaching the opening 53 are not evenly distributed across it. Consequently, it should be understood that this opening can be moved off center, i.e., ofi the center of the diaphragm and still obtain a very satisfactory result.

Venting the opening of the Thuras tube directly into the sound opening 58 of the lid 36 may also be undesirable. Applicants test indicates firstly, that the Thuras tube should be vented through the diaphragm at points out of alignment with the sound opening of the case, and secondly, that the venting should take place under such circumstances that it is distributed over the diaphragm, as is accomplished by the structures shown in Figures 9 and 10.

The presence of structural obstacles in the motor cavity is not of theoretical importance. Referring to Figure 4, the principal obstacle for air movements is the bulkhead 34 which is shown with just one opening 50. Where the Thuras tube or tubes are mounted on the diaphragm, for all practical purposes, the main motor cavity 52 plays no part, and the opening 50 need only be suflicient to pass the drive rod 48. Where the drive rod Thuras tube such as 72 in Figure 8 is used, the opening 50 is necessary, but its proximity to the lower opening of the drive rod tube is helpful.

Summary .All of the Thuras tubes described open into a small front cavity. The theory of the Thuras tube is that in an intermediate range of frequencies, the air in the tube tends to move in or out of the tube and thereby to reinforce a sound impulse. It is believed that the confined cavity itself acts as a sort of extension of the Thuras tube so as to slightly change the volume in the cavity and distribute the slightly altered pressure over the entire face of the diaphragm. Using the drive pin as a Thuras tube is particularly desirable because it delivers the air at the vibratory center of the diaphragm. The graph in Figure 1 indicates the actual frequencies in which the Thuras tube is useful for microphones for hearing aids.

It will also be appreciated that where the Thuras tube is mounted on the diaphragm, problems of attaining exact symmetry so as to secure dynamic balance become great. In some instances, this balance Will be obtained by adding weight to a point on the diaphragm which will attain such balance.

On page 16 of the 1943 edition of Dynamical Analogies, D. Van Nostrand Company, inc, New York, New York, Harry Ferdinand Olson defines inertance as follows:

Inertance may be expressed as ,1, 'm LiI" Z where m=mass, in grams s=cross sectional area in square centimeters, over which the driving pressure acts to drive the mass. He gives the formula for a circular inertance tube as follows:

The inertance of a circular tube is 8 where R=radius of the tube, in centimeters, l=efiective length of the tube, that is, length plus end correction, incentimeters, and p=density of the medium in the tube, in grams per cubic centimeter.

Having thus described his invention, what applicant claims is:

1. A transducer comprising a case, a motor cavity within said case, a diaphragm forming one Wall of said motor cavity, a motor positioned in the motor cavity, drive means connected from said motor to said diaphragm, and an inertance tube carried by said diaphragm and having one end opening into that side of the diaphragm adjacent the motor cavity and the other end opening on the other side of the diaphragm.

2. A transducer comprising a case, a diaphragm mounted in the case and forming a closed motor cavity and a shallow front cavity, a small opening through the Wall of the case into the front cavity, a motor mounted in the closed motor cavity, drive means connected from said motor to said diaphragm, and an inertance tube carried by the diaphragm and having one end opening into that side of the diaphragm adjacent the motor cavity and the other end opening on the other side of the diaphragm.

3. An electro-acoustic transducer comprising a case, a diaphragm in said case separating a closed cavity from a semi-closed cavity, a motor in said closed cavity, a drive means connected from said motor to said diaphragm, said diaphragm and drive means constituting a movable assembly, and an inertance tube carried by the movable assembly with one end opening on one side of the diaphragm and the other end opening on theother side.

4. An electro-acoustic transducer comprising a case, a diaphragm in said case separating a closed cavit from a semi-closed cavity, a motor in said closed cavity, a vibratable armature in said motor, a drive means connecting a vibratable portion of the armature to the diaphragm, said diaphragm, drive means and armature con stituting a movable assembly, and an inertance tube carried by the movable assembly with one end opening on one side of the diaphragm and the other end opening on the other side.

5. An electro-acoustic transducer comprising a receptacle, a diaphragm mounted in said receptacle to form a sealed cavity therein, a motor and drive means connecting the motor to the diaphragm mounted in said cavity, and an inertance tube carried by the diaphragm and lying in a plane substantially parallel to that of the diaphragm, one end of the tube opening on one side of the diaphragm and the other end opening on the other side.

6. An electro-acoustic transducer comprising an open sided case, a diaphragm disposed across the open side so as to form a fluid tight back cavity, a motor having vibratable armature mounted in the back cavity, and a tube having one end connected to the armature and the other end to the diaphragm, the armature end of the tube opening into the back cavity and the other end of the tube opening into the outer side of the diaphragm.

7. The transducer of claim 6 in which the case is a closed case so that the diaphragm forms both a back cavity and a front cavity and in which there is a small opening through the wall of the case into the front cavity.

8. The transducer of claim 7 in which the opening from the tube into the front cavity is spaced substantially from the opening through the case into the front cavity.

9. The electro-acoustic transducer of claim 8 in which the diaphragm has a peripheral bend extending continuously around the vibratable center and close to the periphery of the diaphragm itself, and in which the opening through the diaphragm into the front cavity is outside this bend.

10. An electro-acoustic transducer comprising a receptacle, a diaphragm mounted in said receptacle to form a closed cavity therein, a motor in said cavity, and a drive pin connecting the motor to the diaphragm, said diaphragm comprising a sheet of vibratory material having a vibratory center, an inertance tube carried by said diaphragm concentrically with the axis through said center and normal to the surface of the diaphragm and extending into the motor cavity, the end of the tube adjacent to the diaphragm opening solely onto the far side of the diaphragm, and the other end of the tube into the motor cavity.

11. An electro-acoustic transducer comprising a case consisting of a bottom and side wall, a bulkhead disposed over the open side, an electro-acoustic motor including a vibratable armature mounted on the inner side of the bulkhead, a reverse peripheral flange on the bulkhead extending outwardly in a direction parallel to the side wall so as to provide a shallow outwardly directed recess, a diaphragm sealed to the edges of said flange and having a vibratable center, there being an opening through the bulkhead in normal alignment with the vibratable center of the diaphragm, a drive rod having one end fastened to the vibratable center of the diaphragm and the other end to a vibratable portion of the armature, a fiat lid seatable over the edge of the bulkhead to provide a shallow front cavity, a small opening through the lid adjacent the diaphragm, and an inertance tube carried by the diaphragm and having the greater portion of its length substantially parallel to the plane of the diaphragm with one end opening into the front cavity and the other end opening into the back cavity.

12. An electro-acoustic transducer comprising a receptacle, a diaphragm mounted in said receptacle to form a closed cavity therein, a motor in said cavity, and a drive pin connecting the motor to the diaphragm, said diaphragm comprising a sheet of vibratory material having a vibratory center, a plurality of open-ended, inertance tubes mounted through the sheet at an angle thereto and so spaced around the vibratory center as to maintain dynamic balance of the diaphragm.

13. An electro-acoustic transducer comprising a receptacle to form a closed cavity therein, a motor in said cavity, and a drive pin connecting the motor to the diaphragm, said diaphragm comprising a sheet of vibratory material, and an inertance tube having the greater portion of its length lying substantially parallel to the plane of said sheet and carried thereby, one end of the tube opening to one side of the sheet and the other end of the tube opening to the other side of the sheet.

14. An electro-acoustic transducer comprising a receptacle, a diaphragm mounted in said receptacle to form a closed cavity therein, a motor in said cavity, and a drive pin connecting the motor to the diaphragm, said diaphragm comprising two sheets of vibratory material in face-to-face relationship, a groove on the inner face of one sheet forming an elongated inertance tube therein, there being an opening into one end of said tube through one sheet and into the other end of the tube through the other sheet, said groove being in dynamic balance with the vibratory center of the diaphragm.

15. An electro-acoustic transducer comprising a receptacle, a diaphragm mounted in said receptacle, to form a closed cavity therein, a motor in said cavity, and a drive pin connecting the motor to the diaphragm, said diaphragm comprising two sheets of vibratory material in face-to-face relationship, a pair of grooves on the inner surface of one sheet, each forming an elongated, inertance tube, said grooves being in dynamic balance with respect 10 to each other and the center of the diaphragm, there being an opening into one end of each groove to one side of the diaphragm and an opening into the other end of each groove to the other side of the diaphragm.

16. An electro-acoustic transducer comprising a recep tacle, a diaphragm mounted in said receptacle to form a closed cavity therein, a motor in said cavity, and a drive pin connecting the motor to the diaphragm, said dia' phragm comprising a sheet of vibratory material having a vibratory center, a second sheet of heavier material of like but smaller configuration in face-to-face relationship with the first sheet, a groove in the heavier sheet forming an inertance tube between itself and the lighter sheet, the configuration of the groove being such that the walls are in dynamic balance with respect to themselves and the vibratory center of the diaphragm, one end of the tube be ing open through the heavy sheet and the other end being open through the light sheet.

17. The electro-acoustic transducer of claim 15 wherein the grooves are arcuate around the center of the diaphragm.

18. The electro-acoustic transducer of claim 15 wherein the grooves are spirals around the center commencing at two outer points apart and terminating at two points closer to the center, the corresponding ends of each of the grooves being open through one sheet of material and the other ends being open through the other sheet of material.

19. An electro-acoustic transducer comprising a receptacle, a diaphragm mounted in said receptacle to form a closed cavity therein, a motor in said cavity, and a drive means connecting the motor to the diaphragm, said diaphragm comprising a sheet of vibratory material in faceto-face relationship with a second sheet, an inertance tube formed on the inside face of one sheet, an opening into said tube through the sheet wherein it is formed, and an opening into said tube through the other sheet whereby the tube opens onto opposite sides of the diaphragm.

20. An electro-acoustic transducer comprising a case, a front cavity and a back cavity therein, a motor in the back cavity, a diaphragm positioned between the two cavities, and a drive pin connecting the motor to the diaphragm, said diaphragm comprising a sheet of vibratory material having a vibratory center, a plurality of inertance tubes the greater portion of their length lying substantially parallel to the plane or said sheet and mounted thereon, said tubes extending radially from the center in dynamic balance with each other and the center, an end of each tube opening to one side of the diaphragm and the other end of each tube opening to the other side of the diaphragm.

21. The electro-acoustic transducer of claim 20 wherein the ends of the tubes nearest the vibratory center of the diaphragm all open on one side of the diaphragm and the other ends all open on the other side of the diaphragm.

References Cited in the file of this patent UNITED STATES PATENTS 1,366,607 Steinberger Ian. 25, 1921 1,847,702 Thuras Mar. 1, 1932 1,869,178 Thuras July 26, 1932 2,400,281 Anderson May 16, 1946 2,503,857 Warnke Apr. 11, 1950 2,789,161 Brennan Apr. 16, 1957