US2329560A - Acoustic device - Google Patents

Description

Sept. 14, 1943. .1. D. SEABERT ACOUSTIC DEVICE Filed Jan. 51, 1942 Patented Sept. 14, 1943 ACOUSTIC DEVICE John D. Seabert, Indianapolis, Ind., assignor to Radio Corporation oi America, a corporation of Delaware Application January 31, 1942, Serial No. 429,020

6 Claims.

My invention relates to acoustic devices and more particularly to loudspeakers employing large directacting diaphragms. An object of my invention is to provide a loudspeaker having smaller than usual dimensions, that will faithfully and efiiciently reproduce all useful sound waves within the frequency range of speech and music.

More particularly, it is an object of my invention -to provide a loudspeaker with a cone diaphragm having a maximum diameter and a flexible base support rim of greatly improved flexibility without increasing the size of the speaker housing or the size of the baille opening beyond that of the maximum cone diameter.

Heretofore, it has been common practice to suspend a cone diaphragm, at its base, from a speaker housing, or dishpan support, by means of an annular, corrugated centering disc that extends outwardly a substantial distance from the base of the cone into the rim of the housing, or in some cases to the edge of the opening in a baffle. In these prior constructions, considerable valuable space has been Wasted as a result of the space requirements necessary to house the base suspension. For example, a cone diaphragm with a 6 inch base required an 8 inch housing.

According to my invention it is possible to provide a practical speaker having substantially smaller overall dimensions than heretofore, and at the same time improved performance. On the other hand, applying the principles of my invention, it is possible to provide a speaker with greatly improved performance for the same overall dimensions as heretofore.

Further, in accordance with my invention, I have provided a loudspeaker characterized by natural resonance of very low frequency, as of the order of to &0 cycles, using an inwardly extending base suspension that is of the usual annular corrugated paper construction and which is adapted to be supported in a simplified and inexpensive manner and to function as an effective acoustic seal.

More specifically in accordance With my invention, I have provided a highly flexible support for the base of a cone diaphragm wherein the cone base suspension extends radially inwardly from the base of the cone and at the same time seals the cone base against leakage of sound pressure around the edge of the cone between the cone and the baffle without interfering with the useful radiation of sound Waves. Heretofore, the outwardly extending corrugated annular centering discs have been restricted in size because of the necessity of minimizing the size of the cone housing. There is considerable space inwardly on the cone base that is available for a centering disc of a size sufficiently great in width to proivde the flexibility desired, and I have provided means to utilize this space without introducing other disadvantages. I am aware that attempts have been made heretofore to utilize the space inside the cone periphery, possibly for the purpose of increasing flexibility, but such attempts have introduced other disadvantages or complications, One of the problems not heretofore solved is that of providing an effective acoustic seal around the periphery of the cone diaphragm. As an example of prior attempts, reference is made to the following: British Patent 294,285 of 1928, Jensen 2,014,621. It will be observed that, in these prior attempts,

flexible spiders were attached to the cone diaphragm and supported centrally by a rod anchored to the central pole piece. Such an arrangement is necessarily complicated and expensive, interferes with effectively placing the dustcap at the smaller end of the cone, and complicates the acoustic sealing between the base of the cone and a surrounding bafile.

My invention possesses other features and objects, but these will all become obvious from the following detailed description of my invention as illustrated in the accompanying drawing. Referring to the drawing, the single figure is a side elevational view, partly in section, of a loudspeaker embodying my invention.

Referring to the drawing, a frusto-conical dishpan housing I, preferably of sheet metal, having sound openings 3 surrounds a frusto-conical diaphragm 5, preferably of a fairly hard finish paper or other flbrous material suitable for sound radiation, carries an inwardly-extending sheet metal supp rt member 1. This member is provided with a flange 9 for clamping the member to a similar flange II on the cone housing as by means of bolts l2. A flexible, corrugated, annular centering disc or web I3 is cemented at its outer periphery to the base peripheral edge of the cone at l5 and extends radially inwardly. It is cemented at its inner peripheral edge to the inner flanged portion 11 of the supporting member 1 with an interposed paper gasket It.

This flexible disc 13 is preferably of soft, pliable paper formed with the corrugations therein by a felting process, 1. e. by the acretion of paper and woolen fibers from an aqueous suspension onto a form, and subsequently pressing, drying, and treating with a waterproofing compound.

The smaller end of the frusto-conical diaphragm is attached to the usual cylindrical coil form l9 Which carries a voice coil 2| disposed in an annular magnetic air gap at 23. The voice coil is centrally supported in the air gap with the aid of a concentrically-arranged, corrugated flexible supporting disc 25 attached to the juncture of the smaller end of the conical diaphragm and the form l9 and supported at its outer periphery by a cylindrical metal structure 21, flanged at 28. This type of support is disclosed in Boudette Patent 1,893,049. This disc 25 is preferably made of cloth impregnated with a resinous compound. A bolt 38, extending through holes in pole plate 31, extending through holes in pole plate 31 and dishpan housing I, engages the flange 28 to clamp the disc 25 in place after the coil 2| has been centered. An

annular piece of felt 28 is disposed around the interior of the cylindrical support 21. A dustcap 29, preferably of paper, is attached adjacent to the smaller end of the diaphragm for closing the opening in the smaller end. I have there fore provided an outwardly extending supporting disc for the smaller end of the diaphragm, of extensive radial dimension, and an inwardly supporting disc for the large end thereof, also of large radial extent and unobstructed area,

thereby resulting in a suspension system which is highly flexible in the axial direction of normal vibratory movement, while exerting the usual restraint against lateral movement.

The remainder of the speaker assembly is of conventional design and consists of a U-shaped yoke 3| of mild steel, and an inner pole 33 of similar material around which there is disposed a field coil 35. Fastened to the ends of the U- shaped yoke by means of screws 30 is the top pole plate 37 having a circular opening at 23 which forms with the center pole piece 24 an annular air gap, mentioned above, into which the voice coil extends. The top pole plate 37 is also bolted to the smaller end of the dishpan or cone housing. Instead of a field coil, a permanent magnet may of course be used.

The finished speaker is adapted to be positioned against a bailie of conventional form as illustrated at 39, the outer cylindrical portion of the support element 1 preferably fitting into the baffle opening 4|. The bafile may be of the usual flat type, directional type or the cabinet type.

It will be apparent that, in accordance with my invention, the inwardly extending base suspension arrangement results in greater flexibility and greater fidelity of reproduction of lower audio frequency waves, gives the usual acoustic seal,

and results in a smaller outside diameter of loudspeaker housing for a given si Ze cone diaphragm. Although the present support structure for the base of the cone would appear to obstruct a substantial. effectively radiating area of the cone, it has been found that there is substantially no obstruction in the acoustic sense. For low frequency waves such an obstruction has no effect, so long as the corrugated suspension I3 is soft and pliable and thereby avoids introducing an out-of-phase cancellation effect with respect to the sound pressure from the adjacent diaphragm portion. For high frequencies, the central portion of the cone is most effective and such portion is not obstructed by the supporting structure.

In assembling my loudspeaker the following procedure is used: The annular supporting element 1 is placed horizontally with the inner and outer peripheral edges or flanges facing upwardly. Cement is applied around the top surface of flange I? which is then horizontal. A paper gasket is applied to this portion as shown at l8. More cement is applied to this gasket. The corrugated centering disc l3, to which the base of the cone has been attached at 15, is dropped into the position shown with the inner circular edge of the disc fitting closely around the then upstanding flanged edge 20 of the metal member 7 in guiding relation. The lower surface of the inner edge portion of the disc I3 is caused to be pressed against the member l8, until the cement hardens, by reason of the vertical weight of the cone diaphragm which carries the voice coil and centering disc 25 and its support structure 27.

A sub-assembly is thereby formed consisting of diaphragm 5, centering disc I3, support element 1, voice coil 23 and its centering structure 25 and 21. The sub-assembly, as a unit, is thereafter assembled into the position shown on the cone housing and clamped in place by means of bolts l2 and 38 after centering of'the voice coil in the airgap. The dustcap 29 is finally cemented in place.

My present invention may be applied to a standard dishpan, or housing, by providing a flanged supporting element like I, which can be bolted to the flange of the standard housing dishpan heretofore used for securing the outer edge of the annular flexible supporting disc of the diaphragm and, of course, making the cone of larger diameter. In other words, referring to'the drawing, the supporting element 1 may be considered as an adapter to be used on a standard cone housing to make it possible to substitute a larger cone diaphragm for the one ordinarily used. It will, therefore, be appreciated that my invention is of a highly simplified and practical nature. While I have shown a conical diaphragm of circular contour, it may be made in other suitable shapes, such as elliptical in contour. Furthermore, the diaphragm may be of the so called morning glory shape. Instead of frustoconical, it may terminate in a point, i. e., it may be conical. The particular shape of the diaphragm is no part of my present invention, and it is intended that the terms conical, or dished, shall define any of these Well known types. Since this invention is capable of considerable variation and modification and should not be construed as limited to the particular structure described and illustrated, but only in accordance with the appended claims.

I claim as my invention:

1. In an acoustic device, a direct acting conical diaphragm, means remote from the base of said diaphragm for vibrating said diaphragm in accordance with audio frequency energy; means for flexibly supporting the base periphery of said diaphragm for axial vibratory movement and against lateral movement comprising an annular stationary member extending inwardly from the region of said base periphery, and a flexible annular disc member attached to said base periphery and extending inwardly into acoustic sealing engagement with said stationary member.

2. In an acoustic device, a direct acting dished diaphragm having a peripheral edge and a central portion, an actuating device connected to the said diaphragm central portion, a flexible annular disc member having its outer periphery attached around the peripheral edgeof said diaphragm as an acoustic seal and extending a substantial distance radially inwardly therefrom, a

housing disposed externally of said diaphragm, and a supporting member carried by said housing and extending inwardly beyond said peripheral edge and secured to said disc member at its inner periphery and substantially free of contact with the remaining portions thereof.

3. In an acoustic device, a frusto-conical diaphragm, a frusto-conical housing for said diaphragm, the base of said housing being disposed adjacent the base of said diaphragm, an annular rigid supporting member attached to said housing base and extending radially inward beyond said diaphragm base, and an annular flexible disc member attached at its outer periphery to said diaphragm base and attached at its inner periphery to said support member, thereby providing a base suspension of substantial unobstructed radial area.

4. In an acoustic device, a frusto-conical diaphragm of stiff fibrous material, a frusto-conical sheet-metal housing disposed externally of said diaphragm with its base terminating adjacent the base of said diaphragm, an annular sheet metal supporting member carried by said housing and extending radially inwardly beyond the base periphery of said diaphragm whereby to form an opening concentric with and smaller than the base periphery of said diaphragm, and a pliable web member attached to said supporting member around said opening and extending radially outwardly into operative engagement with the base of said diaphragm.

5. In combination, an acoustic bafiie having a sound opening, a stationary supporting member disposed inoperative engagement with said baflle around said opening and extending inwardly toward the center of said opening, a dished acoustic diaphragm having a base diameter exceeding the inner dimension of said supporting member, a housing disposed around said diaphragm, said housing being attached to said supporting member, and an annular flexible Web member attached at its outer periphery to the base of said diaphragm and extending radially inwardly into fixed engagement with said supporting member only at an annular portion thereof substantially spaced radially from said diaphragm base.

6. In combination, an acoustic bafiie having a sound opening therein, housing structure for supporting a dished diaphragm substantially in concentric relation with respect to said opening, an annular supporting member carried by said housing structure and disposed around said opening and extending inwardly therefrom, a dished diaphragm within said housing, said diaphragm having a base dimension exceeding the inner dimension of said supporting member, and an annular flexible web member attached around the base of said diaphragm and extending inwardly into engagement with the inner periphery of said supporting member and free of contact with the remaining portions of said supporting member, whereby said web member and said supporting member constitute an acoustic seal between said diaphragm and said baffle.

JOHN D. SEABERT.

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