US3350513A - Cone loudspeakers - Google Patents

Description

C- 31, 957 A. H. ROBERTS 3,350,53

GONE LoUDsPEAKERs Filed June 22, 1964 2 Sheets-Sheet l 5056/71/ SPP/4 y H547 new United States Patent O 3,350,513 CONE LOUDSPEAKERS Alfred H. Roberts, 1615 Monk Road, Gladwyne, Pa. 19035 Filed .lune 22, 1964, Ser. No. 376,621 Claims. (Cl. 179-1155) ABSTRACT 0F THE DISCLOSURE A cone loudspeaker having a voice coil, inner and outer radiators activated by the voice coil which is integral with at least one of the radiators, the outer radiator comprising a winding of strands upon a facing.

This invention relates to cone loudspeaker.

Realism in the reproduction of sound has been prized since the birth of the telephone and the phonograph. The invention by Kellogg of the magnetic cone loudspeaker has been an enduring contribution. Rugged and economical, it still stands numerically supreme among loudspeakers but it has serious defects and shortcomings.

The cone speaker is subject to acoustical laws which limit the frequency range over which a direct radiator can be endowed with smooth response and wide diffusion. To these it adds aflictions of its own, chiefly inherent in the diaphragm. These are irregular frequency response, intermodulation, unsought and undamped local resonances, harmonic radiation and restricted diffusion. Fortunately, the home listening environment masks some of these and the human ear is charitable to all.

In translating electric oscillations into sound, the voice coil of a magnetic loudspeaker will attempt to follow the input voltage until it is carried out of the region of uniform magnetic flux. The geometry of the voice coil and magnetic gap, and the expense thereof, therefore set the limit upon the ability of a speaker to handle surges of power without generating distortion products. This situation is critical in the bass range.

The voice coil must also overcome the inertia of the moving system, principally that of the cone. This effect limits efficiency and rolls off high-frequency response. At low frequencies the stiffness of the suspension and of the enclosure -volume must be overcome. To move much air at these frequencies, a large cone and long magnetic gap are desirable but expensive.

At the limit of cone travel there will be an overshoot caused by inertia, followed by a decaying transient oscillation which seeks the self-resonance of the moving system. This will be damped out, most of the damping being supplied by a braking action of the electro-magnetic loop. y

The foregoing describes a perfect piston, which implies a-perfectly stiff face. The cone, however, also behaves as a membrane and a free-edge stiff plate. Parasitic or local modes of vibration arise which do not respond to damping applied to the voice coil.

The local modes may be described as follows:

Radial: Each impulse at the voice coil produces a exural waveacross the cone. When the velocity of this wave falls below that of sound in air, reflections inward from the rim produceaudible standing waves on the cone, evidenced by concentric surface rings vibrating in phase opposition. These patterns change rapidly with frequency; they produce dips and peaks in the frequency response, together with the radiation of some harmonics and irregularity in the radiation pattern. An attack and a release transient accompany each mode.

Annular: The cone exhibits cloverleaf patterns of vibration, also accompanied by transients.

Combination: These appear as cloverleaf patterns within rings:

Shock wave: So called for want of a better term, this is a phenomenon of stiff plates. The shock wave outstrips the radial wave and therefore is not harmonically related to it.

As a practical matter, some of these modes can be employed to extend the frequency response of a cone radiator, but the coloration effects are unpleasant and difficult to control in manufacture. When cost is not a major factor, the designer will usually divide the frequency spectrum among two or more speakers.

In the low-frequency speaker, the tendency of the designer is to exchange translation efficiency for smoothness and a low primary ressonance, obtained with a heavy self-damping cone. This requires a heavy magnet. The result is often an expensive speaker with an upper limit of l or 2 kc./s. requiring, at an efficiency of well under one percent, an amplier of 40 watts (RMS) output. It is not uncommon to find in the home an amplifier with 1GO-watt stereophonic capability driving two speakers radiating a few milliwatts.

An obvious improvement, if suitable materials and structure can be devised, is to stiffen and lighten the diaphragm and incorporate within it sufficient damping of those parasitic modes which fall within the range of interest. A further gain is available if the voice coil can be wound integrally surface.

Lightening the cone gives the designer the option of trading efficiency for linearity by adding length to a voice coil which overhangs the magnetic gap. This design, while wasteful, provides an increase in undistorted lowfrequency output.

Stiifening and lightening the cone will move upward the troublesome frequencies of cone breakup. This permits allotting more of the spectrum to the primary radiator. Thus the high-frequency speaker may be made smaller and better. In some designs, the need for a midrange radiator will be obviated, the more so if a coaxial tweeter assembly serves to diffuse the upper frequencies radiated by the primary cone.

One expedient known in this art makes use of a compliant concentric ring as part of the cone surface to decouple the outer frustum of the cone from the voice coil above a chosen midfrequency. This is called a mechanical crossover. Above the crossover frequency the inner portion of the cone vibrates independently, the compliant ring absorbing the vibrations rather than transmitting them to the outer frustum. This avoids some breakup modes and reduces beaming of high frequencies -by the larger cone.

In one embodiment of the present invention mechanical decoupling is combined with horn loading of the inner radiator. This embodiment employs (l) a central dome radiator driven by the voice coil, (2) an elastic ring cemented to the periphery of the dome, acting as a compliant decoupling element and (3) an outer diaphragm cemented to the decoupling ring and having a flare rate approximating that of a -catenoidal or exponential horn. Be-

`low the mechanical crossover frequency the two diaphragms vibrate together; above the crossover, the outer diaphragm is inert and becomes a horn for the central dome. The -crossover frequency is chosen to relieve the youter diaphragm of its radiation function at the point it begins to beam. The cutoff flare rate of the horn is set below the crossover frequency. At l kc./s. for a crossover/ flare-rate-cutof, the cone geometry is compatible with that of a 3-inch inner dome and a 10-inch horn mouth (nominal 12-inch loudspeaker).

with its form and with the driven It is the principal object of this lpresent invention to provide a cone loudspeaker which is free from the diiiiculties heretofore found in available loudspeakers.

It is another principal object of this invention to extend and smooth the transmission band Iof cone loudspeakers. v

It is another object of this invention to improve the translation efficiency of cone loudspeakers.

It is another object of this invention to improve the damping of parasitic resonances and transients in cone loudspeakers. Y

It is another object of this invention to provide for loudspeakers, cones and voice coils which are resistant to moisture, heat, decay, corrosion and impact.

It is another object of this invention to provide close quality control of cone loudspeakers.

It is another object of this invention to provide automatic and semiautomatic fabrication methods for loudspeaker diaphragms, including the winding of voice coils thereon.

Other objects of the invention will be apparent from the description and claims.

The nature and characteristic features of the invention will be more readily understood from the following description taken in connection with the accompanying drawings forming part thereof, in which:

FIGURE l is a central sectional view of a loudspeaker in accordance with the invention;

FIG. 2 is an elevational view of an acoustical piston radiator in accordance with the invention, shown upon a mandrel at the completion of the making thereof;

FIG. 3 is a fragmentary perspective View, greatly enlarged, of a part -of the radiating surface;

FIG. 4 is a lperspective view of a mandrel from which a radiating surface is being removed after completion thereon;

FIG. 5 shows dies for shaping the material for the mandrel of FIG. 4;

FIGS. 6A to 6G, inclusive, show diagrammatically a typical processing sequence for making the radiator;

FIG. 7 is a central sectional view of another form of loudspeaker in accordance with the invention;

FIG. 8 is a transverse sectional view taken approximately on the line 8-8 of FIG. 7; and

FIG. 9 is an enlarged view of a portion of FIG. 7.

It should, of course, be understood, that the description and drawings herein are illustrative merely, and that various modifications and changes can be made in the structure disclosed without departing from the spirit of the invention.

Like numerals refer to like parts throughout the several vlews.

Referring now more particularly to FIG. l of the drawings, the loudspeaker 10 there shown includes a diecast alloy -frame 11 and a diecast alloy casing 12 carried by the frame. The frame 11 has a rim 13 with a felt gasket 14 carried thereby. The casing 12 carries a yoke 15A and 15B of magnetic or magnetic responsive material, with a -central cylindrical portion 16, and a lmagnet 17 of ring shape spaced from the cylindrical portion 16 to provide a cylindrical gap 18. The speaker 10 includes a radiator 20 which has a :plurality of sections including a voice coil section 21 cylindrical in form and interposed in the gap 18 with a tightly wound voice coil helix 22 of electrically conductive material therein with leads 26, a shoulder 23 which approximates a short catenoid of revolution and the radiating section 24 which is a figure of revolution or an ellipsoid having typically the profile of a cone, hyperbola, catenoid or exponential curve. A central dome 2S integral with yor secured to the voice coil section 21 is also provided.`

The radiating section 24 and voice coil section 21 preferably includes a -facing 30 which can `be a film or deposited layer of viscoelastic material, or metal, or zigzag winding of random fibers. It fibers are employed they are bonded together. The facing 30, dependent upon its constituents can be deposited by plating, evaporation, spraying, brushing, dipping lor electrostatic precipitation. The facing 30 may be coated with a protective coating and overwound with the voice coil helix 22.

In overlying relation to the facing 30 a plurality of windings or layers of fibers in strands 31 are provided applied in crossed or zigzag relation and Iby universal or progressive winding. The strands 31 may be of fine glass fibers or ends of rovings sprayed lightly with an epoxy resin cement and cured by heat.

The facing 30 and winding of strands 31 may form a closed or porous structure, the latter being employed -for local damping. The strands 31 can be intermixed or interwoven with stiffer or softer materials.

The radiator 20 may be formed upon a mandrel 32 (FIG. 2) by deposition and winding, or alternatively may be formed first upon a cylinder 33 as in FIG. 4, then slit and stripped therefrom as a sheet 34.

In the cylinder method of production the sheet 34 can vbe formed as shown in FIG. 5 into the final radiator 20 and an integral central dome 25 lbetween the dies 35 and 36. This method is available when a relatively at radiator 20 is desired. The voice coil 22 is then wound separately and cemented to the radiating surface at the junction of the radiating section 24 and dome 25.

In the mandrel method of production, as illustrated diagrammatically in FIGS. 6A to 6G, the mandrel 32 is successively advanced through electrostatic precipitators, a voice coil Winder, cone Winder, cement sprayer, oven and trimmer. As indicated in FIGS. 6A and 6B a fibrous facing 30 with a semi-random grain or nap may be laid on by spinning the mandrel 32 or cylinder 33 in proximity to a blower nozzle or other injector (not shown).

Further stiifening of the radiating surface 24 may be achieved by winding it onto an expansible mandrel and before the cement has set, expanding the mandrel by mechanical, hydraulic or thermal means, thus exerting tension upon the windings. Radiator 20 is then cured under tension, leaving the final structure prestressed and tempered.

When a particular parasitic mode must be either suppressed or encouraged, control may be exercised by periodically varying the pitch of the strands 31 of the winding so as to deposit more or less material in chosen annuli. The surface of the radiator 20 will normally thin out toward the rim and the taper may be regulated, if desired, by varying the winding pitch cyclically.

Referring now to FIGS. 7, 8 and 9, in the embodiment or form of the invention there shown, the radiator is provided including the central dome which is integral with the voice coil form 121. Cemented peripherally to the central dome 125 is a decoupling ring 119 of elastic material, and cemented to the decoupling ring 119 is the outer radiator and horn 124 which is given a catenoidal or exponential flare rate. The outer radiator and horn 124 is supported at the apex and at the rim by two iiexible bellows-type annular rings 40 and 41, attached to the frame 111. The voice coil helix 122 is wound as hereinbefore described and embedded in the surface of the voice coil section 121. The voice coil 122 and its form lie within the gap 118 defined by the magnetic structure including the magnet 117 and the yoke 116 andthe voice coil 122 is accessible electrically by means of two leads 126 and two termina-ls 127 to which the input signal is applied. The central dome 125 may be closed or, as shown in FIGS. 7 and 8, may have a small circular opening 43 at its apex, in which is cemented a damping button 44 of soft material such as finely foamed polystyrene for the suppression of parasitic vibrations.

The decoupling ring 119 is chosen so as to absorb vibrations from the central dome above a designed crossover frequency. The lower cutoff frequency of the horn fiare lies below the crossover frequency.

From the foregoing it will be seen that the acoustical piston radiator 20 is composed of cemented thin light- Weight fibrous and viscoelastic layers 30 and strands 31 some wound progressively and some deposited, the radiating section 24 being integral either with the voice coil section 21 or the central dome 25. The acoustical properties of stiffness, unit mass and damping factor are controlled by choice and distribution of material, plus prestressing, if desired. In another embodiment, a driven central `dome 125 is elastically decoupled above a midfrequency from the outer radiator 124, which then functions as a horn.

Various materials can be employed for the facing 30, chopped short lengths of glass or synthetic plastic filaments of the order of a half mil in diameter, being preferred. For the strands 31 it is preferred to employ continuous glass, metal or synthetic plastic filaments or ends of rovings, and tungsten is the preferred metal. Cornmercial epoxy cements, butyl rubber and the like are suitable for the cement referred to above.

The voice coil helix 22 or 122 is preferably of copper or aluminum Wire or edgewise Wound strip.

Glass fibers exhibit the highest ratio of stiffness to mass of any light nonmagnetic material readily available. Interweavings and interdeposit of viscoelastic fiber will improve damping, and interwound tungsten filament will increase stiffness. Microscopic porosity, being resistive to air passage, is proposed for further control parasitic vibration particularly near the rim.

I claim:

1. An acoustical radiator comprising an outer radiator member and a central dome member and a voice coil integral with at least one of said members, said outer radiator comprising a winding of strands upon a facing.

2. An acoustical radiator as defined in claim 1 in which said outer radiator member is bounded by a surface of revolution.

3. An acoustical radiator the outer radiator member flare,

as defined in claim 2 in which has an exponential rate of 4. An acoustical radiator as defined in claim 2 in which the outer radiator member has a catenoidal rate of flare.

5. An acoustical radiator as defined in claim 2 in which the outer radiator member has a conical rate of flare.

6. An acoustical radiator as defined in claim 2 in which the outer radiator member is an hyperboloid.

7. An acoustical radiator as defined in claim 1 in which the facing is of fibers.

8. An acoustical radiator as defined in claim 1 in which the facing is a thin continuous sheet.

9. An acoustical radiator as defined in claim 1 in which the strands are of a material selected from the group consisting of glass fibers and synthetic plastic filaments.

10. An acoustical radiator as defined in claim 9 having additionally incorporated therein an elasticity modifier.

11. An acoustical radiator as defined in claim 1 in which the outer radiator member is impervious to air.

12. An acoustical radiator as defined in claim 1 in which the outer radiator member is pervious to air.

13. An acoustical radiator as defined in claim 1 in which the voice coil is integral with the outer radiator member.

14. An acoustical radiator as defined in claim 1 in which the dome member is integral with the outer radiator member.

15. An acoustical radiator as defined in claim 1 in which the thickness of the winding s non-uniform to vary the suppression of parasitic modes.

References Cited UNITED STATES PATENTS 1,466,427 8/1923 Drummond 181-32 2,007,747 7/1935 Ringel 181-32 2,231,479 2/1941 Perry 181-31 2,371,951 3/1945 Cook 181-31 2,942,071 6/ 1960 Witchey 181-32 KATHLEEN H. CLAFFY, Primary Examiner. A. MCGILL, Assistant Examiner.

Claims (1)

1. AN ACOUSTICAL RADIATOR COMPRISING AN OUTER RADIATOR MEMBER AND A CENTRAL DOME MEMBER AND A VOICE COIL INTEGRAL WITH AT LEAST ONE OF SAID MEMBERS, SAID OUTER RADIATOR COMPRISING A WINDING OF STRANDS UPON A FACING.

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