Temperature/Environment-Resistant Transducer Suspension

The present invention relates to acoustic transducers and more particularly it relates to improvements in the structure and manufacture of diaphragm-suspension members of actuators and loudspeakers of the moving voice coil/diaphragm type for use in harsh environments at unusually low and high ambient temperatures.

The moving voice coil/diaphragm type transducer configuration has been utilized widely in loudspeakers, where the main moving diaphragm is typically but not necessarily conical in shape with its inner edge attached to a voice coil bobbin. Typically there are two resilient suspension members: a spider attached to the voice coil bobbin generally near the region where the bobbin is attached to the cone, and a surround member forming an annular border strip having its inner edge attached around the outer edge of the cone and its outer edge attached to a peripheral flange of the speaker frame. Both of these suspension members are required to provide the cone/voice coil assembly with elastic freedom to move back and forth over large excursions along the central axis while at the same time providing sufficient lateral stiffness to hold the voice coil and bobbin concentrically aligned within the gap between the magnetic poles.

It is quite difficult to find suitable materials that will act satisfactorily as suspension members in a hostile environment such as an automotive exhaust pipe where the temperature can range from -40 to +600 deg. F. (-40 to 316 deg. C.), compared to a much narrower range, typically 0 to 150 deg. F. (-18 to 66 deg. C.) over which ordinary loudspeakers are designed to operate.

For this severe duty, generally most materials will be found unsuitable in either fatigue endurance, acoustic properties or flexural properties, when subjected to extreme heat and cold.

U.S. Pat. No. 3,767,004 to Liebscher discloses a loudspeaker utilizing a thin flat surround member around the outside of the cone, which receives its full support from a double spider rear suspension system, thus removing all stresses from the surround member so that it serves merely as an air separator rather than as a suspension member. In Liebscher's flat surround configuration, the material is unstressed in an inactive condition, and is required to stretch isotropically, i.e. omnidirectionally, with cone displacement.

The foam material utilized by Liebscher is specified to be of the closed cell type (i.e. containing bubbles); such material is essentially not moldable into a required shape because necessary pressurization would make it impossible to control the resulting thickness of the surround material when the pressure is released. Thus the usefulness of foam material in loudspeaker surrounds is limited to flat non-supportive members such as the Liebscher surround which requires no lateral stiffness and thus can be made inexpensively from sheet foam stock cut to the required outline and used in flat form.

Many loudspeakers of known art use a type of surround suspension which is cut from a sheet of fibrous material such as soft paper and press-formed to have annular corrugations so as to provide some degree of lateral stiffness along with flexibility to vibrate axially; however such suspensions are well known to be fragile and tend to have short life expectancy especially under harsh environmental conditions including temperature extremes.

It is a primary object of the present invention to provide a robust loudspeaker surround suspension member in which a specially selected material is particularly configured to provide uniform resilience in the direction of vibration and sufficient lateral stiffness to provide concentric control of the voice coil, and which can withstand large temperature variations while providing the required amount of excursion without sacrificing fatigue resistance and/or acoustic efficiency.

It is a further object of the invention that the material selected for the surround suspension member be suitable for manufacturing methods which allow freedom and flexibility in shaping the selected material to optimal form for best realization of the primary object of the invention.

The above objects have been met in the present invention of voice coil/cone suspension members made from silicone rubber or a silicone rubber composite of dense non-cellular structure as distinguished from an open porous or cellular structure such as is found in foam rubber materials, that is molded into the shape of a suspension member in a constant temperature molding process, preferably transfer molding. The ability to mold silicone rubber composite in thin cross sections adds a great degree of freedom in designing a surround system. The molding techniques of this material allow injection, transfer and compression molding of a suspension member with good control of the shape, mechanical properties and thickness, particularly with regard to the surround suspension member. Molding enables the cross-section of the surround to be made in the shape of one or more arcuate half-rolls which operate on the principle of anisotropic hoop tension to hold the voice coil well centered while allowing freedom to vibrate.

The above and further objects, features and advantages of the present invention will be more fully understood from the following description taken with the accompanying drawings in which:

FIG. 1 is a perspective view of a typical transducer showing the location of the surround member.

FIGS. 2-5 are cross sectional views of the surround support region of a transducer as implemented in different embodiments of the present invention:

FIG. 2 shows a half-roll surround member molded onto a diaphragm edge.

FIG. 3 shows a half-roll surround member adhesively attached to the diaphragm edge.

FIG. 4 shows a half-roll surround member formed with variations in thickness.

FIG. 5 shows a multiple roll surround member molded onto a diaphragm edge.

In FIG. Is a typical transducer 10 is depicted in a perspective frontal view showing the cone shaped diaphragm 12 attached around its outer edge to the inner edge 14 of a surround member 16 which is in turn attached by its outer edge 18 to the peripheral flange 20 of the frame of transducer 10.

FIG. 2 is a cross sectional view of an edge region of transducer 10 as viewed at radial axis 2--2' of FIG. 1. In this embodiment of the invention, a surround member 16a is molded from silicone rubber or a composition thereof; the inner edge bifurcated to form two lips 14a and 14b, between which diaphragm 12 is sandwiched and bonded in place by a molding process or by an adhesive.

The surround member 16a is seen to have a half-roll arched portion 22a which provides the main axial compliance, and a flat outer edge 18 by which it is attached to flange 20 of the transducer 10, typically by adhesive bonding.

FIG. 3 shows a surround member 16b having a half roll arched portion 22 similar to that of FIG. 2, however in this embodiment the inner edge 14 is made flat and attached on its lower surface to diaphragm 12 by adhesive bonding.

FIG. 4 illustrates another embodiment of the invention wherein the surround member 16c is made to have different thickness at different regions of the cross section. The thickness dimension B in a central region of arched portion 22 is made less than the thickness dimension A at outer edge 18; also the arched portion 22 is made thicker at dimension C near the bifurcated inner edge 14a, 14b. In this example the inner edge is shown as in FIG. 2, i.e. bifurcated with lips 14a and 14b for molded attachment to the diaphragm 12; however, the concept of varying the thickness is also applicable in the case of one-sided adhesive edge attachment as shown in FIG. 3. As an example of varying the thickness of the surround member 16c, for a 6" round loudspeaker, dimensions A and C may be made 0.03" (0.76 mm) while dimension B is made to be 0.015" (0.38 mm). The values selected will depend on several design parameters such as the size of the transducer, the molding process and the composition of the silicone rubber material. This variation in thickness, which is readily implemented in a molding process by shaping the cavities as required, provides design flexibility to increase the axial compliance and its linearity while preserving lateral stiffness for centering purposes and providing a robust region for attachment at the outer edge 18 and the inner edge 14.

FIG. 5 shows an example of an alternative configuration for the surround member 16d in which the resilient portion 22d is made with multiple rolls as shown in place of the half roll configuration of the foregoing embodiments. In this instance there are three half rolls approximating three adjacent semicircles: this concept can be extended to any number of multiple rolls, typically there will be an odd number of half rolls. The concept of multiple rolls may be applied in the same manner as described above for a single half roll: i.e. in combination with molded attachment, adhesive attachment and/or thickness variation.

In implementing this invention silicone rubber alone will provide improvements over prior art. Further enhancement may be obtained by adding other material to the silicone to form a composite silicone molding material, typically pulp and weave fiber grades of the mix materials such as those sold under Kevlar, Nylon, Nomex and Fiberglass, as selected to provide desired mechanical, acoustic and fatigue resistant properties of the silicone suspension member. In an example of a viable composite material for this purpose, fiber pulp sold under Kevlar is added to the silicone elastomer material in an amount of 1.5% by weight.

Regarding molding processes, transfer molding has been found most satisfactory and economical, and is typically performed at a pressure of 200 to 300 psi and a temperature around 350 degrees F. (177 deg. C.). As an alternative constant temperature molding process, compression molding could be utilized. It would also be possible to injection mold preheated material in a thermal cycle process. Material thickness typically ranges from 0.017" (0.43 mm) to 0.025" (0.63 mm).

A round speaker configuration is shown in FIG. 1 as illustrative: the invention is equally applicable to other speaker shapes such as elliptical or rectangular.

The invention may be embodied and practiced in other specific forms without departing from the spirit and essential characteristics thereof. The present embodiments are therefore to be considered in all respects as illustrative and not restrictive, the scope of the invention being indicated by the appended claims rather than by the foregoing description; and all variations, substitutions and changes which come within the meaning and range of equivalency of the claims are therefore intended to be embraced therein.