Vibratory Elements for Audio Equipment

This invention relates to elements for use in audio equipment, and particularly to acoustic vibratory elements such as diaphragms for speakers and microphones and cartridge cantilevers for record players, the elements being lightweight, highly stiff and highly elastic and having a sufficient internal loss and an improved temperature resistance.

In general, acoustic vibratory elements such as acoustic diaphragms and cantilevers are required to have low density high stiffness, high elasticity, and sufficient internal loss. Low density, high stiffness and high elasticity are necessary to provide efficient reproduction up to a high frequency range without causing partial vibration while sufficient internal loss is effective for preventing sound pressure from rapidly rising at about resonance frequencies in the high range as well as improving damping characteristics. Acoustic vibratory elements are known which are formed from lightweight, highly elastic materials, for example such as aluminum, titanium and beryllium, and composite materials such as carbon fiber reinforced plastics (CFRP). None of these known materials are free of the problem of poor internal loss. Acoustic diaphragms with sufficient internal loss are also known which are formed from paper, synthetic resins or composite materials thereof. Though these materials have sufficient internal loss, they show a low modulus of elasticity and hence, a low specific modulus (ratio of modulus of elasticity E to density .rho., simply referred to as "E/.rho."). There is need for material which has not only a low density and high elasticity, but also sufficient internal loss.

Recently, the inventors proposed in U.S. Ser. No. 147,866 (filed May 8, 1980) cantilevers and diaphragms which are characterized by low density, high elasticity and high internal loss, the cantilevers and diaphragms being formed from a kneaded mixture comprising polyvinyl chloride, polyvinylidene chloride and flaky graphite powder. The proposed material has improved physical properties which are not found in prior art materials. In various application of the previously proposed material, the inventors encountered a problem that it is insufficient in temperature resistance. Elements made of this material tend to be deformed particularly when used in automobile audio sets which may be exposed to the summer sun or possibly used in the tropics.

It is therefore an object of the present invention to provide an improved acoustic vibratory element which has satisfactory temperature resistance as well as excellent qualities of low density, high elasticity and sufficient internal loss.

According to a first aspect of the present invention, there is provided an acoustic vibratory element fabricated from a kneaded mixture comprising post-chlorinated polyvinyl chloride and flaky graphite powder, the graphite flakes being oriented substantially parallel to the surface of the element.

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

FIG. 1a is a cross-sectional view of a kneaded mass of graphite flakes in a resin matrix;

FIG. 1b is a cross-sectional view of a rolled sheet in which graphite flakes are oriented parallel to the surface according to the present invention;

FIG. 2 is a perspective view of a laminate having four rolled sheets placed one on top of the other;

FIG. 3 is a cross-sectional view of an embodiment of the cone-shaped diaphragm according to the present invention;

FIGS. 4a and 4b are radial and axial cross-sectional views of a cylindrical member formed from the rolled sheet, respectively.

FIG. 5 is an axial cross-sectional view of a cantilever prepared from the cylindrical member shown in FIGS. 4a and 4b;

FIGS. 6a, 6b and 6c illustrate successive steps of molding the kneaded mixture into a desired article;

FIGS. 7a, 7b and 7c are perspective views of a speaker enclosure, a record player base and a radio/cassette player casing, respectively;

FIG. 8 is an exploded view of a speaker enclosure; and

FIGS. 9a, 9b, 9c, 9d and 9e are perspective views of a speaker frame, a horn, an acoustic lens, an acoustic equalizer and a grille, respectively.

The acoustic vibratory element according to the present invention is fabricated from a kneaded mixture mainly containing post-chlorinated polyvinyl chloride and flaky graphite powder. The resin component may be post-chlorinated polyvinyl chloride alone or in admixture with polymethyl methacrylate (PMMA).

The post-chlorinated polyvinyl chloride used herein may be prepared by chlorinated polyvinyl chloride (PVC) as disclosed in U.S. Pat. No. 2,996,489 which is incorporated herein by reference. Such post-chlorinated PVC has a softening point which is higher by 20 degrees Centigrade or more than the softening point of 80.degree. C. of normal PVC.

Preferred examples of polymethyl methacrylate are those having a softening point which is higher by about 10.degree. C. than normal PVC.

Further, a rubber-like material may also be added to the post-chlorinated PVC. It is also contemplated in the present invention to add both a rubber-like material and polymethyl methacrylate to the post-chlorinated PVC. Examples of the rubber-like material are acrylonitrile-butadiene rubber (NBR), butyl ruber (IIR), styrene-butadiene rubber (SBR), etc. It is to be understood that the rubber-like materials contribute to a further improvement in internal loss.

In general, the modulus of elasticity of a resin gradually decreases as the surrounding temperature is raised. The temperature at which modulus of elasticity decreases to one-half of the original value at room temperature is referred to as half-modulus temperature in this specification. When the half-modulus temperatures of resins of interest are compared, the half-modulus temperature of post-chlorinated PVC is higher by about 10-20 degrees Centigrade than that of normal PVC, and PMMA is higher by 10 degrees Centigrade than the latter. Accordingly, molding materials comprising flalky graphite powder, post-chlorinated PVC and optionally, PMMA have a higher half-modulus temperature and hence, improved temperature resistance as compared with the previous PVC- based molding materials.

The flaky graphite powder contributes to an improvement in modulus of elasticity, which cannot be expected in the case of resin components per se. The modulus of elasticity is substantially increased when graphite flakes are oriented in one direction, preferably parallel to the surface of a molded product. Graphite flakes have a thin disc- or platelet-form and preferably have an average particle size of about 0.1 to about 20 microns, particularly about 0.1 to about 5 microns. An improvement in modulus of elasticity attributable to graphite flakes are satisfactory moldability can be expected and moldings are free of embrittlement when about 10 to 90 parts by weight of graphite powder is combined with 90 to 10 parts by weight of the resin component. An outstanding improvement is achieved when about 50 to 75 parts by weight of graphite powder is combined with about 50 to 25 parts by weight of the resin component.

PMMA is widely used with post-chlorinated PVC as its processing aid and the amount of PMMA to be added may be varied in the range of 1-15%, preferably 5-10% by weight of the post-chlorinated PVC, depending on the shape and properties of an intended product. The rubber-like material may be added in an amount of about 2 to 50 parts per 100 parts by weight of the resin component. The addition of about 10 pph of the rubber-like material will result in a 20% reduction of modulus of elasticity while internal loss is increased about 1.5 to 2.0 times.

The present invention will be more fully understood by referring to the preferred embodiments in conjunction with the drawing.

Acoustic vibratory elements may be fabricated by first mixing flaky graphite powder with a resin component which may be post-chlorinated polyvinyl chloride alone or its admixture with polymethyl methacrylate and an optional rubber-like material. Also, a plasticizer and/or stabilizer may optionally be added. The thus obtained mixture is fully kneaded by means of a conventional kneader or roll mill while heating to a temperature of 130.degree.-180.degree. C. at which the resins will soften or melt. The kneaded mass is designated at 3 in FIG. 1a as containing graphite flakes 2 in a resin matrix 1. As seen from FIG. 1a, graphite flakes 2 are randomly distributed throughout the resin marix 1.

The kneaded material 3 is then repeatedly rolled by means of a roll mill into a sheet 4 as shown in FIG. 1b. The rolling of the kneaded material causes graphite flakes 2 to be oriented parallel to the surface of the sheet 4. As a result of graphite orientation, the modulus of elasticity of the rolled sheet is increased two or three times over kneaded materials containing randomly distributed graphite flakes.

One sheet 4 may be fabricated into an acoustic vibratory element of a desired shape by any suitable method including vacuum forming, air-pressure forming or press molding while heating to about the softening temperature of the resin. If desired, two or more sheets 4 may be placed one on top of the other to form a laminate having a desired thickness. FIG. 2 illustrates such a laminate 11 having four sheets 4. Heat bonding or adhesive application may be effected to bond the adjoining sheets.

In fabricating a speaker diaphragm 12 as shown in FIG. 3, the sheet 4 or the laminate 11 of some sheets laminated to a desired thickness in press molded between upper and lower mold halves at temperatures of 100.degree.-200.degree. C. The resulting diaphragm 12 exhibits a high stiffness and a high modulus of elasticity as graphite flakes 2 are oriented parallel to the surface as shown in FIG. 3. Although the diaphragm is of cone shape in this example, it amy be of dome shape.

A cantilever may be fabricated by rounding the single sheet 4 or the lam inate 11 of some sheets into a cylinder as shown in FIGS. 4a and 4b while heating to temperatures of 150.degree.-200.degree. C. The abutting edges of the rounded sheet are heat bonded under pressure or bonded with an adhesive at an interface 5. The thus integrated cylindrical member may be deformed into a cantilever 13 as shown in FIG. 5 using suitable deforming means.

Examples of the present invention are described below.

A sheet having a thickness of 100 microns obtained by rolling in each Example is vacuum formed at 100.degree. C. into a cone-shaped diaphragm as shown in FIG. 3.

In addition, each sheet is heated to 100.degree. C. before it is rounded on a cylindrical core. The abutting edges of the rounded sheet are heat bonded and the core is then withdrawn. The resulting cylindrical member is deformed into a cantilever shape as shown in FIG. 5.

The thus fabricated diaphragms and cantilevers exhibit a high modulus of elasticity and a high specific modulus as graphite flakes are oriented parallel to the surface. As seen from the Table, they show a high specific modulus E.rho. exceeding the specific modulus of metals such as aluminum and a large internal loss approximating the internal loss of paper. Due to these improved properties, the diaphragms of the present invention exhibit more flat frequency response over a wide frequency range without inducin partial vibration. In addition, the temperature resistance of the diaphragms according to the present invention is improved by about 20 degrees Centigrade over the previously proposed material as proved by a heat distortion temperature of above 120.degree. C.

It is to be noted that the kneaded mixture 3 of the resin matrix 1 having graphite flakes 2 randomly distributed therein as shown in FIG. 1a is ready for use in molding various articles which need not necessarily have a substantially increased modulus of elasticity. Such articles are casings for acoustic equipment, for example, speaker enclosures, record player bases and potable radio/cassette player casings. Other examples are non-vibratory elements for speaker systems, for instance, speaker frames, horns, equalizers, acoustic lens, and grilles. These articles may be molded directly from the kneaded mixture 3 by any suitable molding techniques including compression molding, injection molding and press molding. A process of compression molding, for example, is illustrated in FIGS. 6a-6c. Lower and upper mold halves 21 and 22 define a cavity having a configuration corresponding to the shape of a desired article, when mated together. A metered amount of the kneaded mixture material 3 is placed on the cavity-defining surface of the lower mold half 21 as shown in FIG. 6a, and then the upper mold half 22 is moved downward to compress the material while heating, allowing the material to flow throughout the cavity. Thereafter, the mold halves 21 and 22 are cooled and separated. A molded article 23 is then removed.

The molded article 23 may take the form of a speaker enclosure 24, a record player base 25, and a radio-cassette player casing 26 shown in FIGS. 7a, 7b, and 7c, respectively.

Although the kneaded material containing randomly distributed graphite flasks or the article 23 directly molded therefrom has a modulus of elasticity which is about one-half or one third of that of a material of the same composition, but having graphite flakes oriented. However, the former and the latter are equal in internal loss. Therefore, acoustic equipment casings molded from the kneaded material are satisfactory because only improved vibration damping characteristics are required for such casings.

Of course, the sheet 4 obtained from the kneaded material by rolling and having graphite flakes 2 oriented may be used to fabricate casings. Usually, a plurality of sheets are placed one on top of the other to form the laminate 11 as shown in FIG. 2. For example, a speaker enclosure may be fabricated by assembling a front panel 31a, a top panel 31b, side panels 31c and a bottom panel 31d as shown in FIG. 8. The assembly may be completed by heat bonding or adhesive bonding.

A record player base may be fabricated by superposing a number of sheets 4 to a required thickness. The laminate 11 is then provided with openings (not shown) for receiving a turntable assembly and a tone-arm base.

It will be understood that the article 23 may be either molded directly from the kneaded material or assembled from some components or rolled sheets. For example, a one-piece integral enclosure 24 shown in FIG. 7a can be easily molded directly from the kneaded material whereas the enclosure assembled from panels 31 of rolled sheets as shown in FIG. 8 is rigider than the former.

FIGS. 9a to 9e illustrate a speaker frame, a horn, an acoustic lens, an acoustic equalizer, and a grille which are typical examples of articles molded using the compression molding technique as shown in FIGS. 6a to 6c. Such an article may be either a one-piece article molded from the kneaded material or an article assembled from two or more shaped components. As the kneaded material has sufficient internal loss, these speaker elements molded therefrom are substantially free of resonance and exhibit sufficient vibration damping characteristics.