GB2084784A - Sound reproducing apparatus - Google Patents

Abstract

In order to avoid resonances within the material of a conventional disc record (10), the turntable or platter (1) is made from a material having an acoustic impedance which is similar to that of the disc record material (usually P.V.C.). This encourages acoustic energy incident on the interface between the record and the turntable to travel into the turntable where it is dissipated in the form of heat. The record needs to be in intimate contact with the turntable for which purpose a clamp 12 is provided. If desired, the turntable can have a slightly concave surface, shown exaggerated in the drawing. <IMAGE>

Description

SPECIFICATION Sound reproducing apparatus This invention relates to sound reproducing apparatus, particularly apparatus of the type for reproducing sound from a disc having a grooved surface formed with irregularities constituting a record of sound to be reproduced.

The invention arose from an appreciation of the fact that imperfections in high quality record playing equipment are to some degree attributable to acoustic resonance within the disc itself. Hitherto such resonances have been kept to a minimum by resting the disc on a relatively soft mat of rubber, synthetic rubber or similar material. The mat has discrete points, e.g. defined by ribs on its surface, on which the disc is supported resiliently in an effort to dampen vibrations. The dampening effect is not however perfect.

The inventor has evolved the theory that resonances in the disc are more effectively reduced by supporting the disc on a material with which it makes intimate contact over a major part of its surface, and by choosing the material of the supporting platter such that it has a similar acoustic impedance to that of the disc so that, when acoustic waves travelling in the disc are incident on an interface between the disc and the supporting platter, they pass mainly into the said material where they are dissipated in the form of heat. Only a small part of the energy is thus reflected back into the disc and resonance is thereby avoided.

Accordingly the invention provides sound reproducing apparatus comprising: a disc or other member constituting a record of sound to be reproduced; a relatively rigid platter for supporting the record; the platter being formed from a material having an acoustic impedance which is substantially matched to that of the record so as to reduce acoustic reflections from an interface between the disc and the platter.

Conventional discs are made from polyvinyl chloride which has an acoustic impedance of about 30 x 104 grams per second per square centimeter. Therefore in order to reduce acoustic reflections to a reasonable level the acoustic impedance of the platter must be matched to this value. Acoustic impedances within the range 15 X 1to 60 1 X 104 are considered acceptable though a range of 20 X 104 to 40 x 104 is preferable. The opti- mal range is considered to be 25 x 104 to 35 x 104.

It is notable that most synthetic plastics materials have acoustic impedances within the ranges given above. Synthetic plastics are therefore preferred and some materials considered suitable are: acrylic resins, e.g. polymethy methacrylate (perspex R.T.M.) which is considered particularly suitable; and thermoplastic polymers, particularly vinyl polymers such as P.V.C. and P.V.A.

Some woods, particularly hard woods, also have suitable impedances, for example Ash (impedance 29 X 104); Beech (impedance 25 x 104); Elm (impedance 23 x 104); Fir (impedance 24 x 104); Mahogany (impedance 27 X 104); Maple or Oak (impedance 29 X 104); Poplar (impedance 21 X 104); Sycamore (impedance 23 X 104); Walnut (impedance 26 X 104). The foregoing impedance values are for propagation with the grain. The values across the grain are much lower. For this reason, and for the purpose of avoiding undue risk of warping, the grain should lie substantially perpendicular to the interface.

Materials considered not suitable are metals which have very high impedance values varying between about 100 X 104 and about 600 X 104, ceramics which similarly have too high an impedance (between about 60 X 104 and 200 X 104). Rubber has too low an impedance, (varying between about 0.6 x 104 and 15 x 104 according to composition).

The apparatus preferably includes means such as a clamp or suction device for retaining or pressing the record into contact with the platter.

The supporting platter can be machined to the desired shape or alternatively moulded e.g. from a suitable resin. It is desirable that the disc be held in intimate contact with the turntable or other support in order to ensure that acoustic energy is transmitted effectively from the disc to the platter or turntable. This requirement for intimate contact is not easily met because conventional discs can warp and therefore do not always have a perfectly flat surface. This problem can be overcome if the turntable or other platter has a concave surface, e.g. a conical or part-spherical surface, and if some suitable clamp or other device is used to press a central part of the disc into contact with the centre of the concave sur face.

According to a second aspect of the invention there is provided sound reproducing apparatus comprising: a platter for rotating a flexible disc or other member constituting a record of sound to be reproduced, the platter having a concave surface for supporting the flexible member; and means for pressing and holding a central part of the concave surface so that the disc conforms to the shape of the concave surface.

Alternatively the concave surface could be replaced by a convex surface in which case the apparatus would need to include means for pressing and holding an outer part of the flexible member against the outer part of the convex surface.

Another advantage of using a concave surface is that energy reflected from a flat undersurface of the platter is directed outwardly: away from the stylus.

One way in which the invention may be performed will now be described by way of example with reference to the accompanying drawing which is a cross-section through part of a record player constructed in accordance with the invention showing a record placed loosely thereon before being tightened in position but not showing the stylus, pick-up arm, motor and associated mechanical equipment.

Referring to the drawing, there is shown a circular turntable 1 which is machined from a solid homogeneous sheet of Perspex (R.T.M.) so as to have a flat bottom surface 2 formed with a circular groove 3 and a frusto-conical surface 4 (shown greatly exagerated on the drawing) on its top side. A shallow recess 5 is provided at the centre of the surface 4.

A platter constituted by a turntable is supported for rotation on a spindle 5 and is driven in a manner known per se by a drive belt 6 passing around a hub portion 7 defined within the groove 3 and around a drive wheel 8 connected by a shaft 9 to a motor (not shown).

A A conventional grooved disc 10 constituting a sound record is supported on the conical surface. As is usual the disc 10 has a paper label 11 fixed to its central part on upper and lower surfaces.

In order to reproduce the sound recorded on the disc 10 the latter is firmly clamped to the conical surface 4 by a clamp 12 so as to conform with the conical surface 4, and to make intimate contact therewith despite the fact that the disc 10 may be warped or otherwise be imperfectly flat. When the disc is thus held in position it is constantly pressed over its whole recorded surface against the turntable. This means that any minute momentary movements away from the turntable as the stylus passes along the groove, are avoided. The recess 5 accomodates the paper label 11.

Since conventional discs are grooved on both sides it will be appreciated that the insides of the grooves will not contact the surface 4. This is not however of critical importance since the widths of the grooves are relatively small compared with the wavelength of sound waves travelling across the interface between the disc and the turntable.

Also, the widths of the grooves may be small compared with the widths of the spaces between them.

The clamp 12 comprises a rigid plate 13 carrying a felt pad 14. This plate 13 is acted on by an external threaded member 15 which slides on the spindle 5 and is slotted, e.g. as shown at 16. A knurled knob 17 has an internal thread which co-operates with the external thread of the member 15. By pressing the knob 17 downwardly pressure is exerted on the centre of the disc via the plate 13 and pad 14 so as to press it into the required intimate contact with the turntable.

The knob 17 is then turned so as to compress the member 15 against the spindle 5 and to hold the plate 13 in position whilst the disc is being played.

Claims (21)

1. Sound reproducing apparatus comprising: a disc or other member constituting a record of sound to be reproduced and a relatively rigid support for supporting the record; the support being formed from a material having an acoustic impedance which is substantially matched to that of the record so as to reduce acoustic reflections from an interface between the disc and the support.

2. Apparatus according to Claim 1 wherein the support has an acoustic impedance within the range 15 x 104 to 60 x 104 grams per second per square centimeter inclu size.

3. Apparatus for playing records made from polyvinyl chloride and comprising a support for supporting the record; the support being formed from a material having an acoustic impedance within the range 15 x 104 to 60 x 104 grams per second per square centimeter inclusive.

4. Sound reproducing apparatus according to Claims 2 or 3 wherein the range is 20 x 104 to 40 X 104 grams per second per square centimeter.

5. Sound reproducing apparatus according to Claim 4 wherein the range is 25 x 104 to 35 x 104 grams per second per square centimeter.

6. Sound reproducing apparatus according to any preceding Claim wherein the support is primarily of synthetic plastics material.

7. Sound reproducing apparatus according to Claim 6 wherein the synthetic plastics material is an acrylic resin.

8. Sound reproducing apparatus according to Claim 7 wherein the synthetic plastics material is polymethyl methacrylate.

9. Sound reproducing apparatus according to Claim 6 wherein the synthetic plastics material is a thermoplastic polymer.

10. Sound reproducing apparatus according to Claim 9 wherein the thermoplastic polymer is a vinyl polymer.

11. Sound reproducing apparatus accord-ing to Claim 10 wherein the vinyl polymer is P.V.C.

12. Sound reproducing apparatus according to Claim 11 wherein the vinyl polymer is P.V.A.

1 3. Sound reproducing apparatus according to any preceding Claim wherein the support is machined.

14. Sound reproducing apparatus according to any of Claims 1 to 12 wherein the support is moulded.

15. Sound reproducing apparatus according to any one of Claims 1 to 5 wherein the support is of wood.

16. Sound reproducing apparatus according to Claim 15 wherein the wood is selected from the group consisting of Ash, Beech, Elm, Fir, Mahogany, Maple, Oak, Poplar, Sycamore and Walnut.

17. Sound reproducing apparatus according to Claims 15 or 16 wherein the grain of the wood extends substantially perpendicular to the interface between the support and the record, when the record is on the support.

18. Sound reproducing apparatus according to any preceding Claim including means for retaining or pressing the record in contact with the support.

1 9. Sound reproducing apparatus according to any preceding Claim in which the support has a concave surface for supporting the record.

20. Sound reproducing apparatus according to any one of Claims 1 to 18 in which the support has a convex surface for supporting the record.

21. Sound reproducing apparatus substantially as described with reference to the accompanying drawing and substantially as illustrated therein.