GB2138648A - Moving coil loudspeaker - Google Patents

Abstract

A loudspeaker having a diaphragm which has a flat, front sound radiating surface defined by a balsa wood sheet (16), the rear surface of which carries a cellular plastics backing (18). The front surface of the balsa sheet is covered by a thin plastics film (20) for, inter alia, protecting the balsa wood from atmospheric moisture. <IMAGE>

Description

SPECIFICATION Moving coil loudspeaker The present invention is concerned with moving coil loudspeakers.

Conventional moving coil loudspeakers comprise a circular coil immersed in a magnetic field and attached to the apex of a conical diaphragm, which is usually made of paper or a plastics material such as bexdrene or polypropylene. The cone is mounted at its outer edge by a flexible plastics or rubber ring and at its apex by a cambric "spider". The conical shape of the diaphragm is chosen as it makes possible a degree of strength and lightness which belies the fragile nature of the materials used.

The behaviour conical diaphragms has been extensively studied and it is known that when the coil is excited by vibratory forces of various frequencies and wavelength the cone moves in a piston-like manner, at least for wavelengths which are large compared with its diameter. However, where the wavelength is smaller, parts of the cone move out of phase with other parts. These out of phase vibratory patterns create resonances and anti-resonances in the system and are responsible for uneven frequency response.

Attempts have been made previously to provide diaphragms which are essentially flat and these have met with varying degrees of success. It is an object of the present invention to provide a new form of flat diaphragm structure which is simple and cheap to manufacture but which provides a smooth frequency response over the whole frequency range.

In accordance with the present invention, there is provided a moving coil loudspeaker having a diaphragm whose front, sound radiating surface is defined by a flat balsa wood lamina, whose rear surface carries a cellular plastics backing portion.

Preferably, the flat lamina comprises a single sheet of balsa wood, at least the front, sound radiating surface of which is sealed by a plastics film.

Thus, a moving coil loudspeaker in accordance with the present invention has a diaphragm made of balsa wood with a cellular plastics backing.

Preferably, the loudspeaker includes a rigid coil former which extends through the cellular backing portion and is firmly connected to the back of the balsa lamina. For this purpose, the coil former can be received in one or more slots formed in the cellular backing portion.

The invention is described further hereinafter, by way of example only, with reference to the accompanying drawings, wherein: Fig. 1 is a perspective diagrammatic view of part of a loudspeaker embodying the present invention; Fig. 2 is a transverse section on A-A in Fig. 1; and Figs. 3 to 5 are perspective diagrammatic views of parts of second, third and fourth loudspeaker configurations embodying the invention.

With reference to Figs. 1 and 2, the illustrated loudspeaker in accordance with the invention comprises a diaphragm 10 coupied to a coil former 14 carrying a coil 12. In this embodiment, the diaphragm happens to be of elongate configuration having mutually parallel longitudinal sides. Although not illustrated, to complete the loudspeaker, the illustrated parts would be mounted flexibly in a rigid frame together with a permanent magnet structure, Since these latter parts are of a conventional nature, they have not been illustrated but in a practical loudspeaker they would of course be present.

Fig. 2 shows in more detail the special structure of the diaphragm 1 0 which comprises a single sheet of balsa wood 1 6 whose grain is arranged to lie in a direction transverse to the length of the diaphragm, i.e. in the direction of the arrow A in Fig. 2. Securely fixed to the back of the balsa sheet 1 6, for example by glueing, is a mass of cellular plastics foam material 1 8. As shown in Fig. 2, the balsa sheet is arranged to extend beyond the periphery of the foam 18 to form a peripheral lip portion 1 6a. The whole of the front surface of the balsa sheet 1 6 and the underside of the lip portion 1 6a are covered by a thin plastics skin 20 to provide additional stiffness to the balsa and to overcome problems of moisture variations and warping.Preferably, the skin is tightly stretched over the balsa sheet and bonded to it by heat treatment, such as hot ironing. One such plastics film which can be used in practice is a self adhesive film called "Solarfilm" (Trade Mark) which is applied with a flat iron at a low heat setting.

The coil former 14 comprises a pair of planar rectangular ribs 22a, 22b which are firmly attached to the back of the balsa wood sheet 1 6 by suitable adhesive so as to lie in mutually parallel planes perpendicular to the plane of the balsa sheet. The ribs pass through respective slots 26a, 26b in the cellular foam 18 but are preferably adhered to the sides of the slots, again by a suitable adhesive. During assembly, it is preferred for the cellular foam to be squeezed laterally in the directions of arrows B in order to ensure a good joint with the ribs 22. The foam 1 8 can conveniently be in the form of three strips 1 8a, 1 8b and 1 8c which are squeezed together as described above for attachment to the ribs.

It will be appreciated that as a result of the aforegoing construction, the balsa wood sheet is wholly encapsulated and isolated from the atmosphere so as to avoid problems, such as warping, arising from exposure to moisture.

The coil 28 of the speaker is mounted on the outside of the coil former 1 4. In other embodiments, particularly where higher powers have to be handled, a second coil can be mounted on the inside of the coil former 14. The two coils would then be connected in parallel.

Although the diaphragm has been shown in Fig.

1 to have a parallel sided configuration, other configurations are also possible, for example as shown in Figs. 3, 4 and 5, wherein like and equivalent parts are indicated by the same reference numerals. In the case of the embodiment of Fig. 5, the slot in the cellular backing material which receives the circular coil former 14' would have to be correspondingly circular.

The ribs 22a, 22b are preferably made of a hard, brittle material, such as paxoline. Preferred material for the foam 18 is polurethane foam.

An advantage of the aforegoing structure is that the balsa lamina 1 6 can be sufficiently stiff to provide adequate sound radiation without undue mechanical distortion of the diaphragm whilst the backing material 18, which provides acoustic damping for the sound radiating part 16, is of sufficiently small mass for the diaphragm as a whole to retain the necessary'lightness.

Claims (11)

1. A moving coil loudspeaker comprising a diaphragm which has a flat, front sound radiating surface and which is made of balsa wood having a cellular plastics backing.

2. A moving coil loudspeaker having a diaphragm whose front, sound radiating surface is defined by a flat balsa wood lamina, the rear surface of which carries a cellular plastics backing portion.

3. A loudspeaker as claimed in claim 2 wherein the flat lamina comprises a single sheet of balsa wood.

4. A loudspeaker as claimed in claim 3 wherein at least the front, sound radiating surface of said sheet of balsa wood is covered by a plastics film.

5. A-loudspeaker as claimed in claim 4 wherein the balsa wood lamina extends laterally beyond the cellular plastics backing to form a peripheral lip portion having no plastics backing, the whole of the front surface of the balsa sheet and the lateral edge and underside of the lip portion being covered by said plastics film to provide additional stiffness to the balsa-and to protect the balsa wood from atmospheric moisture.

6. A loudspeaker as claimed in claims 2, 3, or 4, including a-rigid coil former which extends through the cellular backing portion and is firmly connected to the back of the balsa lamina.

7. A loudspeaker as claimed in claim 6 wherein the coil former comprises a pair of ribs which are firmly attached to the back of the balsa wood sheet by adhesive so as to lie in spaced, generally parallel planes perpendicular to the plane of the balsa sheet.

8. A loudspeaker as claimed in claim 7 wherein the ribs pass through respective slots in the cellular plastics backing.

9. A loudspeaker as claimed in claim 8 wherein the ribs are adhered to the sides of the respective slots in the backing material through which they extend.

10. A loudspeaker as claimed in any of claims 1 to 9 wherein the cellular plastics material is a polyurethane foam.

11. A moving coil loudspeaker substantially as hereinbefore described with reference to and as illustrated in the accompanying drawings.