GB2127650A - Moving coil loudspeaker - Google Patents

Abstract

A loudspeaker diaphragm is laminated from a plurality of Balsa wood sheets 10a, 10b, 10c which are hermetically enclosed within a film of plastics material 12, 18. The grain directions of the individual laminations are preferably not mutually aligned and may be disposed symmetrically. Various shapes of diaphragm are described. <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 of conical diaphragms has been extensively studied and it is known that when the coil is excited by vibratory forces of various frequencies and wavelengths 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.

In order to attempt to overcome this unevenness of response, most modern loudspeaker system comprise two or more units of diminishing size, each required to operate over a frequency range in which it exhibits the desired piston-like behaviour. The total audio signal containing frequencies from, say 40 Hz to 20,000 Hz, is divided by filter networks which pass on the appropriate frequency band to each unit.

There are several disadvantages to the latter system. Firstly, the audio signal is inevitably degraded by electrical phase shifts in the dividing networks. Secondly, the phase relationships between drive units are frequency variable because the units are spatially separated by distances which are substantialy compared with the wavelengths of the transmitted sound. Filter networks introduce two further undesirable factors. They cause large variations of electrical impedance as seen from the amplifier output and they destroy a large part of the electrical damping which an amplifier can exert on the system when it is directly connected to a voice coil.

All these latter problems, which are often ingeniously minimised in well designed systems, lead towards three undesirable acoustic characteristics, namely: (1) Ragged frequency response.

(2) Poor spatial stereo imaging.

(3) Chesty colouration of the sound and lack of naturalness and clarity.

It is an object of the present invention to provide a loudspeaker having a single drive unit which can operate over the whole audio frequency range without the problems associated with the known conical diaphragms discussed above.

A loudspeaker in accordance with the present invention comprises a coil immersed in a magnetic field and centrally attached to a flat diaphragm made of laminated Balsa wood hermetically sealed within a thin plastics envelope.

In order for the diaphragm to be uniformly stiff in all directions, it is advantageous, but not essential, for successive laminations to be arranged so that the orientations of their grains are distributed evenly around an imaginary circle, i.e. with three laminations the grains are set at 1200 to one another, with four laminations they are set at 900 etc.

The Balsa wood laminations are, in themselves, fairly stiff and dimensionally stable, but a skin of a plastics material over the whole surface forming said envelope gives them a great deal of additional stiffness and overcomes the problem of moisture variations and hence of warping. The plastics skin also aids damping.

Preferably, the plastics material is tightly stretched over the laminations and bonded by heat treatment. One such plastics material that has been used in practice is a self-adhesive film called "Solar film" (trade Mark) which is applied with a flat iron at a low heat setting.

Advantageously the diameter or width of the coil is large in relation to the diameter of the diaphragm, preferably, but not necessarily, of the order of 75% the diameter of the diaphragm.

The invention is described further hereinafter, by way of example, only, with reference to the accompanying drawings, wherein: Fig. 1 shows diagrammatically the arrangement of three Balsa wood laminations during the fabrication of one embodiment of a diaphragm for a loudspeaker in accordance with the present invention; Fig. 2 illustrates diagrammatically the fabrication of the diaphragm from the Balsa wood stack of Fig. 1; Fig. 3 shows diagrammatically the manner in which a coil former and coil are attached to the assembly of Fig. 2; Fig. 4 is a diagrammatic illustration of one possible diaphragm, cone and coil configuration for a loudspeaker in accordance with the present invention; Fig. 5 is a diagrammatic illustration of a second possible diaphragm, cone and coil configuration for a loudspeaker in accordance with the present invention;; Fig. 6 is a diagrammatic illustration of a third possible diaphragm, cone and coil configuration for a loudspeaker in accordance with the present invention; Fig. 7 is a diagrammatic illustration of a fourth possible diaphragm, cone and coil configuration for a loudspeaker in accordance with the present invention; Fig. 8 is a transverse sectional view of a first loudspeaker incorporating the diaphgram, cone and coil configuration of Fig. 4; Fig. 9 is a partial sectional view of a second loudspeaker incorporating the diaphragm, cone and coil configuration of Fig. 4; Fig. 10 is a transverse sectional view of a loudspeaker incorporating the diaphragm, cone and coil configuration of either Fig. 5, Fig. 6 or Fig.

7; and Fig. 11 is a longitudinal sectional view of the loudspeaker of Fig. 10.

Attempts to design a single Hi Fi loudspeaker drive unit, which obviates the above discussed problems, by dispensing with crossover networks, have been largely abandoned for the last twently years or so because of the difficulty of designing a conical diaphragm which could perform equally well over the whole frequency range. The three design variables to be controlled are size, shape and material.

Too small a diaphragm will not be able to move sufficient air particles to create adequate sound power without moving through impracticably large to and fro excursions. Too large a diaphragm will offer a frequency balance which favours the low frequencies and also will transmit sound in a narrow beam at high frequencies.

The normal circular shape of a cone means that the most severe dimension-related resonance in diaphragms is emphasised by the constant dimension between the coil and the edge of the cone. In multiple drive unit systems, the severe resonance is designed to fall outside the frequency bands over which each unit is required to operate. Another characteristic of cones is that they lose their ability to hold their shape if they are not made circular.

For these and other reasons, the present Applicant has moved away from the use of conical diaphragms and has turned to the possibility of using a diaphragm which is essentially flat. This in itself presents a number of different problems in practise.

If one wishes to substitute a flat disc for a cone or dome, the inherent stiffness which the latter shape provides is lost and the necessary stiffness must be achieved in other ways. Clearly, a flat sheet of material will not prove stiff enough unless it is thick and heavy or is made of some naturally light and stiff substance. It is also desirable that the diaphragm material should possess a degree of self damping so that such resonances that might occur will not be pronounced, i.e. of high Q. With modern materials science it may be possible to produce a plastics materials which is very light, very stiff and non resonant, but at present none is known.

Foamed plastics all exhibit unfavourable acoustic transmission characteristics. For instance, polystyrene is a good diaphragm material for frequencies up to about 1 500 Hz, but above this it absorbs energy very rapidly. Light metals such as titanium give a very 'perky' response.

The lightest material occurring naturally is Balsa wood and its strength in respect of its weight is extremely favourable. It is believed that experiments with plain Balsa wood sheet for loudspeaker diaphragms may have been attempted many years ago but the material was rejected because, inter alia, Balsa wood is very hygroscopic and it therefore warps easily with changes of moisture content. Another reason why Balsa wood was rejected previously was possibly that sound transmission along its grain is found to be both rapid and efficient whereas across its grain sound travels more slowly and is soon absorbed. Thus, the use of plain Balsa wood sheet can result in an uneven transmission across the face of the diaphragm with consequential poor sound distribution.

In the diaphragm of a speaker in accordance with the present invention, several layers of Balsa wood are arranged to form a laminated structure hermetically sealed within a plastics envelope Figs. 1 to 3 illustrate one method of fabricating such a structure using, in this embodiment, three layers of Balsa.

The three Balsa wood sheets 1 Oa, 1 0b, 1 Oc are cut from Balsa wood stock and are shaped as shown in Fig. 1, with their grain lying along one axis. The sheets are stacked one on top of the next such that the grain of the middle sheet 1 Ob lies, in this embodiment, at right angles to that of the outer sheets 1 0a, 1 Oc. A quick-hardening epoxy resin glue is used to bond the three sheets 1 Oa, 1 Ob, 1 Oc together. An epoxy resin glue is used because, when used in its very viscous form, it does not significantly soak into the Balsa wood and thus there is little expansion and contraction of the sheets during bonding. This also reduces warping.

The assembly of Balsa sheets is held flat while the adhesive is hardening to prevent warping.

When the adhesive has hardened, both surfaces of the Balsa wood assembly are covered with a thin, light, self-adhesive, plastics film such as "Solar film" (Trade Mark). This is ironed onto the surface of the Balsa wood with a "cool" flat-iron or a heated press which activates the adhesive.

As indicated diagrammatically in Fig. 2, on the rear side of the diaphragm (top side as viewed in Fig. 2) the film consists of a circular sheet 1 2 of film surrounded by a narrow ring 14 of film so as to leave an annular ring 1 6 of Balsa wood exposed. On the front side of the diaphragm, the whole surface is covered by a sheet 18 of film which is taken over the peripheral edge of the Balsa wood stack and terminates a few mms inside the peripheral edge of the rear surface. In practice, the composite film 12, 1 4, 1 8 lies closely against the Balsa wood surfaces to tightly enclose same.

As indicated in Fig. 3, the exposed ring 1 6 of Balsa is formed to enable a cylindrical coil former 20 to be adhered directly to the Balsa wood by means of adhesive 22, such as an epoxy resin, which covers the exposed ring 1 6 whereby to complete the hermetic sealing of the Balsa wood. The cylindrical former 20 may be made of paper and supports a coil 24.

It is essential that the coil 24 and its former 20 should remain accurately circular and should not deform from this shape under the thermal and mechanical stresses of operation. Moreover, should the diphragm suffer even the slightest deformation (warping) it must not be allowed to deform the coil 24. To this end, the diaphragm/coil assembly is fitted with a rear suspension formed by a cambric "spider" 26 which is trimmed into a cruciform shape but a ring of cambric is left at the centre and is firmly glued to the exterior of the coil former 20 at 28.

This is found to provide adequate restraint in this embodiment.

It is a preferred feature of a loudspeaker constructed in accordance with the invention that the coil diameter is large in relation to the size of the diaphragm. This feature is adopted for the following reasons.

In general, as sound energy travels outwards from the edge of a coil through the diaphragm material to the outer edge of the diaphgram, two effects occur. Firstly, the upper frequencies are rapidly absorbed in the diaphragm material, leaving the outer areas with only low frequency energy to transmit. Secondly, as discussed above, there is a change of phase between the inner and outer edges of the diaphragm. Both of these phenomena can be minimised by reducing the distance between the coil and the outer edge of the diaphragm. Thus, there is advantage in making the coil larger and the diaphragm smaller.

Also, when the energy reaches the outer edge of the diaphragm, what is left of it is reflected back and forms standing waves. In speakers of conventional dimensions, this happens at frequencies where they are least desirable i.e. in the range 1000 to 3000 Hz. There is a great deal of musical energy and the ear is most sensitive in this range.

Furthermore, as high frequencies are only transmitted by the area of the diaphragm close to the coil, the high frequency performance is better sustained by increasing this effective area which results from having a larger coil.

Another feature of the present diaphragm is that it need not be of circular shape and indeed, theoretically at least, is preferably not circular, as shown by way of example, in Figs. 4 to 7 discussed further below. An advantage of this shape is that the distance from the coil to the outer diaphragm edge is non-constant around the coil so that the single, pronounced standing wave that would result from a constant distance is replaced by a plurality of standing waves at different frequencies, thus lowering the Q of the associated resonances.

A still further advantage of using a relatively large coil diameter can be appreciated by considering the stresses incurred in moving coil speaker diaphragms. All force bearing elements and especially vibratory ones are subject to tensional, compressional, bending and sheer forces. Such forces cause distortion of size and shape and in a loudspeaker these can result in distortions of the emanated sound. Such distortions cannot be entirely eliminated but they need to be minimised.

A small coil driving a large cone or diaphragm suffers the worst possible deformations from the latter stresses. Bending is magnified by having a long lever arm between the point of application of the force to the diaphragm and the location of support for the diaphragm at its outer edge. What is less evident is that with a small coil, the driving force is applied along a relatively short coil edge and the crushing stress (sheer) is distributed over a small quantity of cone material. As the coil size is increased, both bending and sheet stresses are reduced and this reduces deformations of shape.

It is noteworthy here that the deformations in a cone are different in the two half cycles of a to and fro vibratory movement, whereas in a flat diaphragm the displacements are symmetrical.

The approximate relative dimensions of four examples of the diaphragm and coil of loudspeakers embodying the present invention are shown diagrammatically in Figs. 4, 5, 6 and 7.

In each case the diaphragm 30 is attached to a coil former 32 carrying a coil 34.

The configurations of Figs. 5 to 7 have much to recommend them as discussed below but having a non-circular coil and hence a non-circular magnet structure, they are more difficult to construct accurately than the configuration of Fig.

4 which has a circular coil former 32 and coil 34 nut non-circular diaphragm 30 of the shape used in the example described in connection with Figs.

1 to 3 above.

The configuration of Fig. 4, while theoretically less favourable than that of Figs. 5 to 7 in that its greater lateral width should lead to inferior lateral polar distribution of high frequency sound and hence weaker stereo imaging, doey not in reality prove significantly inferior in these respects and the relative ease of its construction is attractive.

It appears that so long as the lateral dimension is kept smaller than the dimension which separates the listener's ears, i.e. less than bout 5", the stereo image remains good.

The configuration shown in Fig. 6, while not having the production advantages of a circular coil, nevertheless imitates that shown in Fig. 4 in that the dimension from coil to diaphgram edge is non-constant.

Fig. 7 shows a further variation in which, not only is satisfactory coil winding facilitated but both the lateral dimension across the coil and from coil to diaphragm edge is made nonconstant and hence the Q of these dimensional modes is reduced.

It should be noted that longitudinal stability and stiffness of the diaphragms in the configurations of Figs. 5, 6 and 7 is enhanced by the ribs forming the channel-shaped coil former 32 and which carry the coil. These ribs must be made stiff as is possible, commensurate with minimal mass. A suitable material is, for example, carbon fibre laminate or SRB paper. Curving of these ribs as in the case of the configuration of Fig. 7 tends to add to their stiffness and also to the torsional stiffness of the whole diaphragm assembly.

Figs. 8 to 11 shows examples of how the diaphragm/former/coil configurations of Figs. 4 to 7 can be incorporated in practical loudspeaker structures.

Fig. 8 shows a loudspeaker incorporating the configuration of Fig. 4. The outer periphery of the diaphragm 30 is supported in a chassis 38 in a conventional manner by means of a flexible surround 40, made of a suitable flexible rubber or plastics material. Mild steel pole pieces 42, 44 and a mild steel tube 46 are supported on the chassis 38 around a toroidal magnet 48 by spacers 50 so as to define a flux path containing an annular air gap into which the coil 34 extends.

To ensure that the flat diaphragm 30 is retained in positions parallel to its fixed assembly both when at rest and when in movement, a flexible rear suspension (spider) 52 is provided.

This can be located, as shown in Fig. 9, at the rear end of the coil 34, behind the magnetic gap and its pole plates. This is done in order to increase the effective lever arm between front and rear suspensions which would otherwise be shortened when changing from a conical to a flat diaphragm.

Where, however, the length of the coil and its support are sufficient to provide an adequate lever arm, the rear suspension can be positioned in front of the pole plate 42 as shown in Fig. 8.

Alternatively, where only a small excursion of the diaphragm is required (e.g. where the bass response of the unit is restricted) a short coil 34 and former 32 can be used. In this event, the front suspension 40 can be made sufficiently stiff to prevent "rocking" and the rear suspension 52 can be omitted altogether.

In Fig. 10 and 11, similar parts have been given the same reference numerals as used in Fig. 8.

Whatever the shape of the diaphragm 30, experiment suggest that a minimum surface area of about 80 sq.cms. may be necessary in order to produce enough acoustic power for loud domestic listening without undue diaphragm excursion.

It should be emphasised that the invention is not iimited to diaphragms having the peripheral configurations illustrated nor to the particular type of plastics film described. The plastics film need not be self adhesive and in other embodiments can be attached by additional adhesive. Good results have been obtained with diaphragms formed with three Balsa laminations, each 1/32" (0.79 mm) thick. However, four, five or six laminations of thinner Balsa can work equally as well or better.

Claims (13)

Claims

1. A loudspeaker comprising a coil immersed in a magnetic field and centrally attached to a flat diaphragm made of laminated Balsa wood sealed within a thin plastics envelope.

2. A loudspeaker as claimed in claim 1, wherein successive Balsa wood laminations are arranged so that the orientations of their respective grains are distributed evenly around an imaginary circle, i.e. with three laminations the grains are set at 1 200 to one another, with four laminations they are set at 900 etc.

3. A loudspeaker as claimed in claim 1 or 2, wherein the diameter or width of the coil is large in relation to the diameter or width of the diaphragm.

4. A loudspaaker as claimed in claim 3, wherein the diameter or width of the coil is approximately 75% of the diameter or width of the diaphragm.

5. A loudspeaker as claimed in any of claims 1 to 4, wherein the diaphragm and the coil are shaped such that the distance from the coil to the outer edge of the diaphragm is not constant around the diaphragm.

6. A loudspeaker as claimed in any of claims 1 to 5, wherein the coil is carried by a coil former made of carbon fibre laiminate or SRB paper and having a generally U-shaped transverse sectional configuration.

7. A loudspeaker comprising a coil immersed in a magnetic field and centrally attached to a flat diaphragm made of laminated Balsa wood sheets which are adhered together and enclosed by means of a plastics film.

8. A loudspeaker as claimed in claim 7, in which there are three Balsa wood sheets, the intermediate one of which is disposed with its grain running substantially at right angles to the grains of the outer sheets.

9. A loudspeaker as claimed in claim 7 or 8, wherein the coil and coil former are supported by a cambric spider.

10. A loudspeaker as claimed in claim 9, in which the cambric spider is of cruciform configuration.

11. A method of forming a loudspeaker, comprising forming a stack of flat sheets of Balsa wood of similar peripheral shape, enclosing the Balsa wood stack in a film of plastics material and attaching centrally to the Balsa wood a coil former carrying a coil.

1 2. A loudspeaker having a diaphragm, coil and coil former constructed substantially as hereinbefore described with reference to and as illustrated in Fig. 4, or Fig. 5 or Fig. 6 or Fig. 7 of the accompanying drawings.

13. A loudspeaker substantially as hereinbefore described with reference to and as illustrated in Figs. 8 and 9, or in Figs. 10 and 11 of the accompanying drawings.

1 4. A loudspeaker having a diaphragm constructed substantially as hereinbefore described with reference to and as illustrated in Figs. 1 to 3 of the accompanying drawings.