GB2297218A - A low frequency loudspeaker voice coil with at least eight layers of wire - Google Patents

Abstract

A voice coil element for a loudspeaker, especially for bass reproduction, comprises a former 10 on which at least eight layers are wound to form the voice coil, the coil turns 14 and layers being bonded by thermally activated adhesive. The invention also provides a method of producing the voice coil element.

Description

Improvements in Low Frequency Loudspeaker Drive Units This invention relates generally to loudspeaker drive units and more especially to the voice coil element within low frequency drive units.

A conventional loudspeaker system, for example bass reflex, transmission line or sealed box, employs a drive unit which includes a diaphragm to produce the audible frequency range.

Such a drive unit comprises a diaphragm fixed to a voice coil former to which is bonded, around its diameter, a coil of wire with normally two and exceptionally four layers.

In use, the voice coil is mechanically suspended within a constant magnetic field which, when an alternating current (A.C.) is applied to the voice coil wire, induces a forward or backward displacement of the voice coil along its axis and hence the diaphragm, causing a movement of air.

In a perfect drive unit the frequency of the applied A.C.

would be exactly duplicated by the displacement of the diaphragm throughout the audible frequency range.

However, due to physical constraints this is not possible, so conventional loudspeakers consist of a multiplicity of drive units, each reproducing the A.C. over the frequency for which it works efficiently. The split of frequencies is determined by a series of electronic filters called a crossover and the physical restraints of the drive units.

Unfortunately, interaction between different drive units over their overlapping crossover regions, where two or more drive units may be operating at once, can cause sound wave cancellation and phase shifting, leading to a general degradation of sound quality. This can be partly improved by clever design of the crossover, but interaction between the electronic components within the crossover can also lead to distortion and phase changes.

A primary object of the present invention is to reduce these electronic and physical distortions.

According to one aspect of the present invention, there is provided a voice coil element comprising a voice coil former around the circumference of which is wound at least eight layers of wire bonded to itself and to the voice coil former by means of a thermally activated adhesive.

The voice coil element in accordance with the invention has the advantage that it limits the upper frequency response of the drive unit and accords an earlier roll off of frequency response than can be achieved through passive electronic crossovers.

The arrangement also has the advantage that it reduces the number of electronic crossover components thereby reducing distortion and cost.

Some high cost domestic Hi-Fi and professional public address systems use a complex active filter system to achieve similar results to that produced by the present invention, but at much higher cost.

An earlier roll off of frequency also limits the overlap period of two drive units over the crossover frequency range, lessening wave physical wave cancellation and thus offering a further advantage.

The voice coil former may be made of aluminium, paper, Kapton (Trade Mark), Nomex (Trade Mark) or other suitable plastics material.

The wire used in the voice coil element may be of copper, aluminium, silver or suitable conductive alloys.

According to a further aspect of the invention, there is provided a voice coil element as above defined, produced by use of a precision mandril and sleeve.

A reason for the uniqueness of the eight or more layer coil element is the difficulty in winding a coil with more than four layers without kinking, physical distortion or layer separation due to inadequate bonding.

In the method of production in accordance with this aspect of the invention, the wire is brought onto the mandril through a tunnel or slot in the sleeve in such a way as to prevent the wire from kinking on each subsequent wound layer. This keeps the layers of coil turns tight together, enabling the bonding of many layers. Additionally, a plug and collar exert a pressure along the axis of the voice coil former during the bonding process.

The present invention therefore also extends to a precision mandril and sleeve designed to ensure exact manufacturing of the voice coil element having eight or more coil layers as hereinbefore defined.

An embodiment of the voice coil element and means of manufacture thereof in accordance with the invention are exemplified in the following description, making reference to the accompanying drawings, in which: Figure 1 is a side elevational view of an eight layer voice coil element; Figure 2 is a side elevational view of a two layer coil element; Figure 3 is a front end view of an eight layer coil element; Figure 4 is a cross sectional view of a sleeve and mandril; and Figure 5 is a cross sectional view of an alternative sleeve and mandril.

Referring to the drawings, the illustrated voice coil former (Figure 1) has an aluminium, Kapton, plastics Nomex, paper or suitable material body in the shape of a tube 10, the diameter and length of which are variable according to requirements.

The former has a long front section 11 which is fixed to the diaphragm and a curled over rear section 12.

The former carries a coil of eight or more layers of copper, aluminium, silver or other suitable conductive wire wound around its diameter, bonded to itself and to the former with thermally activated adhesive. The coil turns 14 are wound so as to be all fully bonded with each other and not to be kinked or physically misaligned. The ends of the wire 15 are led away at the same forward end of the coil Figure 2 shows a typical conventional two layer voice coil and former. The thickness difference of the many layered coil of Figure 3 can clearly be seen, and the difficulty in winding the coil of Figure 3 appreciated.

A precision mandril 26 and sleeve 27 (Figure 4) is used accurately to wind the coil, and to provide a constant axial pressure during the curing of the adhesive bond.

The wire 16 is fed through a tunnel 17 in the sleeve 27 and then wrapped around a securing peg 18 or similar restraining device. An alternative is shown in Figure 5 using a slot 19. The wire is then wound onto the periphery of the former 20 building up the layers as shown by the arrow 21.

The first layer is bonded to the former 20, the successive layers to each other, as the coil is built up. Bonding is effected by a thermally activated adhesive, and a removable core plug 22 and collar 23 allow axial pressure to be applied to the coil, more especially while curing of the adhesive takes place.

Various further modifications of the above described arrangement are possible within the scope of the invention hereinbefore defined.

Claims (7)

Claims

1. A voice coil element comprising a voice coil former and a voice coil comprising, around the circumference of the former, at least eight layers of wire bonded to itself and to the former by means of a thermally activated adhesive.

2. A voice coil element according to claim 1, wherein the former is made of aluminium, paper, or plastics material.

3. A voice coil element according to claim 1 or claim 2, wherein the voice coil is wound with wire of copper, aluminium, silver or a conductive alloy.

4. A method of producing the voice coil element of any of claims 1 to 3, according to which the former is located in a mandril and surrounded by an apertured sleeve behind the region of the former on which the coil is to be wound, and an end portion of the wire is anchored to the sleeve and extends from the anchorage through the aperture in the sleeve to the region at which the coil is to be wound, thereby to prevent kinking of the wire as it is reversed from the end of one layer to the beginning of the next.

5. A method as claimed in claim 4, according to which a collar projects over the end of the former remote from the sleeve to exert axial pressure on the coil, at least during curing of the thermally activated adhesive.

6. A voice coil element substantially as hereinbefore described with reference to Figures 1 and 3 of the accompanying drawings.

7. A method of producing a voice coil element substantially as hereinbefore described with reference to Figures 4 or 5 of the accompanying drawings.