GB2205715A - Loud Speaker - Google Patents

Abstract

A moving coil loudspeaker has two ring magnets 14 and 16 arranged back to back, e.g. with their North poles adjacent, but separated by a plate 20 having a central core 18 extending into the hole on one of the magnets to form one side of an annular voice-coil gap 34, and a magnetic path connecting the two South poles with the other side of the gap 34. <IMAGE>

Description

TITLE: Loudspeaker This invention relates to a loudspeaker, and in particular to a magnet system for a moving coil loudspeaker.

In a moving coil loudspeaker, a voice coil attached to the loudspeaker diaphragm moves in a gap, and a permanent magnet is arranged to produce a magnetic field across the gap.

The sound pressure level depends upon the magnitude of the magnetic flux across the gap. Conventionally the level of flux across the gap can be increased by increasing the physical size of the permanent magnet.

According to the present invention, there is provided a magnet system for a moving coil loudspeaker, the system comprising a first permanent ring magnet, a bottom plate held against one face of the magnet with a central core extending through the hole in the magnet so as to leave an annular gap, a second permanent ring magnet held against the opposite side of the bottom plate to that against which the first magnet is held with the adjacent magnetic poles of the magnet being like poles, and a connection forming a magnetic path between the remote, like poles of the magnets.

The use of two ring magnets arranged in this way, with their adjacent and their remote pairs of like poles connected by a magnetic path produces a magnetic flux level in the voice coil gap of the system at a very much higher level than would be possible using a single magnet with a weight equal to the combined weights of the two ring magnets in accordance with the invention.

The magnets and the bottom plate are preferably enclosed in a casing of magnetic material which provides the magnetic path between the remote poles of the magnets. The casing may be a continuous casing, which provides a shielding effect around the magnet system shielding the magnets from stray flux and extraneous magnetic fields.

The voice coil gap is preferably formed between the central core and an aperture in an end face of the casing through which the core projects.

The magnets preferably have their North poles adjacent one another, and their South poles remote from one another.

The magnets are preferably held against the bottom plate by means of adhesive, and the casing is preferably also held onto the magnets by means of adhesive. The casing may be a two-part casing with each part being in the form of a cup, one part having a central aperture to form the voice coil gap with the central core.

The magnets are preferably ferrite ring magnets.

The invention also provides a loudspeaker having a magnet system with the features set forth above.

The invention will now be further described, by way of example, with reference to the accompanying drawings in which: Figure 1 is a cross section through part of a loudspeaker with a magnet system in accordance with the invention; and Figure 2 is a similar cross section showing the path of the lines of magnetic flux.

The drawings show a magnet system 10 and the inner end of a diaphragm 12. The magnet system comprises a first annular magnet 14 and a second annular or ring magnet 16. Both these magnets are permanent ferrite magnets arranged with their magnetic N-S poles as shown. A magnetic core 18 includes a bottom plate 20 against which the North pole faces of both magnets 14 and 16 are abutted. The magnets are secured to the bottom plate by adhesive. The core 18 then projects up through the central aperture 22 of the magnet 14, so that there is a gap between the aperture 22 and the wall of the core 18. The core itself and the bottom plate 20 are made of magnetic material so that they provide a conducting path for lines of magnetic force.

This arrangement of the magnets 14 and 16 is housed inside a casing 24 also of magnetic material such as iron or steel.

This casing is made of an upper section 26 and a lower section 28 which meet at a joint 30. The lower section 28 is in contact with the South-pole face of the second magnet 16, and the upper section 26 is in contact with the South-pole face of the first magnet 14. The two sections 26 and 28 are connected to one another at 30, and the upper face of the section 26 has an aperture 32 in which the top end of the core 18 is received. A magnetic gap 34 is formed between the aperture 32 and the core 18, and a voice coil 36 of the loudspeaker sits in this gap. The coil 36 is connected to the bottom end of the diaphragm 12 and when an audio signal is applied to the turns of the voice coil, movement of the coil in the magnetic field across the gap . > r3t occurs and produces movement of the diaphragm resulting in audible sound.

The magnets 14 and 16 are glued to the opposite faces of the bottom plate 20 and also to the internal faces of the casing sections 26 and 28. Note that the bottom plate 20 does not extend as far as the periphery of the magnets 14 and 16, and also that the casing 24 is in contact with the pole faces of the magnets 14 and 16, but not with their edge surfaces.

Figure 2 shows the path followed by lines of magnetic flux, and it will be seen that the flux resulting from the second magnet 16 reinforces the flux from the first magnet 14 and produces a high level of flux across the gap 34.

It has surprisingly been found that the flux in the gap achievable by the use of two ring magnets as described is substantially greater than that which could be achieved using one single magnet having a weight equal to the combined weight of these two magnets. Thus, for the same output rating, a magnet system in accordance with the invention can be very much lighter in weight and very much cheaper than a conventional single magnet system.

Claims (11)

1 A magnet system for a moving coil loudspeaker, the system comprising a first permanent ring magnet, a bottom plate held against one face of the magnet with a central core extending through the hole in the magnet so as to form an annular gap, a second permanent ring magnet held against the opposite side of the bottom plate to that against which the first magnet is held with the adjacent magnetic poles of the magnets being like poles, and a connection forming a magnetic path between the remote, like poles of the magnets.

2 A magnet system as claimed in Claim 1 wherein the magnets and the bottom plate are enclosed in a casing of magnetic material.

3 A magnet system as claimed in Claim 2, wherein the casing is continuous around the magnets.

4 A magnet system as claimed in Claim 2 or Claim 3, wherein a voice coil gap is formed between the central pillar and an aperture in an end face of the casing.

5 A magnet system as claimed in any preceding claim, wherein the diameter of the bottom plate is less than the diameter of the magnets.

6 A magnet system as claimed in any preceding claim, wherein the magnetic path between the remote, like poles of the magnets extends from one pole to the other out of contact with any other part of the magnet system.

7 A magnet system as claimed in any preceding claim, wherein the magnets are glued to the bottom plate and to the magnetic path connections.

8 A magnet system as claimed in any preceding claim, wherein the magnets have their North poles adjacent one another and their South poles remote from one another.

9 A magnet system as claimed in any preceding claim, wherein the magnets are ferrite ring magnets.

10 A magnet system substantially as herein described with reference to the accompanying drawings.

11 A loudspeaker having a magnet system as claimed in any one of the preceding claims.