GB2348563A

GB2348563A - Snap-acting over-centre springs assist loudspeaker diaphragm motion

Info

Publication number: GB2348563A
Application number: GB9907650A
Authority: GB
Inventors: Peter Alexander Fryer
Original assignee: B&W Loudspeakers Ltd
Current assignee: B&W Loudspeakers Ltd
Priority date: 1999-04-01
Filing date: 1999-04-01
Publication date: 2000-10-04
Anticipated expiration: 2019-04-01
Also published as: GB9907650D0; GB2348563B

Abstract

A loudspeaker drive unit comprises a diaphragm mounted so as to be movable back and forth, a voice coil connected to the diaphragm, and a magnetic system to provide a magnetic field across the winding of the voice coil, and a mechanical spring arrangement 1 is provided to assist the displacement of the diaphragm in both the back and forth directions of movement. The mechanical spring arrangement comprises an over-centre spring arrangement having an unstable neutral position between two stable end positions and comprises a multiplicity of snap-acting elements 2. The snap-acting elements 2 each comprise a respective bowed strip of resilient material and the bowed strips are arranged next to each other with the directions of bow alternating. The snap-acting elements 2 are arranged radially about a common centre and extend radially inwards from an outer common ring member 3 to an inner common hub member 4. Means including a sensor to sense the position of the diaphragm and including a negative feedback control loop to adjust the neutral position automatically can be provided to adjust the neutral position to compensate for changes in the internal pressure in the enclosure relative to the external air pressure.

Description

LOUDSPEAKER DRIVE UNITS AND LOUDSPEAKER SYSTEMS This invention relates to loudspeaker drive units and to loudspeaker systems.

Patent specification GB 2 083 974 A discusses the problems of non-linearity in the relationship between the forces acting upon the voice coil of a loudspeaker drive unit and the displacement of the voice coil from its rest position. The effects of the compression of the air in the magnet enclosure and the rarefaction of air trapped beneath the diaphragm are discussed. Although not discussed explicitly in that specification, it is also true that the compression and rarefaction of air enclosed in the whole of the enclosure in which the loudspeaker drive unit is mounted creates problems; the air acts as a spring resisting the movement of the diaphragm.

Furthermore, it is well known to the designers of loudspeaker systems that, other factors being equal, the bigger is the enclosure, the better are the bass response and the sensitivity of the speaker. Unfortunately, however, very large enclosures are expensive to construct and, for domestic usage, unpopular on the grounds of weight, aesthetic appearance and space requirement.

The proposal contained in the above-mentioned specification of superimposing positive feedback compensation on the acoustic signal fed to the voice coil is unsatisfactory when a small enclosure is used because of problems of dissipating the heat created within a small confine by the current flowing as a result of the compensation signal.

The object of the invention is to provide a loudspeaker drive unit allowing reduction of the disadvantageous effect of the springiness of the air in a loudspeaker enclosure whilst at the same time reducing or avoiding problems of heat dissipation.

The present invention provides a loudspeaker drive unit comprising a diaphragm mounted so as to be movable back and forth, a voice coil connected to the diaphragm, and a magnetic system to provide a magnetic field across the winding of the voice coil, wherein a mechanical spring arrangement is provided to assist the displacement of the diaphragm in both the back and forth directions of movement.

Because the mechanical spring arrangement assists the displacement of the diaphragm in both the back and forth directions of movement it counteracts the spring effect of the air in an enclosure in which the loudspeaker drive unit is mounted, the air tending to impede the movement of the diaphragm. In effect, the spring constant of the air is reduced by this means which might, indeed, be considered as adding a negative spring effect to the loudspeaker drive unit. At the same time, because the arrangement assisting the movement of the diaphragm is a mechanical spring arrangement, the undesirable heating effects caused by the flow of an electrical current is avoided.

The mechanical spring arrangement advantageously comprises an over-centre spring arrangement having an unstable neutral position between two stable end positions. Such an arrangement can be produced very simply and the tendency of the over-centre arrangement to move towards the other of its stable end positions once it has been moved beyond the unstable neutral position from one of its stable end positions provides the desired assistance of the movement of the diaphragm.

It is preferred that the over-centre spring arrangement comprises a multiplicity of snap-acting elements. The use of a multiplicity of snap-acting elements provides a simple way of achieving an over-centre spring arrangement with particularly suitable characteristics for use in a loudspeaker drive unit. Surprisingly, snap-acting elements are capable of having a sufficiently linear spring characteristic to provide overall an enhancement of the sound from a loudspeaker drive unit rather than, as might have been supposed, adding distortion to it.

The snap-acting elements may each comprise a respective bowed strip of resilient material. Such a construction is particularly simple and easy to manufacture.

Stiff plastics material or spring steel may, for example, be used for making the snap-acting elements.

Preferably, the bowed strips are arranged next to each other with the directions of bow alternating. Such an arrangement has very suitable characteristics for use in a loudspeaker drive unit.

Advantageously, the multiplicity of snap-acting elements are arranged radially about a common centre. Such an arrangement is readily applicable to loudspeaker drive units since they almost invariably have voice coils of circular cross-section.

The multiplicity of snap-acting elements may extend radially inwards from an outer common ring member to an inner common hub member. That arrangement provides a simple way of mounting the snap-acting elements and connecting them to the diaphragm.

Preferably, the radially outer ends of the snap-acting elements are fixed in position and their radially inner ends are connected to the diaphragm.

The diaphragm may be cone-shaped and the connection to the snap-acting elements provided at the narrow end of the cone. That is a particularly simple arrangement.

A loudspeaker system according to the invention comprises a loudspeaker drive unit as claimed in any preceding claim mounted in a loudspeaker enclosure.

Advantageously, the interior of the enclosure is sealed with respect to the surroundings of the enclosure and the pressure of the air within the enclosure is such that the over-centre spring arrangement is substantially at its neutral position when the loudspeaker has no drive signal. In such an arrangement, the"air spring"has a neutral position corresponding substantially with that of the over-centre spring arrangement so that when the air is being compressed (and cannot escape from the enclosure) the mechanical spring arrangement aids the compression, and when the air is being rarefied (and no more can be drawn into the enclosure) the mechanical spring arrangement aids the rarefaction.

It is not, however, to be assumed that the invention is applicable only to sealed enclosures. The mechanical spring arrangement can be used simply to reduce the spring constants of a given loudspeaker drive unit and moreover may provide some benefit even in a vented enclosure.

When a sealed enclosure is used, changes in atmospheric pressure will cause the neutral position of the over-centre spring arrangement to become displaced with respect to the neutral position of the"air spring". Means may be provided to adjust the neutral position to compensate for changes in the internal pressure in the enclosure relative to the external air pressure. If desired, a small leak may be provided in the enclosure (additionally or instead of the last mentioned means) to enable the pressure within the enclosure to follow changes in atmospheric pressure.

The means to adjust the neutral position comprise means to generate a biasing force electromagnetically by interaction with the magnetic field of the magnetic system. That is a particularly simple way of providing the adjustment.

The means to generate a biasing force electromagnetically may comprise means to supply a d. c. or very low frequency bias current to the voice coil.

The means to adjust the neutral position may include a sensor to sense the position of the diaphragm and means including a negative feedback control loop to adjust the neutral position automatically. By that means the neutral position of the over-centre spring arrangement can be adjusted automatically and, if desired, negative positional feedback used to reduce distortion at audible frequencies Even though electromagnetic means be used for these purposes, the heat generation will generally be less than when a positive feedback electromagnetic system such as described in patent specification GB 2 083 974A is used.

If desired, an electromagnetic positive feedback system may provided to supplement the mechanical spring arrangement so as to obtain the benefits of both an electromagnetic and a mechanical system for combating the effects of the springiness of air in a loudspeaker enclosure.

Ways of carrying out the invention will now be described, by way of example only, with reference to the accompanying drawings, in which: Figure 1 is a diagrammatic representation of a mechanical spring arrangement for use in the invention; Figure 2 is a diagrammatic representation of one element of the mechanical spring arrangement; Figure 3 is a diagrammatic representation of an adjacent element of the mechanical spring arrangment; and Figure 4 is a diagrammatic representation of an over-centre spring arrangement with improved linearity of spring characteristic.

Referring to the accompanying drawings, a mechanical spring arrangement 1 is shown for addition to a loudspeaker drive unit (not shown) of otherwise conventional form.

The loudspeaker drive unit may, for example, be as disclosed in patent specification GB 2 083 974A and thus comprise a diaphragm mounted so as to be movable back and forth, a voice coil connected to the diaphragm, and a magnetic system to provide a magnetic field across the winding of the voice coil. The mechanical spring arrangement 1 is used to assist the displacement of the diaphragm in both the back and forth directions of movement.

The mechanical spring arrangement 1 is an over-centre spring arrangement having an unstable neutral position between two stable end positions and comprises a multiplicity of snap-acting elements 2. The snap-acting elements 2 each comprise a respective bowed strip of resilient material, the bowed strips being arranged next to each other with the directions of bow alternating. That is shown in the drawings by the use of the signs"."and"x". As shown in Figures 2 and 3, the snap-acting elements 2 have"waists"5 at their ends to facilitate their movement. The spring arrangement 1 is made by deforming a basic cartwheel shape cut out of a sheet or resilient plastics or metal material.

The multiplicity of snap-acting elements 2 are arranged radially about a common centre, the elements extend radially inwards from an outer common ring member 3 to an inner common hub member 4. The spring arrangement 1 is applied to the loudspeaker drive unit by the radially outer ends of the snap-acting elements being fixed in position and having their radially inner ends connected to the diaphragm by means of the hub member 4. For example, in the loudspeaker drive unit shown in patent specification GB 2 083 974A the diaphragm is cone-shaped and the connection to the snap-acting elements can be made at the narrow end of the cone.

The loudspeaker drive unit with the mechanical spring arrangement 1 is mounted in conventional manner in a loudspeaker enclosure (not shown) of known construction. The interior of the enclosure is sealed with respect to the surroundings of the enclosure and the pressure of the air within the enclosure is such that the overcentre spring arrangement is substantially at its neutral position when the loudspeaker has no drive signal. That result can be achieved by pumping air into the enclosure or extracting air from it (depending on which of the two stable states the over-centre spring arrangement was in initially) until the over-centre spring arrangement is just about to move over-centre, the enclosure then being sealed.

Means (not shown) are provided to adjust the neutral position to compensate for changes in the internal pressure in the enclosure relative to the external air pressure and comprise means to generate a biasing force electromagnetically by interaction with the magnetic field of the magnetic system. The means to generate a biasing force electromagnetically comprises means to supply a d. c. or very low frequency bias current to the voice coil.

The means to adjust the neutral position includes a sensor to sense the position of the diaphragm and means including a negative feedback control loop to adjust the neutral position automatically.

An electromagnetic positive feedback system can, if desired, be provided to supplement the mechanical spring arrangement.

The technology disclosed in patent specification GB 2 083 974 can be readily adapted to provide the means described in the three preceding paragraphs.

Figure 4 shows an alternative form of over-centre spring arrangement 10 comprising two springs 12 and 14, each compressed in a respective tube 16,18 and arranged opposite to each other, the remote ends of the tubes being closed. Each of the springs 16,18 is connected to a respective lever arm 20,22 by a respective articulation 24,26. The distal ends of the lever arms 20,22 are connection to each other and to one end of a rod 28 by an articulation 30 and the other end of the rod 28 is connected to diaphragm 32 of the loudspeaker drive unit. The position of the rod 28 and the lever arms 20 and 22 on going over centre is shown in broken outline.

With suitable dimensioning of the parameters of the arrangement 10, a substantially linear spring characteristic can be achieved.

Claims

CLAIMS:

1. A loudspeaker drive unit comprising a diaphragm mounted so as to be movable back and forth, a voice coil connected to the diaphragm, and a magnetic system to provide a magnetic field across the winding of the voice coil, wherein a mechanical spring arrangement is provided to assist the displacement of the diaphragm in both the back and forth directions of movement.

2. A loudspeaker drive unit as claimed in claim 1, wherein the mechanical spring arrangement comprises an over-centre spring arrangement having an unstable neutral position between two stable end positions.

3. A loudspeaker drive unit as claimed in claim 2, wherein the over-centre spring arrangement comprises a multiplicity of snap-acting elements.

4. A loudspeaker drive unit as claimed in claim 3, wherein the snap-acting elements each comprise a respective bowed strip of resilient material.

5. A loudspeaker drive unit as claimed in claim 4, wherein the bowed strips are arranged next to each other with the directions of bow alternating.

4. A loudspeaker drive unit as claimed in claim 3, wherein the multiplicity of snap-acting elements are arranged radially about a common centre.

5. A loudspeaker drive unit as claimed in claim 3, wherein the multiplicity of snap-acting elements extend radially inwards from an outer common ring member to an inner common hub member.

6. A loudspeaker drive unit as claimed in claim 4 or claim 5, wherein the radially outer ends of the snap-acting elements are fixed in position and their radially inner ends are connected to the diaphragm.

7. A loudspeaker drive unit as claimed in claim 6, wherein the diaphragm is cone-shaped and the connection to the snap-acting elements is provided at the narrow end of the cone.

8. A loudspeaker system comprising a loudspeaker drive unit as claimed in any preceding claim mounted in a loudspeaker enclosure.

9. A loudspeaker system as claimed in claim 8 when dependent on claim 2, wherein the interior of the enclosure is sealed with respect to the surroundings of the enclosure and the pressure of the air within the enclosure is such that the over-centre spring arrangement is substantially at its neutral position when the loudspeaker has no drive signal.

10. A loudspeaker system as claimed in claim 9, wherein means are provided to adjust the neutral position to compensate for changes in the internal pressure in the enclosure relative to the external air pressure.

11. A loudspeaker system as claimed in claim 9, wherein the means to adjust the neutral position comprise means to generate a biasing force electromagnetically by interaction with the magnetic field of the magnetic system.

12. A loudspeaker system as claimed in claim 11, wherein the means to generate a biasing force electromagnetically comprises means to supply a d. c. or very low frequency bias current to the voice coil.

13. A loudspeaker system as claimed in claim 11, wherein the means to adjust the neutral position includes a sensor to sense the position of the diaphragm and means including a negative feedback control loop to adjust the neutral position automatically.

14. A loudspeaker drive unit as claimed in any of claims 1 to 8 or a loudspeaker system as claimed in any of claims 9 to 13, wherein an electromagnetic positive feedback system is provided to supplement the mechanical spring arrangement.