GB2368484A

GB2368484A - Distributed mode loudspeaker including pistonic diaphragm

Info

Publication number: GB2368484A
Application number: GB0026123A
Authority: GB
Inventors: Stuart Michael Nevill
Original assignee: B&W Loudspeakers Ltd
Current assignee: B&W Loudspeakers Ltd
Priority date: 2000-10-25
Filing date: 2000-10-25
Publication date: 2002-05-01
Anticipated expiration: 2020-10-25
Also published as: GB2368484B; GB0026123D0

Abstract

A distributed mode panel type loudspeaker 1' comprises a panel 4 and an electro-mechanical transducer 2, connected to the panel 4, arranged to produce the sound output of the loudspeaker by exciting bending waves in the panel (4). A diaphragm 30 is mounted within the area of the panel 4, is able to move relative the panel 4, and is arranged to be driven pistonically to improve the bass response of the loudspeaker.

Description

Distributed mode panel type loudspeakers

This invention relates to distributed mode panel type loudspeakers.

In recent years, distributed mode panel type loudspeakers have been extensively developed and marketed for hi fi sound reproduction. Such loudspeakers are described, for example, in patent specification WO 97/09842 and transducers for use within them are described, for example, in patent specification WO 98/34320. In those loudspeakers, the sound is produced by the excitation of bending waves in one or more panels by one or more transducers.

A known distributed mode panel type loudspeaker comprises a panel formed from a sandwich material made of two skins and a core material. Resonant bending waves are established in the panel by the vibration of an exciter in the form of an electro-mechanical transducer.

In order to improve the reproduction of bass sounds, such distributed mode panel type loudspeakers have been sold with conventional sub-woofer loudspeakers. The use of a separate sub-woofer loudspeaker takes up additional space, however, and adds to the expense of the loudspeaker system.

A proposal for driving a panel pistonically at bass frequencies is described in patent specification Wo 97/09849 but the arrangement is complicated and requires a subsidiary enclosure working as an air pump.

The bass response of a distributed mode panel type loudspeaker can be improved by using a larger panel but that is open to the objection of the loudspeaker's becoming too big for domestic use.

The problem underlying the invention is to improve the bass response of a distributed mode panel type loudspeaker of given size by simple and inexpensive means or, alternatively, to enable the same bass response to be achieved with a panel of smaller size.

The present invention provides a distributed mode panel type loudspeaker comprising a panel and an electro-mechanical transducer, connected to the panel, arranged to produce the sound output of the loudspeaker by exciting bending waves in the panel, characterized in that a diaphragm is mounted within the area of the panel, is able to move relative the panel, and is arranged to be driven pistonically to improve the bass response of the loudspeaker.

The invention is based on the realization that the bass response of the known distributed mode panel type loudspeakers was limited by low frequency cancellation occurring round the edges of the panel acting as a finite dipole radiator. Additionally, the invention is based on the realization that the use of such a known loudspeaker close to a boundary (for example, a wall of a room) increased the cancellation losses and further reduced efficiency at low frequencies.

The above-mentioned problem is solved by the diaphragm mounted within the area of the panel, able to move relative thereto, and arranged to be driven pistonically to improve the bass response of the loudspeaker.

The provision of the diaphragm within the area of the panel can be achieved very simply and inexpensively.

Moreover, the space required for a separate sub-woofer or sub-enclosure air pump is saved. The diaphragm is capable of radiating sound, at the frequencies where the panel radiation arising from bending wave excitation is deficient, in the manner of a plane piston mounted on an infinite baffle. Such radiation of sound is not limited by dipole cancellations.

Preferably, the diaphragm is connected to the transducer to be moved pistonically thereby. Such an arrangement is economical in that the expense of a separate transducer to drive the diaphragm is thereby avoided. Instead, a separate transducer may be provided to drive the diaphragm and that separate transducer may drive the diaphragm exclusively and be of the type used in conventional diaphragm type loudspeakers or it may be another transducer for exciting bending waves in the panel.

Advantageously, the panel and the diaphragm are connected to opposite sides of the transducer. By this means, the front of the transducer can be used for exciting bending waves in the panel and the rear of the transducer can be used for driving the diaphragm. The fact that the

diaphragm is then at the rear of the loudspeaker is relatively unimportant because the bass sounds which the diaphragm is to produce are not perceived by the ear as coming from a particular direction as are mid-band and high frequency sounds.

Advantageously, the diaphragm is connected to a magnet assembly of the transducer. The transducer of a distributed mode panel type loudspeaker is usually mounted on the rear face of the panel and comprises a compliantly mounted magnet assembly and voice coil rigidly attached to the panel. In use, the forces on the coil and magnet assembly are equal and the magnet assembly vibration is limited only by the inertia of its own mass and the stiffness of the compliant suspension. Relative to the panel, the acoustic output of the magnet assembly vibrations is normally very low owing to its small size but, in the invention, advantageous use is made of them to drive the diaphragm.

Preferably, the diaphragm is connected to the transducer by a compliant coupling. By that means, a roll-off of the frequency response can be achieved above the bass frequencies which it is desired to augment.

Instead or additionally, the diaphragm itself may be compliant.

Preferably, the periphery of the diaphragm is connected to the panel by a compliant surround.

Thus, the diaphragm and/or associated components may be arranged to achieve a natural roll-off above the required augmentation frequencies so as not to interfere with the distributed modal output, and this may be accomplished by the use of a floppy (that is, compliant) diaphragm material and/or by a compliance between the magnet assembly and the diaphragm. This compliance may be part of a tuned mass system to further augment the bass output. A compliance may also be located between the transducer and the panel to reduce the distributed mode excitation of the panel relative to the bass output of the diaphragm.

Preferably, the panel comprises first and second outer skins sandwiching a core material, and the transducer is connected to one of the skins and the diaphragm is mounted on the other skin.

Advantageously, the core material is porous and, preferably, defines a multiplicity of interconnected cells within the interior of the panel. The porosity of the core material may be anisotropic so as to define one or more waveguides in the core material. The rearward radiation of a conventional loudspeaker piston is usually trapped in the volume of a box or dissipated in the length of a waveguide.

Here, the provision of a porous core allows the space between the two impermeable skins to serve as the box volume, or; with anisotropic porosity; as a waveguide, or both. In both cases, the bass performance is limited merely by the volume

enclosed by the panel skins and the peripheral sealing ; not by the area of the panel or its proximity to a boundary. In fact, such proximity can augment the bass performance in an arrangement according to the invention.

The outer periphery of the panel may be completely sealed all round, may be left open in part to define a port, or may even be open all round, particularly if one or more waveguides are defined within it.

Suitable core material includes polymer foam, an aluminium or other honeycomb, corrugated materials and other low density fillers.

The core material may comprise one or more layers of corrugated material.

The core material may comprise a multiplicity of layers of corrugated material with the corrugations of adjacent layers running transversely to each other.

The internal spaces of the corrugations of a or the layer may be joined to each other by an insert of corrugations running transversely to the corrugations of the core material, the insert meeting the surrounding material at a bias.

Thus, dipolar radiation can be augmented through the utilization of a core material porous in some or all the directions in the plane of the panel and by the addition of the diaphragm driven by the vibrations of the magnet assembly of the transducer.

Most advantageously, the periphery of the diaphragm is sealed to the rear skin of the panel by a compliant surround, or, as already stated, the diaphragm itself may be compliant, or an area of compliant skin in the panel may be used.

Distributed mode panel type loudspeakers constructed in accordance with the invention will now be described, by way of example only, with reference to the accompanying drawings, In which: Figure 1 is a schematic cross-section through a prior art distributed mode panel type loudspeaker; Figure 2 is a schematic cross-section to a larger scale of a first distributed mode panel type loudspeaker in accordance with the invention; Figure 3 is a schematic cross-section of a second distributed mode panel type loudspeaker in accordance with the invention; Figure 4 illustrates a piece of dimpled core material for use in the invention; and Figure 5 is a cut-away perspective view illustrating the use of corrugated core material in a distributed mode panel type loudspeaker in accordance with the invention.

Referring to the accompanying drawings, Figure 1 shows the construction of a typical distributed mode panel type loudspeaker 1 of the prior art including an electromechanical transducer 2, more specifically an electro-dynamic inertial vibration exciter, of the type described in patent specification WO 98/34320.

The distributed mode panel type loudspeaker 1 comprises a panel 4 to which the electro-mechanical transducer 2 is connected to produce the sound output of the loudspeaker by exciting bending waves in the panel.

The transducer 2 has a magnet assembly consisting of a disc-like magnet 6 sandwiched between opposed pole pieces 8 and 10. The pole piece 8 is cup-like and formed with a downturned flange 12 surrounding the magnet 6. The pole piece 10 is disc-like and co-extensive with the magnet 6. Between the pole pieces 8 and 10, an annular gap 13 is formed in which a voice coil 14 is located. The voice coil 14 is suspended by a resilient annular suspension diaphragm 15. Terminals 16 are provided for making electrical connection to the voice coil 14. The voice coil is wound on a tubular coil former 17 which is supported at its lower end in an annular groove (not shown) in an annular coil carrier 18. The annular coil carrier 18 forms a foot by which the voice coil assembly is secured to a face of the panel 4.

The panel 4 comprises first, 20, and second, 22, outer skins sandwiching a core material 24, and the transducer is connected to the skin 20.

Turning now to Figure 2, a modification of the distributed mode panel type loudspeaker 1 is shown and comprises a distributed mode panel type loudspeaker 1'with enhanced bass response according to the invention. Parts of Figure 2 which correspond to parts of Figure 1 have been given corresponding reference numerals and will not be described again. Because Figure 2 is shown to a larger scale than Figure 1, the opportunity has been taken to show the attachment of the outer periphery of the suspension diaphragm 15 in somewhat more detail yet still schematically. For a more detailed illustration and description of the transducer 2 itself, the reader is referred to the drawings and text of specification WO 98/34320.

In the distributed mode panel type loudspeaker 1', a rigid dome-shaped diaphragm 30 is mounted within the area of the panel, is able to move relative the panel, and is arranged to be driven pistonically to improve the bass response of the loudspeaker. The diaphragm is made from resin-impregnated woven Kevlar & which is a polyaramide made by Dupont but virtually any of the materials customarily used for making loudspeaker diaphragms could be used.

The diaphragm 30 is connected to the transducer 2 to be moved pistonically thereby, the panel 4 and the diaphragm

being connected to opposite sides of the transducer. The centre of the diaphragm 30 is connected to the pole piece 8 of the magnet assembly of the transducer 2 by a compliant coupling 32, and the periphery of the diaphragm is connected to the skin 20 of the panel 4 by a compliant surround 34.

Adhesive bonding is preferably used to make the connections to the compliant coupling 32.

The compliant coupling 32 is preferably made of foam plastics material but could alternatively be made of solid or foam rubber. The compliant surround is preferably made of foam or solid rubber or foam plastics material but virtually any of the materials customarily used for suspending loudspeaker diaphragms can be used.

The core material 24 of Figure 1 is replaced in Figure 2 by a porous material 24'defining a multiplicity of interconnected cells within the interior of the panel. The construction is of one of the forms described in patent application GB 0017995.2 such as a honeycomb with slots or holes 36 on two or more cell walls 38. The communicating holes between cells can, if desired, be used to create internal volumes or waveguides or, advantageously, waveguides incorporating the cells as elements of an acoustic delay line (as described in the said patent application).

Radiation from the diaphragm 30 enters the internal volume between the two skins 20,22 by an aperture 34 in the skin 20 of the panel 4, in addition, the core material 24'is

removed in the area under the diaphragm. It is also to be noted that the annular voice carrier 18 of the transducer 4 is, in Figure 2, connected to the inner surface of the skin 22 instead of the outer surface of the skin 20 as in Figure 1.

The outer periphery of the panel 4 is closed by a sealing member or similar means so that the panel forms a loudspeaker"box"or enclosure. If desired, one or more ports and/or one or more passive radiators (sometimes called drone cones or auxiliary bass radiators) can be provided.

Figure 3 shows a modification of the loudspeaker of Figure 2 in which a compliant diaphragm 30'is connected directly to the magnet assembly of the transducer (that is to say, the compliant coupling 32 is omitted). The outer periphery of the compliant diaphragm 30'is connected directly to the skin 20 (that is to say, the compliant surround 34 is omitted). Soft rubber can, for example, be used for making the compliant diaphragm 30'.

The annular coil carrier 18 of the transducer is connected to the outside of the skin 20 which has a

multiplicity of apertures 34'in the area beneath the diaphragm 30'to enable radiation from the diaphragm to enter the internal volume of the panel.

The bulk mechanical properties of the skin and core materials can easily be kept within bounds that allow the optimization of distributed mode formation although some

compromise may be needed in respect of the need for increased depth in small, thin panels, and some modal damping resulting when a compliant surround is used.

Many variations to the described and illustrated embodiments of the invention are possible within-the scope of the appended claims. For example, a hinged diaphragm as described in patent specification WO 00/32010 can be used.

The core material can comprise a dimpled sheet 40 as illustrated in Figure 4. Such a dimpled sheet can also be used to form one or both skins of the panel.

The core material can be made of a single layer of corrugated material or several layers of corrugated material.

Advantageously, core material formed by layers of corrugations with the corrugations running at differing angles, preferably transversely, from one layer to the next includes one or more diamond-shaped inserts of corrugations running transversely to the corrugations of the main material to allow the intercommunication of all air-spaces within the corrugations.

In such a construction, the core material comprises a multiplicity of layers of corrugated material with the corrugations of adjacent layers running at an angle or transversely to each other, and the internal spaces of the corrugations of the layers are joined to each other by the one or more inserts of corrugations, the inserts meeting the surrounding material at a bias because of the diamond shape.

An example is shown in. Figure 5 which shows a panel 4' in which six layers 50 of corrugated cardboard are used as the core material. The layers are arranged with the corrugations of one layer running at right angles to the corrugations of the or each adjacent layer and the layers are adhesively secured to each other and to the outer skins 20 and 22. A diamond shaped recess is cut out in the layers of corrugations and a corresponding diamond-shaped insert 52 is located therein. In each layer, the corrugations of the insert 52 run at right angles to the corrugations of the adjacent material. In the centre of the diamond-shaped insert a circular recess is cut through the layers of corrugations beneath the area 54 shown cross-hatched in the drawing. The electro-mechanical transducer of the loudspeaker is mounted in the circular recess. By means of the diamond-shaped insert 52, the corrugations across the full width of a layer are brought into communication with the circular recess. Communication between the different layers is made by way of the circular recess. The circular recess can be large enough to permit the radiation from the diaphragm to enter (as in the construction of Figure 2) or apertures may be provided (as in Figure 3).

The core material can be made of an open cell reticulated polymer, ceramic or metallic open cell foam.

The core material can be formed by protrusions from a moulded outer skin and the protrusions can also define waveguides.

The core material can be a stiff fibrous mat.

Areas of the porous core can include impermeable boundaries to create waveguides within the core material.

Such waveguides can, if desired, include radial horn structures, in which case, the diaphragm area can be made no larger than the magnet assembly.

When it is desired to seal part or all of the outer periphery of the panel, a number of methods can be used, including the use of sealing strips, inserts or by edge crushing.

The or each waveguide, if provided, can take the form of a tuned pipe resonator in the manner of a conventional bass reflex cabinet to augment the bass output.

A second, passive, diaphragm, also connected to the internal core volume, can be used in the manner of a conventional auxiliary bass radiator. Either diaphragm can be of the hinged type.

If desired, the compliance associated with the diaphragm driven by the electro-mechanical transducer can be omitted.

Claims

CLAIMS :

1. A distributed mode panel type loudspeaker comprising a panel and an electro-mechanical transducer, connected to the panel, arranged to produce the sound output of the loudspeaker by exciting bending waves in the panel, characterized in that a diaphragm is mounted within the area of the panel, is able to move relative the panel, and is arranged to be driven pistonically to improve the bass response of the loudspeaker.

2. A loudspeaker as claimed in claim 1, wherein the diaphragm is connected to the transducer to be moved pistonically thereby.

3. A loudspeaker as claimed in claim 2, wherein the panel and the diaphragm are connected to opposite sides of the transducer.

4. A loudspeaker as claimed in claim 3, wherein the diaphragm is connected to a magnet assembly of the transducer.

5. A loudspeaker as claimed in claim 2, claim 3, or claim 4, wherein the diaphragm is connected to the transducer by a compliant coupling.

6. A loudspeaker as claimed in any preceding claim, wherein the diaphragm is itself compliant.

7. A loudspeaker as claimed in any preceding claim wherein the periphery of the diaphragm is connected to the panel by a compliant surround.

8. A loudspeaker as claimed in any preceding claim, wherein the panel comprises first and second outer skins sandwiching a core material, and the transducer is connected to one of the skins and the diaphragm is mounted on the other skin.

9. A loudspeaker as claimed in claim 8, wherein the core material is porous.

10. A loudspeaker as claimed in claim 9, wherein the core material defines a multiplicity of interconnected cells within the interior of the panel.

11. A loudspeaker as claimed in claim 10 wherein the cells are hexagonal cells.

12. A loudspeaker as claimed in claim 9, wherein the core material comprises one or more layers of corrugated material.

13. A loudspeaker as claimed in claim 12, wherein the core material comprises a multiplicity of layers of corrugated material with the corrugations of adjacent layers running transversely to each other.

14. A loudspeaker as claimed in claim 12 or claim 13, wherein the internal spaces of the corrugations of a or the layer are joined to each other by an insert of corrugations running transversely to the corrugations of the core

material, the insert meeting the surrounding material at a bias.

15. A loudspeaker as claimed in any of claims 9 to 14, wherein the porosity of the core material is anisotropic so as to define one or more waveguides in the core material.

16. A distributed mode panel type loudspeaker, the loudspeaker being substantially as herein described with reference to, and as illustrated by, Figure 2 or Figure 3 of the accompanying drawings.

17. A loudspeaker as claimed in claim 16, modified substantially as herein described with reference to, and as illustrated by, Figure 4 or Figure 5 of the accompanying drawings.