LOUDSPEAKER

Abstract: A resonant bending wave panel-form loudspeaker comprising a panel and a vibration exciter mounted to the panel to apply bending wave vibration thereto, characterised in that the loudspeaker is adapted to be mounted to a wall or the like surface and by a vibration exciter arranged for driving contact with the wall or the like surface. From another aspect the invention is a method of operating a panel-form resonant bending wave loudspeaker to enhance low frequency performance, characterised by arranging the loudspeaker adjacent to a junction between at least two room boundaries to form a resonant air cavity between the panel and the said room boundaries.

Description: TITLE: LOUDSPEAKER DESCRIPTION TECHNICAL FIELD The invention relates to a loudspeaker, and more particularly to panel -form resonant bending wave loudspeaker, e.g. as exemplified by the loudspeaker described O97/09842 of New Transducers Limited. It is an object of the invention to improve the low frequency performance of panel -form resonant bending wave loudspeakers . Some resonant panel-form bending wave loudspeakers have no theoretical limit to the useful bandwidth that can be obtained. However, materials that are currently available for use mean that to be able to radiate at high frequencies (10kHz-20kHz) , then there is a practical limit to the lowest, useful frequency, which prevents a full range loudspeaker from operating at the lowest frequencies. It is an object of the invention to provide a loudspeaker of the kind aforesaid and capable of low frequency operation when wall mounted. BACKGROUND ART WO97/09842 describes resonant panel-form bending wave so called "distributed mode" loudspeakers which are also arranged to move pistonically, see the embodiments of Figures 7 and 8, to aid the low frequency response of the loudspeakers. DISCLOSURE OF INVENTION According to the invention, there is provided a resonant bending wave panel-form loudspeaker comprising a panel and a vibration exciter mounted to the panel to apply bending wave vibration thereto, characterised in that the loudspeaker is adapted to be mounted to a wall or the like surface and by a vibration exciter arranged for driving contact with the wall or the like surface. The loudspeaker may comprise a frame surrounding the panel and suspension means mounting the panel in the frame, and the said wall driving vibration exciter may be mounted to the frame. A plurality of the said wall driving vibration exciters may be mounted to the frame. The frame may be rectangular and a said wall driving vibration exciter may be provided in at least two adjacent corners of the frame . The panel may be transparent to light, and at least one vibration exciter may be mounted to a marginal portion of the panel; i.e. so as to obscure visibility through the panel to a minimum extent . The said wall driving vibration exciter may be adapted for low frequency operation. The frame may be adapted to span between two adjacent wall or the like surfaces together defining a corner, to define with the panel a resonant cavity to enhance low frequency response. Wall driving vibration exciters may be mounted to the frame and adapted to drive the two adjacent walls. The frame may comprise a back panel to define an enclosed shallow cavity, e.g. as taught in W099/52322. Means may be provided sealing the frame to the wall or the like surface. From another aspect the invention is a method of operating a panel -form resonant bending wave loudspeaker to enhance low frequency performance, characterised by arranging the loudspeaker adjacent to a junction between at least two room boundaries to form a resonant air cavity between the panel and the said room boundaries. The method may comprise arranging the loudspeaker in contact with at least one room boundary to drive the said at least one room boundary in bending wave vibration. BRIEF DISCRIPTION OF DRAWINGS The invention is diagrammatically illustrated, by way of example in the accompanying drawings, in which:- Figure 1 is a cross section of a full range panel-form loudspeaker of the present invention; Figure 2 is a front view of a modified version of the loudspeaker of Figure 1 ; Figure 3 is a graph of the low frequency response curve of the loudspeaker of Figure 2 ; Figure 4 is a front view of a second embodiment of loudspeaker of the present invention; Figure 5 is a graph of the low frequency response curve of the loudspeaker of Figure 4 ; Figure 6 is a perspective view of a loudspeaker of the present invention positioned across a room corner, and Figure 7 is a plan view of the arrangement of Figure 6. BEST METHODS OF CARRYING OUT THE INVENTION In the drawings, and referring more particularly to Figure 1, a full range resonant bending wave panel-form loudspeaker 1, generally of the kind described in WO97/09842, W099/37121, or W099/52322 all of New Transducers Limited, is adapted for mounting on a wall 2 or the like, e.g. as a picture loudspeaker. The loudspeaker 1 comprises a generally rectangular frame 3 surrounding a resonant panel 4 which is mounted in the frame by means of peripheral suspension 5. An inertial electrodynamic vibration exciter 6 is mounted on the panel 4 at a near- centre position as taught in WO97/09842 to apply bending wave energy to the panel to excite it to resonate to produce an acoustic output in response to an audio signal applied to the exciter in known matter. The frame also supports a fixed back panel 7, whereby the loudspeaker defines a shallow enclosure 8, e.g. as taught in W099/52322. The frame can be suspended from the wall in the manner of a picture frame by conventional suspension wire (not shown) fixed to the top of the frame, whereby the bottom edge of the frame rests against the wall 2 on a bead 9 which forms a generally horizontal pivot for the frame on the wall. At its top, the frame 3 is formed with a bracket 10 projecting into the enclosure 8 and on which an electrodynamic vibration exciter 11 is mounted, the exciter extending rearwardly through an aperture 12 in the back panel 7 so as to rest against the wall 2 for driving contact therewith. The portion of the exciter 11 projecting through the back panel 7 is sealed to the back panel with a flexible sealing member 13. In addition the frame 3 is sealed to the wall 2 round its periphery by means of a flexible seal 14, e.g. of foamed plastics. The exciter may be connected to the wall by a suitable coupler 15, e.g. of soft viscous adhesive e.g. of the kind sold under the RTM "BLU-TACK" of Bostik Ltd, Great Britain. In operation, an audio signal is applied to the exciter 6 to cause the panel 4 to resonate to provide an acoustic output and via the exciter 11 to the wall 2 to drive the wall in vibration to enhance the low frequency response of the loudspeaker. A pair of such exciters 11 can be provided, one in each of the top corners of the frame . This wall drive will depend on the nature of the wall construction and where it is of a lightweight dry plasterboard and timber or metal stud-work, i.e. soft, construction, the extent of the drive will be greater than when the wall is of high impedance, e.g. of a dense material such as brick or blockwork, i.e. hard construction. Where the wall is hard, the effect can be to drive the loudspeaker pistonically, which again may enhance low frequency performance. The loudspeaker arrangement shown in Figure 2 is similar to that of Figure 1, only in this case the exciters 6 for exciting the panel 4 are arranged at the top edge of the panel as taught in W099/37121. In this case the panel 4 may be transparent to light and the loudspeaker may form part of a visual display. As shown in Figure 3 , the low frequency response curve of a loudspeaker as shown in Figure 2 is indicated by the dotted line, and without the wall drivers 11 by the solid line. It will be seen that the wall drive exciters 11 increase the low frequency output of the loudspeaker. Figures 4 and 5 correspond to Figures 2 and 3 and are in respect of an embodiment of loudspeaker generally as shown in Figure 1, but with wall drive exciters 11 in all four corners of the frame 3. In Figures 6 and 7 there is shown a loudspeaker 1 generally of the kind shown in Figure 1 placed across a room corner to form an enclosed cavity 16 which will be caused to resonate in the manner of a Helmholtz resonator at low frequencies to enhance the low frequency performance of the loudspeaker, and which is also arranged to drive the walls as previously described. INDUSTRIAL APPLICABILITY A well designed distributed mode loudspeaker can operate over most of the audio frequency range with excellent sound distribution and low distortion. At the lower audio frequencies however the distributed mode loudspeaker panel modes become similar in size to the dimensions of the panel and the panel starts to move as a whole unit making distributed mode radiation impossible. The frequencies just above that are therefore the lowest frequencies that the panel can reproduce in the distributed mode of operation. In most normal size panels this frequency range is around the 100 to 200 Hertz region. Wall mounted distributed mode loudspeakers can be made to reproduce lower frequencies by arranging that the wall on which the unit is mounted can itself be driven be one or more extra exciters mounted within the loudspeaker assembly. This offers the advantage of exploiting a weakness of stud walls that might otherwise detract from low frequency fidelity. Stud walls (made from plasterboard mounted on a timber or metal frame used in most modern buildings) vibrate in sympathy with low frequency sound being propagated in a room. The resulting wall generated absorption and reflection interferes with the wanted low frequencies . Driving the stud wall using purpose designed exciters contained within the loudspeaker assembly causes the wall to radiate in synchronisation with the wanted room low frequency sound thereby overcoming this limitation and providing a self contained solution to the limited low frequency extension of smaller size distributed mode loudspeakers adding over one octave in a typical application. The loudspeaker panel to be low frequency enhanced may be of a type that is driven in the normal optimum near centre position or which is edge driven. The low frequency excitation can be applied with multiple exciters near the corners of the panel or along the edge or with a single exciter or more arranged on the centre of mass of the loudspeaker assembly. Corner bass excitation allows easy fixing of the exciter magnets to the loudspeaker frame assembly and the extra mass of the frame makes it possible to obtain good inertia referencing down to low frequencies. Each low frequency exciter can be placed with its voice coil mount facing backwards so that the voice coil excitation is applied to the wall on which the loudspeaker is mounted. The push-pull action of the low frequency exciter/exciters drives the wall making it produce a distributed mode output using the inertial reaction of the mass of the exciter magnet or the exciter magnet and the loudspeaker frame assembly if the exciter magnet is grounded to the frame. This may be necessary to transfer enough force into the wall at the lower frequencies. The method of fixing the low frequency exciter to the wall can be important because of the requirement for easy fixing, easy removal and firm, hard attachment to prevent buzzing and losses. Blu-Tack (RTM) made a good attachment medium for experimentation but more permanent fixing would require a material such as double sided tape. The hold of double sided tape is greater than required for the application which might make removal difficult and repositioning impossible. Another solution to the problem can be achieved by fixing (using e.g. double sided tape) smooth surfaced thin metal or plastic disks to the wall coated with a non-drying viscous fluid-like heat sink compound, silicone grease or the like. A similar disc fitted permanently to the exciter would be arranged to make contact with the wall disk when the loudspeaker frame is hung in position on the wall. The resulting fluid coating and air lock formed between the two disks provides a firm hold perpendicular to the wall, with no possibility of buzzing but allows easy re-positioning or removal when required. A hinge or universal joint may be provided between the exciter and one of the disks to ensure proper alignment with the wall surface. Less wall mass or impedance (in relation to the fixed mass of the loudspeaκer assembly) results in more movement of the wall for a given exciter force. The mass of the loudspeaker can be made higher which will consequently excite the wall more effectively at lower frequencies. Greater wall mass or impedance results in less wall movement but instead of wall movement, greater pistonic movement of the whole loudspeaker assembly occurs. In this case it may be an advantage to reduce the mass of the whole loudspeaker assembly so that the exciter power is applied more to the air load rather than to accelerating the mass. The hard wall version of the enhanced low frequency panel may benefit from air sealing between the loudspeaker frame and wall as shown in Figure 1 of the accompanying drawings. In both versions the low frequency output is enhanced compared with no wall excitation at all. The invention thus provides a simple way of improving the bass response of a resonant panel -form loudspeaker intended for wall mounting.

Claims: CLAIMS 1. A resonant bending wave panel-form loudspeaker comprising a panel and a vibration exciter mounted to the panel to apply bending wave vibration thereto, characterised in that the loudspeaker is adapted to be mounted to a wall or the like surface and by a vibration exciter arranged for driving contact with the wall or the like surface. 2. A loudspeaker according to claim 1, characterised by a frame surrounding the panel and suspension means mounting the panel in the frame, and in that the said wall driving vibration exciter is mounted to the frame. 3. A loudspeaker according to claim 2, characterised by a plurality of the said wall driving vibration exciters mounted to the frame. 4. A loudspeaker according to claim 3 , characterised in that the frame is rectangular and by a said wall driving vibration exciter in at least two adjacent corners of the frame . 5. A loudspeaker according to any preceding claim, characterised in that the panel is transparent to light, and in that at least one vibration exciter is mounted to a marginal portion of the panel. 6. A loudspeaker according to any proceeding claim, characterised in that the said wall driving vibration exciter is adapted for low frequency operation. 7. A loudspeaker according to any one of claims 2 to 6 , characterised in that the frame is adapted to span between two adjacent wall or the like surfaces together defining a corner, to define with the panel a resonant cavity to enhance low frequency response . 8. A loudspeaker according to claim 7, characterised by 5 wall driving vibration exciters mounted to the frame and adapted to drive the two adjacent walls. 9. A loudspeaker according to any one of claims 2 to 8, characterised by a back panel in the frame to define an enclosed shallow cavity. 10 10. A loudspeaker according to any one claims 2 to 9, characterised by means sealing the frame to the wall or the like surface. 11. A method of operating a panel -form resonant bending wave loudspeaker to enhance low frequency performance, 15 characterised by arranging the loudspeaker adjacent to a junction between at least two room boundaries to form a resonant air cavity between the panel and the said room boundaries . 12. A method according to claim 11, characterised by 20 arranging the loudspeaker in contact with at least one room boundary to drive the said at least one room boundary in bending wave vibration. 25

Inventors: AZIMA HENRY, MORECROFT DENIS, AZIMA, HENRY, MORECROFT, DENIS
Applicants: AZIMA HENRY, MORECROFT DENIS, NEW TRANSDUCERS LTD, AZIMA, HENRY, MORECROFT, DENIS, NEW TRANSDUCERS LIMITED
Application year: 1999
Main classification: H04R7
IPC codes: H04R1, H04R7