LOUDSPEAKERS

Abstract: A bending-wave panel loudspeaker comprising a bending-wave panel and an electrodynamic exciter attached to the panel to vibrate the panel, the exciter comprising a magnet assembly and a voice coil assembly, the exciter being attached to the panel via the voice coil assembly, characterised in that the panel within the ring contact of the voice coil is dished.

Description: TITLE: LOUDSPEAKERS DESCRIPTION TECHNICAL FIELD The invention relates to loudspeakers and more particularly to bending-wave panel loudspeakers, e.g. resonant panel speakers of the kind described in International patent application O97/09842. It is an object of the invention to improve the working frequency bandwidth of a bending-wave or resonant panel loudspeaker. BACKGROUND ART In normal operation, an electrodynamic vibration exciter having a tubular voice coil fixed as a continuous or discontinuous ring contact to a bending-wave or resonant panel to excite the panel can cause the disc of panel material directly underneath and enclosed by the ring contact of the voice coil to vibrate in the manner of a drum, and the frequency at which this localised resonance, occurs depends upon the exciter voice coil diameter and the panel material properties. For some panels, there may be little or no modal panel behaviour above this resonant frequency and therefore this resonant frequency may form the effective upper frequency limit of the panel. For thin panels used with 25mm diameter exciters this can be a particular problem. It has previously been proposed to counter the problem by removing panel material enclosed within the ring contact of the exciter voice coil, but such a solution is not always practicable. DISCLOSURE OF INVENTION According to the invention the panel material underneath or within the ring contact of the voice coil of an electrodynamic exciter is dished, e.g. formed or deformed to extend out of the plane of the panel, e.g. is formed into a raised or depressed concave or convex structure e.g. a dome or pyramidal shape. Since such a depressed or raised shape, whether curved or faceted, forms an inherently stiffer structure than the flat panel material, and this causes the local resonant frequency to be shifted to a higher frequency. The degree of frequency shift is dependent upon the radius or height, and diameter of the raised or depressed structure. The simplest form is a spherical contour which may be raised as in a dome or concave, the latter partially dependant on the depth available to the moving part of the exciter. The height of the raised element may be altered to suit, and its shaped may be based on other curves e.g. hyperbolic, or parabolic, or combinations of straight and curved sections. A simple cone shape may also be effective . Depending on the manufacturing/constructional method the 'dome' element may be formed integrally with the panel, or be additional to the panel, where it may reinforce or replace the original disc section. In the panel forming process, a section of the core may be processed for increased thickness in the region of the exciter, or the panel may be moulded to conform to the desired local contour. In the case of monolithic, moulded bending panel radiators, the 'dome' feature may be part of the overall moulding profile. In addition the controlling feature may have more complex profiles such as a series or an array of smaller 'domes' proportioned so as to give finer control of the resultant acoustic output. The invention is also applicable to the conventional paper/felted speaker diaphragm technology where the bonded felted matrix is formed as a bending wave panel with the dome section for the exciter made integrally by design of the profile of the former. The internal diameter of the dome or other raised or depressed structure may match the internal diameter of a foot or mounting ring by which the exciter voice coil is mounted on the panel, or that of the voice coil itself if directly mounted on the panel, to enhance the improvement in frequency bandwidth. Thus, for example, a 22mm diameter dome may be suitable for use with 25mm diameter exciters whilst for 19mm exciters, a 16mm diameter dome may be more suitable . BRIEF DESCRIPTION OF DRAWINGS The invention is diagrammatically illustrated, by way of example, m the accompanying drawings, m which: - Figure 1 is a partial side view of bending wave loudspeaker; Figure 2a to 2f are partial perspective views of bending wave loudspeaker panels showing alternative embodiments ; Figure 3 is a full spectrum graph of loudspeaker frequency response; and Figure 4 is a part-spectrum graph of loudspeaker frequency response. BEST MODES FOR CARRYING OUT THE INVENTION In Figure 1, a bending wave loudspeaker 1, generally of the kind described m WO 97/09842, comprises a thin stiff lightweight panel 2 excited by an electrodynamic vibration exciter 3, of the kind comprising an mertial magnet assembly comprising a magnet 8 and a pair of pole pieces 6, and a voice coil assembly comprising a coil 10 wound on a former 4 having an annular mounting foot 7 to produce an acoustic output, the exciter 3 being fixed to the panel 2 by the mounting foot 7 of the voice coil assembly to form a contact ring therewith. Tne ring may be discontinuous or continuous as desired. The area of the panel inside the contact ring is bulged away from the exciter to form a domed, part spherical dish 5. The magnet assembly 6,8 of the exciter 3 is resiliently suspended on the panel 2 by a compliant suspension 9, e.g. of foam rubber . Figures 2a to 2f show alternative shapes of the dish 5 m the panel 2. Thus m Figure 2a the dish is conical, m Figure 2b the dish is a part -sphere, while m Figure 2c the part spherical dish of Figure 2b is inverted. In Figure 2d the dish is pyramidal, m Figure 2e the dish is a combined cone and dome shape and in Figure 2f the dish is of hyperbolic shape. Figure 3 shows the improvement m frequency bandwidth of the loudspeaker by using the dome feature or shape compared to a conventional flat panel. The flat panel frequency response (thin line) shows a panel "local or cap" resonance at approximately 6 kHz and a subsequent isolated harmonic of this resonance at approximately 18kHz. The 'dish' feature panel (thick line) shows an extended high frequency range with a minor resonance at approximately 16 kHz. If desired, the height of the dome or dish may be changed to improve the frequency bandwidth. Figure 4 shows the effect of using different dome height (radius) on a different type of felt paper panel, m this case a Black SIG grade of paper with a lacquer treatment . Figure 4 clearly shows that the 5mm dome height (thick line) increases the useful bandwidth by approximately 2 kHz . The aperture resonance frequency for the 5mm height dome is approximately 2 kHz greater than that of the 3mm height dome (thin line) . However, the useful range of heights for the dome feature may vary from about 1 to 7mm. Above 7mm or below 1mm, there is likely to be only a very slight increase in the aperture effect resonance frequency. Clearly, the total thickness of the panel may be of critical importance m some applications which will prohibit the use of very high domes i.e. >7mm. The dome feature is most suited to thinner resonant panels because the "cap or local" resonance frequency is more of a problem on thin materials. A typical panel thickness range for the present invention would be approximately 0.2 -4mm. Clearly only certain panel materials will be able to be deformed m this manner. This conventional pistonic cone speaker material i.e. felted paper, can be formed m this manner, and the process could be extended to foam materials such as polystyrene, polyurethane and polymethylacrylimide (PMI = Rohacell [RTM] ) , and also to injection-moulded thermoplastics such as nylon, ABS (acrylo butadiene styrene) or polypropylene. INDUSTRIAL APPLICABILITY The invention thus provides a simple way of increasing the frequency bandwidth of a bending-wave panel loudspeaker.

Claims: CLAIMS 1. A bending-wave panel loudspeaker comprising a bending- wave panel and an electrodynamic exciter attached tc the panel to vibrate the panel, the exciter comprising a magnet assembly and a voice coil assembly, the exciter being attached to the panel via the voice coil asserr-Dly, characterised m that the panel within the ring contac: of the voice coil is dished. 2. A loudspeaker according to claim 1, wherein the dish is a part -spherical curve. 3. A loudspeaker according to claim 1 or claim 2, wherein the dish is formed integrally with the panel. 4. A loudspeaker according to any one of claims 1 to 3 , wherein the diameter of the dish matches the internal diameter of a foot or mounting ring by which the voice coil is mounted on the panel, or that of the voice coil itself if directly mounted on the panel.

Inventors: BANK GRAHAM, FORDHAM JULIAN, BANK, GRAHAM, FORDHAM, JULIAN
Applicants: BANK GRAHAM, FORDHAM JULIAN, NEW TRANSDUCERS LTD, BANK, GRAHAM, FORDHAM, JULIAN, NEW TRANSDUCERS LIMITED
Application year: 2000
Main classification: H04R7
IPC codes: H04R1, H04R7, H04R9