LOUDSPEAKERS

Abstract: An exciter for exciting bending waves comprises a voice coil (5), a magnet assembly (11) having apertures (21) and a strut member (23) having ribs (27) extending through the apertures (21) for coupling to top and bottom plinths (1, 3).

Description: TITLE: LOUDSPEAKERS DESCRIPTION The invention relates to loudspeakers and more particularly to resonant panel -form loudspeakers e.g. of the general kind described in International patent application WO97/09842, and to vibration exciters for such resonant panel -form loudspeakers. The present invention is particularly concerned with resonant panel-form loudspeakers of the kind in which one or more vibration exciters are embedded within the panel structure. Previous designs of embedded exciter resonant panel loudspeaker have used a variety of set-ups and configurations but have tended to involve time-consuming and difficult manufacturing procedures. In addition it has proved difficult with embedded exciters to produce adequate high frequency performance (< 10kHz) . The major problems are thickness restrictions and the method of creating bending waves m a panel comparable to the usual surface- mounted exciter which is simpler and less time-consuming to manufacture whilst not compromising the requirements for a broad frequency bandwidth panel. Existing embedded exciter designs have also tended to have different frequency responses on each side of the panel due to the lack of symmetry in the panel construction adjacent to the exciter. It is an object of the invention to provide an embedded exciter resonant panel loudspeaker construction alleviating one or more of these difficulties. From one aspect the invention is an exciter for imparting bending wave energy to a resonant panel, comprising a coil assembly having a coil for engaging a first plinth, a magnet assembly having a magnet, an inner pole piece inside the coil, and an outer pole piece outside the coil, the assembly defining apertures therein, a strut member for engaging a second plinth at the opposite end of the coil to the first plinth having projections extending through the apertures m the magnet assembly. From another aspect the invention is a loudspeaker comprising a resonant panel -form member and an exciter as described above embedded therein to cause the panel to resonate to produce an acoustic output. Thus the exciter design may be such that the coil of the exciter is bonded directly to a disc-like plinth below the coil and also to an identical plinth positioned directly above the coil via axial projections m the form of a set of ribs. The coil movement is transmitted to the lower plinth and upper plinth simultaneously and both plinths move in the same direction. The two plinths may be adhesively bonded to the panel core material at the top and bottom surfaces of the panel . The invention is diagrammatically illustrated, by way of example, in the accompanying drawings, in which: - Figure 1 is an exploded view of an embodiment of embedded inertial electrodynamic vibration exciter; Figure 2 is a view of the exciter of Figure 1 when assembled; Figure 3 is a cross section through the exciter embedded in a panel ; Figure 4 shows a cross section through a second embodiment of the exciter embedded in a panel; Figure 5 illustrates the assembly of exciters into a panel ; Figure 6 illustrates the orientation of exciters in a panel ; Figure 7 is a graph of the frequency response of another embodiment of an embedded inertial electrodynamic vibration exciter; Figure 8 is a graph of the frequency response of an exciter of the kind shown in Figures 1 and 2 ; Figure 9 illustrates the power response using a pair of exciters; and Figure 10 shows the structure of the plinths in an embodiment . Referring to Figures 1 to 3 , an electrodynamic exciter is shown. A top plinth (1) and a bottom plinth (3) form the top and bottom faces of the exciter. A coil assembly including a coil (5) mounted on a coil former (7) is mounted on the bottom plinth (3) . A magnet assembly having a magnet cup (11) having a base (13) and sides (15) sits over the voice coil (5) . Inside the magnet cup sit a magnet (17) and an inner pole piece (19) in the form of a disc. Apertures (21) in the base and sides of the cup are provided. A strut member (23) has a ring (25) extending circumferentially around the base of the cup and ribs (27) extending through the apertures (21) . Foam supports (29) for bonding to the core material of the panel (or to separate supports) maintain the magnet cup position relative to the coil while still allowing movement of the magnet cup . The three equi-spaced axial ribs (27) are bonded to both disc-like plinths and to the sides of the coil support. As shown the vertical ribs appear to cross the magnet cup. This is achieved by using a cut-away magnet cup as shown in Figure 2 which allows the ribs to connect to the coil assembly at three positions. Figure 3 shows a cross section of the exciter (4) embedded in a panel (31) . The foam supports (29) hold the magnet cup (11) . The panel comprises skins (33) around a core (35) ; the exciter is flush with the faces (34) of the panel . An alternative embodiment is shown in Figure 4. The strut member (23) may be integrally formed with the coil assembly (5) . Indeed, a single plastics moulding (39) may include the coil former (7) and ribs (27) . In this case the ring may be omitted; it may be possible to glue the ribs to the top plinth or slots (37) may be provided in the top plinth to accept the ribs. This design is not symmetrical and therefore there may be a difference m frequency response (perhaps only minimal) between the two sides. It is, however, possible to achieve a symmetrical design by using two exciters back to back and hence achieve consistent frequency responses on both panel sides, as shown m Figures 5 and 6. Referring to Figure 5, two holes (41) are machined in the panel of a size corresponding to the outer diameter of the plinths (1,3). A slit (43) is cut m the panel and holes are machined prior to laminating the panel. A length of copper track (45) is fitted in the slit, likewise prior to lamination. Epoxy resin is then used to fix first and second exciters (47,49) into position, and the two exciters are wired m phase . Figure 6 illustrates how the second exciter (49) is reversed with respect to the first (47) to produce a more symmetrical structure and hence frequency response. Preferably the exciter is such that the area around the exciter will have comparable bending stiffness to that of the panel. Effectively, the plinths connected by the axial ribs act as a form of sandwich structure which will have a comparable impedance to the rest of the panel. However, because the plinths are thin, aperture effects related to the diameter, density and stiffness of the plinths may occur. To increase the plinth resonant frequency, possibly to a position outside the audible frequency bandwidth, a small diameter, stiff, low density plinth construction is preferable. The construction of the plinth must also allow the effective transmission of bending waves to the panel . By connecting the coil to the axial ribs, the effective coil mass is increased. If too high, this mass will result in high frequency roll -off. To prevent this effect, the mass of the vertical ribs must be minimised whilst also maintaining adequate stiffness to transmit the forces between the two plinths. Investigations to date have used ribs made from sections of polyurethane thermoplastic (1mm thick) although other thermoplastics and perhaps other materials such as light metals could be employed. The critical parameters for this exciter design are as follows : (i) Plinth material, diameter, thickness and density (ii) Axial rib material characteristics, stiffness and density (iii)Coil diameter In the following example, an embedded panel of the following specification was selected: Size : 600 x 530mm Maximum Rohacell Thickness : 10 -12mm There are several limiting factors on the exciter design: (i) Minimum total exciter thickness = 8mm (ii) Preferred exciter size - 25mm diameter The thickness restrictions mean that the plinth thickness is limited to approximately 2mm for a total panel thickness of 12mm. The preferred exciter size is based on the required loudness of the panel i.e. smaller diameter exciters may not put enough force into the panel . This restriction dictates that the minimum plinth diameter is fixed at approximately 40mm. Having fixed the exciter parameters and some of the panel parameters, the plinth options can then be evaluated. The following options for the plinths have been evaluated: Table 1: Plinth Constructions and Upper Frequency Limits Table 1 shows that increasing the plinth thickness improves the high frequency performance of the panel up to the maximum possible thickness of 2mm. Using a thicker panel or decreasing the exciter thickness to allow higher plinth thickness could increase the high frequency limit further . A plinth (1,3) may have a sandwich structure of skins (51) sandwiching a core (53) (Figure 10) . The sandwich structure plinths appear to have improved high frequency performance compared to the monolithic structures. This is probably due to the higher stiffness to weight ratio of these sandwich structures . For these panels, the frequency of the coil side of the panel differs from that of the magnet side of the panel and therefore both sides of the panel need to be measured and compared. For all the embedded panels, there was a difference between the two sides of the panel, mainly in the high frequency performance. Ideally the frequency response needs to be identical; see the results presented later. Figure 7 shows the frequency response for an embedded panel with a single exciter. In this case the high frequency performance can be seen to be uneven, with the magnet side of the panel having a 'suck-out' at approximately 7-8 kHz. The high frequency limit of this material is approximately 10 kHz. The higher frequency performance appears to be due to the nature of the plinths. The sandwich structure plinths have improved higher frequency performance . Preferably in order to simplify the manufacturing process, the plinth, vertical ribs and foam supports may be injection moulded to form a single part. A thermoplastic plinth design may be more suitable for low cost panels where high frequency performance is not required. A sandwich construction plinth has superior high frequency performance and is more suited where broad bandwidth is essential. Thicker, sandwich structure plinths would improve the high frequency performance of the panel . Thinner exciters or thicker panels may be used. Back to back exciters embedded in a panel may be expected to have more similar frequency responses at the respective panel surfaces than using a single double plinth exciter. This turns out to be the case. Figure 8 shows the frequency response on each side of a panel with two exciters, which may be compared with the like graph of Figure 7 for a panel with one exciter. Figure 8 shows an improved match; for example, on the gross asymmetry at 7 - 8 kHz is smoothed. In order to assess the power response of the embedded panel with twin exciters (one reversed) , a power response measurement was taken. This gives a superior representation of how the panel actually sounds to the human ear. This was achieved by taking a series of polar measurements using Missa measurement system and averaging the results to achieve an average power measurement of the panel at IV rms input. This response is shown in Figure 9. The power measurement shows that this embedded panel produces high frequency output up to approximately 17kHz and has a reasonably smooth output . The invention is not limited to the above described embodiment. In particular, although the properties of the exciter are substantially improved by having the strut member located close to the voice coil, which requires the strut member to pass through the magnet assembly and hence for the magnet assembly to have apertures, it may be possible in a less preferred embodiment for the or each exciter to have the strut member located outside the magnet assembly. In that case, the strut member may span the gap between first and second plinths and be fixed to each plinth, for example using adhesive, so that it couples the first and second plinths.

Claims: CLAIMS 1. An exciter for imparting bending wave energy to a panel, comprising a coil assembly (5,7) having a coil (5) for engaging a first plinth (3) , a magnet assembly (11) having a magnet (17), an inner pole piece (19) inside the coil (5) , and an outer pole piece (15) outside the coil (5), the assembly defining apertures (21) therein, a strut member (23) for engaging a second plinth (1) at the opposite end of the coil (5) to the first plinth (3), the strut member (23) having projections (27) extending through the apertures (21) m the magnet assembly (ID - 2. An exciter according to claim 1 further comprising resilient supports (29) to hold the magnet assembly (11) for allowing the magnet assembly (11) to move axially in order that the exciter functions as an inertial exciter. 3. An exciter according to any preceding claim wherein the magnet assembly (11) comprises a magnet cup (13,15) over one end of the coil (5) , the sides (15) of the cup forming the outer pole piece and at least the base (13) of the cup defining the apertures . 4. An exciter according to any preceding claim wherein the projections of the strut member (23) are a plurality of axial ribs (27) . 5. An exciter according to claim 4 comprising a ring (25) linking the ribs (27) . 6. An exciter according to claims 4 or 5 wherein the ribs (27) are attached to the outside of the coil (5) . 7. An exciter according to any of claims 1 to 3 wherein the voice coil (5) sits around a former (7) , and the strut 5 member (23) is integral with the former. 8. An exciter according to any preceding claim further comprising a first plinth (3) at one end of the coil assembly (5,7) and a second plinth (1) engaged by the strut member (23) at the other end of the coil. 10 9. An exciter according to claim 8 wherein the second plinth (1) has at least one slot (39) locating the strut member . 10. An exciter according to claims 8 or 9 wherein each plinth has skins (51) sandwiching a core (53) . 15 11. A loudspeaker comprising a panel (31) for supporting bending wave modes, and an exciter (47) according to any preceding claim embedded in the panel (31) for exciting the bending wave modes to produce an acoustic output. 20 12. A loudspeaker according to claim 11 further comprising a further exciter (49) according to any of claims 1 to 10 embedded in the panel (31) . 13. A loudspeaker according to claim 13 wherein the further exciter (49) is mounted upside down with respect to 25 the first exciter (47) . 14. A loudspeaker according to any of claims 11 to 13 wherein the or each exciter (47,49) is an exciter according to any of claims 8 to 10 and the plinths (1,3) are flush with the front and back faces (34) of the panel. 15. An exciter for imparting bending wave energy to a panel, comprising a coil assembly (5,7) having a coil (5) engaging a first plinth (3) , a magnet assembly (11) having a magnet (17) , an inner pole piece (19) inside the coil (5) , and an outer pole piece (15) outside the coil (5), the coil assembly (5,7) and magnet assembly being arranged for relative motion, and a strut member (23) coupling the first plinth (3) to a second plinth (1) at the axially opposite end of the coil (5) to the first plinth (3) .

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: H04R1
IPC codes: H04R1, H04R7, H04R9