BACKGROUND ART It is known from W097/09855 to provide packaging comprising a distributed mode panel-form loudspeaker.

DISCLOSURE OF INVENTION According to the invention there is provided a loudspeaker comprising a box-form structure made from stiff lightweight sheet material to define a plurality of faces, at least one face of the structure forming a panel-form bending wave acoustic radiator and having an electro- acoustic vibration transducer coupled thereto to apply bending wave energy to the radiator to cause it to radiate

an acoustic output when an input signal is applied to the transducer, the box-form structure being collapsible, so that the box-form structure can be stored and transported in a flat form and erected as a box when required as a loudspeaker.

A stiff material is one which is self-supporting. The box-form structure may be made from folded stiff lightweight sheet material that is sufficiently flexible at the folds to allow flat-packing. Thus, the box-form structure may consist of a single piece of the lightweight material which should greatly simplify manufacture and assembly. The fold between at least two adjacent faces may be a single fold or may comprise a parallel pair of folds.

Such a double fold may provide extra compliance and more decoupling between faces. The folds may be formed by grooving the sheet material and the grooving may comprise local compression of the sheet material.

Alternatively, particularly if the box-form structure comprises a plurality of panels made from stiff lightweight sheet material which is not foldable, the panels may be united at the panel edges by connectors, e. g. adhesive tape. The connectors preferably comprise hinge portions whereby the panels are moveable relative to one another.

The folds or the connectors may be continuous or discontinuous. The folds or connectors may be such as to permit the transmission of bending wave energy between faces. Thus, the faces may be both mechanically and

acoustically coupled. In this way, a transducer need only be attached to one face and adjacent faces may be driven by bending wave energy which is transmitted across the fold. This may be achieved if the fold or connector resists flexing, i. e. has residual bending stiffness after folding.

Alternatively, the fold or connector may be fully flexible whereby the fold or connector acts as a simply supported edge termination of an excited panel. Thus, the faces adjacent the radiator primarily act as baffles whereby bass response of the radiator may be improved.

The baffle may be substantially open or closed.

The box-form structure may be of any suitable geometrical shape, e. g. cuboid, cube-shaped or prism shaped and may be open or closed. For example, the box- form structure may be in the form of a truncated pyramid, preferably having a triangular base. The triangular base means that the side faces adjacent the radiator provide an effective baffle of a greater depth for the radiator than for other shaped bases, e. g. rectangular. The plane of the truncation may be angled, for example at 20°, with respect to the plane of the base of the pyramid.

The stiff lightweight sheet material may be a packaging material such as corrugated cardboard or the like. The corrugated cardboard may be of the kind comprising face skins sandwiching a corrugated core.

Alternatively, the stiff lightweight sheet material may be

vacuum-formed plastics or extruded twin wall polypropylene sheet, e. g. such as that sold under the trade-mark "Correx", the latter being generally equivalent to corrugated cardboard. The corrugations of the corrugated material may be arranged to extend perpendicular or at an acute angle to the base of the structure. Such materials permit the manufacture of very lightweight, portable, low cost and possible disposable speakers. Alternatively, more durable, long lasting or higher performance sheet materials could be used, e. g. that sold under the trade mark"Traumalite".

The panel-form bending wave radiator may be resonant and the loudspeaker may be of the distributed mode kind.

Thus the properties of the panel-form radiator may be chosen to distribute resonant bending wave modes of the radiator substantially evenly in frequency. In other words, the properties or parameters, e. g. size, thickness, shape, material etc., of the panel-form radiator may be chosen to smooth peaks in the frequency response caused by "bunching"or clustering of the modes.

The box-form structure may be of concertina or fold- out form, and image width may be increased by designing for a multiple concertina fold-out action. For example, a face of the box-form structure may be formed with a fold whereby that face can be folded on itself to collapse the box-form structure. The fold in the said face may be substantially central of the face whereby the face can be folded in half to collapse the box-form structure. A tab may be disposed

adjacent to the fold and may be integral with a face of the box-form structure. The tab extends across the fold when the structure is erect to prevent folding of the. said face in one direction. In this way, the face may be only folded inwards and thus the ability to flat pack the speaker does not necessarily lead to a loss of stability or strength.

The box-form structure may comprise a support flap connected to a face of the box-form structure and which can be folded to abut at least one adjacent face to hold the box-form structure erect. The support flap may abut two adjacent faces, e. g. two side faces and may strengthen the overall structure. The support flap may also act as a spacer between the interior surfaces of two adjacent faces when the box-form structure is collapsed, one of which interior surfaces has the transducer coupled thereto, to provide a cavity for receiving the transducer.

The transducer may be a moving coil inertial exciter comprising a magnet assembly and a voice coil assembly.

Since the transducer is mounted on a sloping face, there is uneven weight loading which may lead to unwanted non- axial movement of the magnet assembly. The magnet assembly may thus be supported in a transducer housing mounted to the radiator. The housing may be in the form of a plastic spider which decouples the mass of the transducer from the face. The magnet assembly may be secured to the housing by foam pads which act as a heat sink. The transducer housing discourages unwanted non- axial movement of the magnet assembly and hence voice coil

damage may be alleviated and the transducer excursion may be limited.

Alternatively, the transducer may be an inertial or grounded vibration transducer, a piezoelectric transducer, a magnetostrictive transducer, a bender or torsional transducer (e. g. of the type taught in WO00/13464) or a distributed mode transducer (e. g. of the type taught in W001/54450).

More than one face may form a panel-form bending wave acoustic radiator. A transducer may be mounted on each face which forms a panel-form bending wave acoustic radiator to excite bending wave vibration in the radiator.

By providing transducers on more than one face, stereo sources may be obtained from a single object. A transducer may be mounted to each face of the box-form structure whereby omnidirectivity at high frequencies may be improved.

The loudspeaker may have a pop-up design whereby the loudspeaker may be assembled by a single push or pull action. Alternatively, the speaker may have a snap-out design whereby time and effort required in assembly is reduced. Press studs may be used to maintain the box-form structure, particularly for a pop-up or snap-out design, in its flat-pack arrangement. The speaker may comprise ground engaging feet, which may be pop-up or clip-on feet.

Thus, the invention provides a light-weight fold-away loudspeaker which may be used as a Hi-fi, AV or presentation loudspeaker. Low weight and reduced volume

offers improved distribution with lower shipping and warehousing costs. The loudspeaker is also scalable from desktop use to large floor standing box-form structures.

Applications of the technology include foldable versions of the following: a lightweight subwoofer, a multi-media loudspeaker which wraps around a multi-media monitor, e. g. for a PC or laptop, a PA system, a lectern which may incorporate a PA system, a suspended or pole mounted multi-polar announcement system, a musical wigwam, a musical/talking Wendy house, musical toys/models for children to assemble, promotional display loudspeakers, an expandable baffle for portable conferencing/personal handsfree product to improve low frequency, cot-side travel units with soundchip, personal head-worn systems, walk-in portable listening rooms and lampshades.

The"point of purchase"market generally requires displays to be delivered flat-pack. Particularly for the smaller objects, the improved low frequency performance will be useful when, for example, amplifier headroom and battery life are at a premium. The opportunity for images covering the entire object surface is also attractive to merchandisers. Furthermore, the loudspeaker can be made to look like the product or packaging e. g. Weetabix@ cereal or a Tobleronet chocolate bar.

BRIEF DESCRIPTION OF DRAWINGS The invention is diagrammatically illustrated, by way of example, in the accompanying drawings in which:-

Figure 1 is a perspective view of a partly assembled loudspeaker according to the present invention; Figure 2 is a perspective view of the loudspeaker of Figure 1 in flat-pack form; Figures 3a to 3f are perspective views of loudspeakers according to six alternative embodiments; Figure 4 is a perspective view of a loudspeaker according to another aspect of the invention ; Figures 4a and 4b show upper and lower releasable locking mechanisms for the loudspeaker of Figure 4; Figure 5 is a perspective view of a loudspeaker according to another aspect of the invention ; Figures 5a and 5b are cross sections of the rear spine and the side spines of the loudspeaker of Figure 5; Figure 5c is a plan view of the loudspeaker of Figure 5 before assembly; Figures 5d, 5e and 5f are perspective views of the loudspeaker of Figure 5 at various stages during assembly; Figure 5g is a perspective view of the assembled loudspeaker of Figure 5 in flat-pack form; Figure 6a is a perspective view of a loudspeaker according to another aspect of the invention; Figure 6b is a cross-section through a foot for the loudspeaker of Figure 6a ; Figures 6c and 6f are respective perspective and side views of the connector panel of the loudspeaker of Figure 6a; Figure 6d and 6e are plan and side views of the CONFIRMATION COPY

transducer and transducer housing of the loudspeaker of Figure 6a; Figures 7a, 8a and 9a and 7b, 8b and 9b are exploded cross-sections of alternative hinge mechanisms in the open and closed state respectively; Figure 10a is a perspective view of a loudspeaker according to another aspect of the invention; showing an alternative hinge mechanism; Figure lOb is an exploded cross-section of a hinge showing the transmission of energy across the hinge; Figures 11,12a and 12b are perspective views of alternative speakers; Figure 13 is the modal distribution of two bending wave panels which may be used in the loudspeaker shown in Figure 4; Figure 14 is the acoustic response (sound pressure level in dB versus frequency) for the speaker of Figure 5.

BEST MODES FOR CARRYING OUT THE INVENTION Figures 1 and 2 show a loudspeaker 10 according to the present invention. In Figure 2, the loudspeaker 10 is in flat pack form, i. e. for transport and storage. In Figure 1 the loudspeaker 10 is partially assembled with the loudspeaker 10 being completed by folding upper and lower flaps inwards to form a generally cuboid structure, i. e box. As shown, lower side flap 14 and lower front flap 16 are folded inwards in the directions of arrows A and B respectively. Figure 3e shows the fully assembled form of the loudspeaker which has a cuboid box-structure.

The loudspeaker has a box-form structure with a plurality of faces which define a volume. The front face 12 forms a panel-form bending wave acoustic radiator which is capable of support bending wave vibration, preferably resonant bending wave modes. A transducer 18 is coupled to the front face 12 to drive bending wave vibration in the panel to produce an acoustic output. The transducer 18 is shown in dotted line and is mounted on the inner side of the front face 12, i. e. within the box.

Figures 3a to 3f show six alternative erected box- like loudspeakers. Each loudspeaker may have a base and thus define a closed box. Alternatively the base may be defined by the surface on which the loudspeaker stands.

Figures 3a, 3b, 3c and 3f show prisms each having four side faces extending from a rectangular base. In each of Figures 3a, 3b and 3c two opposed side faces are perpendicular to the base and the prisms have a constant cross-section defined by the opposed faces.

In Figure 3a the prism has a rectangular top face 20 to which the four faces extend and the two opposed side faces 22 of the prism are trapezium-shaped. Figure 3b shows a complex prism comprising the prism of Figure 3a mounted on a cuboid. Figure 3c shows a prism in which the four side faces meet in a line with the two opposed side faces 26 being triangular. Figure 3f shows a prism similar to that of Figure 3c except that each side face is inclined at an acute angle to the rectangular base. Figure 3d shows a tetrahedral in which each face, including the

base, is triangular.

Figure 4 is a loudspeaker 30 having an erected box- form structure in the form of a truncated pyramid having an equilateral triangular base. The plane of the truncation is angled at approximately 20° to the plane of the base of the pyramid. A triangular base shape provides the largest ratio of side face to overall box size. The pyramid is made from a corrugated cardboard having a high stiffness to mass ratio and a high quality clay-coated surface finish which is particularly suitable for printing any desired design on the faces of the pyramid. The cardboard is of the kind comprising face skins sandwiching a corrugated core.

A transducer (not shown) is mounted to one face of the pyramid and is connected to an audio signal by audio connections 32. Since there is only one transducer, only one face 34 of the loudspeaker is excited directly and this face forms a panel-form bending wave acoustic radiator. The other two side faces 36, the base and the top face 38 are mechanically coupled to the excited face by folds 40 whereby the excited face is simply supported along all of its edges. The other two side faces 36 primarily act as baffles for the excited face. There may be transmission of vibrational energy across the folds 40 whereby the other faces, in particular, the other two side faces 36 are also acoustically coupled to the excited front face 34 and may thus be excited.

As with previous embodiments, the box-form structure is intended to fold flat for ease of transport and/or storage. Thus pairs of faces are connected by single continuous folds which act as hinges whereby the two faces are rotatable relative to each other. The loudspeaker 30 comprises upper and lower releasable locking mechanisms 42,44 which connect to a flange 46 which extends along the fold joining each of the two side faces 36. Figure 4a shows the upper mechanism 42 which comprises a flap 48 which folds down from the top face 38 in the direction of arrow A and is secured to the flange 46 by a fastener.

Figure 4b shows the lower mechanism 44 which comprises a flap 50 which folds across from the side face and is secured to the flange 46 by a fastener. The fasteners may be VelcroTM or the like or fastener disks whereby easy assembly and disassembly of the loudspeaker is achieved.

Figure 5 shows a loudspeaker 100 which is generally similar to that of Figure 4 and thus features in common have the same reference numbers. In contrast to Figure 4, in Figure 5 the front face 34 is connected to the two side faces 36 by a fold 102 having a parallel pair of folds.

The audio connections 32 are connected to a connector panel 108 (see Figure 6c).

Figure 5a shows the rear spine which connects the two side faces 36 and which is in the form of a double fold 102 permanently attached by a glued joint 114 joining a flap on one side face with the other side face. Figure 5b shows one of the folds which connects the front face 34

with a side face 36 and which is in the form of a double fold 102.

Figures 5c to 5f illustrate the manufacture of the loudspeaker of Figure 5 from a specially formed blank of a single piece of cardboard. The blank shown in Figure 5c comprises integral panels which are in the form of generally truncated triangles and which when the loudspeaker is erected as a box form its front and side faces 34,36. The panel forming the front face 34 is integral with an upper central flap 122 and a lower central flap 124 which respectively form the top and base faces of the assembled loudspeaker. The lower central flap 124 is formed with two holes 140 which ensure correction alignment of a transducer mounting jig whereby the transducer may be accurately positioned on the front face 34. The upper central flap is generally triangular with two additional side pieces and is formed with a central hole 132 to assist in assembly.

Each panel forming a side face 36 is integral with an upper and a lower side flap 116,118 and the flaps may be moved relative to the side faces along single folds. One side face 36 comprises a central fold 138 and a small hole 134 towards the top of the face to assist in collapsing the box-form structure. The other side face 36 comprises a hole 136 into which the connector panel is inserted and the face is integral with a side flap 120 which is folded over to form the glued joint. Each lower side flap 118 comprises a slot 130 corresponding to a tab 128 on the

lower central flap 124. The lower side flap 118 integral with the side face 36 having the central fold 138 is formed in two pieces. One piece has a tab 164 which prevents outward movement along the fold 138 when the box- form structure is assembled. A strip of sticky tape 126 is attached to each of the upper and lower flaps.

The cardboard comprises two face skins sandwiching a corrugated core which comprises two fluted layers separated by an intermediate skin layer. The upper fluted layers is formed from 180gsm white top Kraft paper, i. e. paper with a high content of wood pulp mixed with some recycled paper, and the lower fluted layer is formed from 190gsm light-weight-clay coated paper. The cardboard is thus of type BE 190Y 180W. The flutes of the cardboard are arranged perpendicular to the base of the front face orientation whereby the front face is stiffer in a direction parallel to the base than in the direction perpendicular to the base. As a result of the shape of the blank, the flutes of the cardboard in the panels forming the side faces are at an acute angle to the base of each side face.

Each of the folds between the panels is formed by pressing the cardboard to form grooves or creases. The crease may be made when the blank is die-cut by using a strip of steel on the die which has a rounded edge and is set in the die such that the strip pushes in to the sheet only to the required depth. The central fold on one side panel may be formed by pressing a crease, using a rubber

strip on the platen of the press which forms the other creases. The central fold 138 folds in the opposite direction to the other folds between panels and thus the crease is formed on the opposite face of the blank to the other creases.

The box-form structure of the loudspeaker is assembled as follows. The transducer and connector panel are preferably secured to the relevant faces before the box-form structure is assembled. a) Fold upper and lower side flaps 116,118 onto respective side faces 36; the sticky tape 126 bonds the flaps to the faces (see Figure 5D). A hot melt process would achieve the same effect. b) Fold upper and lower central flaps 122, 124 towards the front face 34. By folding over the various flaps, the number of exposed cut surfaces is reduced. Thus, if the speaker is formed from cardboard, the water resistance of the speaker may be improved. c) Fold side faces 36 inwards along the folds 102 having a pair of parallel folds. d) Form the rear spine joining the two side faces 34 by gluing the side flap 120 to one side face. e) Pull lower central flap 124 away from the front face in the direction of the arrow in Figure 5E and lock the tabs 128 into the respective slots 130. The lower central flap 124 abuts both side faces 36 and acts as support flap and strengthens the overall structure. f) Pull upper central flap 122 using central hole 136

away from the front face in the direction of the arrow in Figure 5F. This locks the top face in place and assembly is now complete.

The assembled speaker is collapsible into flat pack form as shown in Figure 5G. First the top face is removed from its locked position by exerting pressure through the hole 134 on the side face 36. The box-form structure is then collapsed inwards along the fold 138 extending along the side face so that 180° of folding is achieved. The collapsed side face together with the other faces defines a W-shaped cross-section. The box-form structure may be erected again as a box by unfolding the collapsed fold and thus the structure may be considered to be of concertina form.

The box-form structure is optionally held flat by press studs 142 or other fasteners. The lower central flap 124 (or support flap) acts as a spacer between interior surfaces of the front and side faces so that a cavity is provided for receiving the transducer.

Alternatively the double folds 102 may act as spacers or holes may be cut in the side faces to allow clearance for the transducer assembly when the speaker is folded down.

Figure 6a shows a loudspeaker 104 which is generally similar to that of Figures 4 and 5 and thus features in common have the same reference numbers. Figure 6a shows the position of the transducer 106 which is mounted on the interior surface of the excited front face 34. The

transducer location is selected so as to optimise the acoustic output from the speaker. The transducer 106 is connected to the connector panel 108 by connections 110.

Ground engaging feet 112 shown in Figure 6b are attached to the base of the loudspeaker 104.

Figures 6c and 6f shows the connector panel 108 which comprises a generally circular plate 148 having three snap-fit connectors 144 whereby the connector panel is secured to a side face of the box-form structure. The plate 148 is integrally formed with a box-like member 150 having a terminal port 146 linking audio connections from an audio source with the connections to the transducer.

The connections may be fitted with a quick release connector to avoid damage to the transducer if the cable is pulled violently. The plate 148 is formed with feet 166 and a rim 168 which all protrude from an interior face to define a cavity which protects the transducer when the box-form structure is flat packed.

Figures 6d and 6e show the transducer 106 and its housing 152. The transducer is a moving coil inertial exciter comprising a magnet assembly 154 and a voice coil assembly 156. The transducer 106 is connected to the connector panel by connections 110. Since the transducer is mounted on a sloping face, there is uneven weight loading which may lead to unwanted movement of the magnet assembly. Thus, in addition to mounting the voice coil assembly 156 direct to a face of the box-form structure, the magnet assembly 154 is supported in a transducer

housing 152.

The transducer housing 152 is in the form of a plastic spider which decouples the mass of the transducer from the face. The spider comprise a cup 155 which covers the transducer 106 and three curved arms 158 extending away from the cup 155. Each distal end of the arms 158 is mounted to the face by resilient sticky pads 160. The magnet assembly 154 is secured to the cup 155 by resilient foam pads 162 which act as a heat sink. The transducer housing discourages unwanted non-axial movement of the magnet assembly and hence voice coil damage may be alleviated and the transducer excursion may be limited.

Figures 7a to 10a show alternative hinge mechanisms or folds for connecting pairs of faces in the loudspeakers. In Figures 7a to 8b and Figure 10a, the hinge is integral with the faces and thus adjacent faces may be formed from a single piece of material. In Figures 9a and 9b the hinge is a discrete member which is connected to both faces and thus both faces may be formed from separate pieces of material.

The loudspeaker may be made from a foldable material, e. g. a monolith or a skinned panel with a collapsible core. A hinge can be made with V-grooving as shown in Figures 7a and 7b. Figure 7a and 7b show the hinge in its open and closed states which correspond to the loudspeaker in flat pack form and assembled box-form respectively.

Each face is made from a composite panel which comprises a core 60 sandwiched between two skins 62. A V-shaped

section of the core, including one skin, is cut-away with the point of the V-shape defining the fulcrum 66 about which the faces are rotatable relative to each other. One face is rotatable in the direction of Arrow B from a position in which both faces are in the same plane (Figure 7a) to a position in which both faces are perpendicular to each other (Figure 7b). Reinforcing tape 64 is added along both sides of the panel in the region of the groove, the tape runs inside the closed hinge.

Figures 8a and 8b show a double hinge comprising two of the V-grooves illustrated in Figures 7a and 7b and thus the same reference numbers are used. Each face is rotated in the directions of arrows C and D from a position in which both faces are in the same plane to a position in which both faces are parallel but not co-planar. Thus 180° of folding is achieved.

Figures 9a and 9b show two faces 52 which are spaced apart so as to define a gap which is approximately equal to the thickness of each face and which are connected by a connector in the form of a strip of self adhesive tape 68 which forms a hinge. One face is rotatable in the direction of Arrow B from a position in which both faces are in the same plane (Figure 9a) to a position in which both faces are perpendicular to each other (Figure 9b).

The tape is chosen to have a high degree of internal damping and a suitable high tack adhesive. If the face is made from a milled core, the tape may prevent loose edges

from rattling and buzzing. This arrangement is appropriate if the faces are not made from a foldable material.

Figure 10a shows a discontinuous single hinge 51 connecting two faces 52. The hinge 51 comprise folds 54 and cutaway sections or openings 56 between the folds.

The hinge or fold should be sufficiently flexible to allow the loudspeaker to be flat packed. The flexibility of the hinge may range from substantially resistant to flexing to fully flexible. If fully flexible the hinge acts as a simply supported edge termination of an excited panel and little or no bending wave energy is transmitted across the hinge. Alternatively, if the hinge resists flexing, i. e. has residual bending stiffness after folding, bending wave energy may be transmitted across the hinge from an excited face to an adjacent face. Although there may be losses as frequencies increase, the hinge may be designed to transmit bending wave energy of all frequencies in the operative range, i. e. at least up to 20KHz.

Figure 10b illustrates the transmission of bending wave energy from a driven face 76 to an adjacent face 78 across a hinge 80. The bending wave energy in the driven face causes a rotational pivoting action (arrow D) about the longitudinal axis of the hinge 80 which drives bending wave energy into the adjacent face 78. Bending waves from the driven face 76 arrive at the hinge 80 as local lateral angular displacements which are translated by the hinge into opposite polarity displacements in the adjacent face

78. The opposite polarity displacements have equal and opposite angles to the original displacements and drive bending waves into the adjacent face 78 as a result of the areal mass, stiffness and inertia of the face 78. As indicated by arrows E and F which shows the direction of local bending wave vibration in the driven face 76 and the adjacent face 78 respectively, the adjacent face 78 is excited in anti-phase to the driven face 76.

Figures 11,12a and 12b show box-form structures which are open, i. e. at least one face is fully or partially missing or removed. In Figure 11, the speaker is generally in the form of a truncated square based pyramid. The speaker has generally triangular shaped front and side faces 82,84 and a transducer 88 is mounted to each of these faces whereby each face forms a separately driven panel-form bending wave acoustic radiator. The rear face 86 is passive but may be modally active via acoustic coupling across the hinge as explained previously. The rear face 86 comprises two sections separated by a gap which acts as a vent to the loudspeaker. The rear face 86 controls the motion of the rear edges of the side faces 84. The rear face adds to the effective baffle size, whereby bass response may be improved.

In Figures 12a and 12b, the loudspeaker comprises a truncated triangular front face 82 and two triangular side faces 84. The front face 82 is driven by a transducer (not shown) and the side faces 84 act as baffles. The rear

edges of the side faces define an open rear face 92,94.

Figure 12a shows a substantially closed baffle in which the rear edges of the side faces almost meet. Thus, the open rear face 92 is small and the lower edge of each side face is at an acute angle a to the lower edge of the front face. Figure 12b shows a substantially open baffle in which the open rear face 94 is large and the lower edge of each side face is at an obtuse angle 0 to the lower edge of the front face. More open baffles generally have greater bass weight.

In each embodiment, each panel-form bending wave acoustic radiator may be a distributed mode radiator as taught in W097/09842 and others to the present applicant, and thus the properties of the panel-form radiator may be chosen to distribute resonant bending wave modes of the radiator substantially evenly in frequency. Turning in particular to the size, as shown in Figure 13, the modal distribution 70 for a large triangular panel-form radiator is more dense, more evenly distributed and extends to lower frequencies than the modal distribution 72 for a radiator of a similar shape which is 50% smaller. In particular, the larger radiator has more evenly distributed low frequency modes (i. e. modes below 500Hz).

Such a substantially even distribution may be achieved by interleaving low frequency modes associated with each conceptual axis of the panel-form radiator.

Appropriate selection of the parameters of the

loudspeaker and transducer location contribute to providing a good acoustic output. Figure 14 shows the frequency response for the speaker of Figure 5 which has a trapezium shaped front face having two parallel sides, i. e. base and top side, of length 515mm and 157mm and height (i. e. distance between the two parallel sides) of 715mm. The transducer is mounted to the inner surface of the front face at a location which is 256mm from the base side and 52 mm from the panel centre line.

Figure 14 shows that the sound pressure level averages 84dB (5dB) over a frequency range extending from approximately 50Hz to 15kHz. The. sound pressure level is measured at 1 metre from the front face for an input of 1 watt. The triangular base means that the side faces provide an effective baffle of a greater depth for the excited side than for other shaped bases, e. g. rectangular. This combined with simply supporting the excited face on all sides may increase the density of modes in the 150Hz to 500Hz region compared to other shaped bases.

Below 100Hz, there are two peaks in the frequency response, the first at approximately 40Hz is caused by the fundamental exciter resonance and the second peak at approximately 70Hz is the first resonant bending mode of the excited face. The first mode is low enough to give a perceived depth of bass. The bass response is also usefully extended by setting the fundamental resonance of

the transducer below that of the radiator.

The invention thus provides a simple and highly portable loudspeaker with a wide variety of applications and markets. Although the invention has been described with reference to packaging materials such as corrugated cardboard, it will be appreciated that more durable, long lasting or higher performance sheet materials could also be appropriate to form the speaker.

In all embodiments, the transducer may be any known exciter or actuator which is suitable. For panel-form bending wave acoustic radiators in the form of distributed mode radiators, the transducer location may be chosen to couple substantially evenly to the resonant bending wave modes. In particular, the transducer location may be chosen to couple substantially evenly to lower frequency resonant bending wave modes. In other words, the transducer may be at a location where the number of vibrationally active resonance anti-nodes is relatively high and conversely the number of resonance nodes is relatively low.