WO2003009219A2

WO2003009219A2 - Electronic article comprising loudspeaker & touch pad

Info

Publication number: WO2003009219A2
Application number: PCT/GB2002/003316
Authority: WO
Inventors: Graham Bank; Neil Simon Owen; Neil Harris; Martin Christopher Cassey; Martin Colloms
Original assignee: New Transducers Limited
Priority date: 2001-07-20
Filing date: 2002-07-19
Publication date: 2003-01-30
Also published as: GB2392797B; AU2002321398A1; CN1529840A; GB2392797A; HK1060931A1; WO2003009219A3; GB0329570D0; JP2005500725A; CN1228700C

Abstract

A touch pad assembly (202) for use in an electronic article (190), the assembly comprising a touch pad (202), a coupler for mechanically coupling the touch pad (202) to casing (198) of the electronic article and a transducer (108) which is mounted by coupling means (66) on the coupler or the touch pad to drive the casing as an acoustic radiator. An electronic article incorporating the same. An electronic article (190) comprising a body in or on which a bending wave loudspeaker is mounted, the loudspeaker comprising a bending wave acoustic radiator (204) and an electromechanical force transducer (108) mounted to the radiator (204) to vibrate the radiator to produce an acoustic output characterised in that the transducer (108) has an intended operative frequency range and comprises a resonant element having a frequency distribution of modes in the operative frequency range and coupling means (66) for mounting the transducer (108) to the radiator.

Description

ELECTRONIC ARTICLE COMPRISING LOUDSPEAKER & TOUCH PAD

DESCRIPTION

TECHNICAL FIELD

The invention relates to electronic articles, in particular low power or self powered articles, for example electronic articles for personal use, such for example, as mobile telephones, personal organisers and pocket radios. BACKGROUND ART

It is known from International patent application WOOO/69212 to provide a personal portable electronic article having a body or casing in or on which a bending wave loudspeaker is mounted. The bending wave loudspeaker comprises an acoustic radiator and a vibration exciter mounted on the radiator to vibrate the radiator to produce an acoustic output. The radiator is formed integrally with the body or casing as an injection moulding and defines a sub- area of the body or casing. By personal portable electronic article it is meant an article which is sufficiently small to be hand-held. The radiator may be a distributed mode acoustic radiator speaker e.g. of the kind described in International application WO97/09842.

In general, personal portable electronic articles are either low power or self-powered devices. SUMMARY OF THE INVENTION

According to the invention an electronic article comprises a body or casing in or on which a bending wave loudspeaker is mounted, the loudspeaker comprising a bending wave acoustic radiator and an electromechanical force transducer mounted to the radiator to vibrate the radiator to produce an acoustic output characterised in that the transducer has an intended operative frequency range and comprises a resonant element having a frequency distribution of modes in the operative frequency range and coupling means for mounting the transducer to the radiator. The coupling means may be mounted on the resonant element .

The electronic article may be a remote powered article, for example, with either a light or infrared power source. The electronic article may thus be selected from a wireless panel, personal PA or solar panel. The electronic article may be a low power article, e.g. cordless devices such as portable radios, alkmans, personal data assistants (PDA) , electronic toys, buzzers, polyphonic or monophonic sounders, chimes, electronic novelties, laptops, computer mouse, keyboard, display case, personal computers, monitors or televisions. The electronic article may further be disposable, e.g. disposable speaker or buzzer, low cost communication devices, credit cards, novelties, books or cards . The acoustic radiator may be moulded or co-moulded integrally with the body or casing. The radiator may be transparent and may, for example, define a display screen area.

The acoustic radiator may be a touch pad. The acoustic radiator may be whole or part of the casing which surrounds the touch pad. The coupling means may comprise the touch pad which is acoustically coupled to the casing, e.g. by a frame surrounding the periphery of the touch pad. A transducer may be mounted to the touch pad either directly on the touch pad or on the frame and the transducer drives the casing. The touch pad may be coupled to the casing by integral locking clips or separate locking components such as bolts, screws or bayonet fixings. Alternatively, the touch pad may be a friction fit on the casing. In this way, an integrated assembly comprising the touch pad and transducer may be used to drive the acoustic radiator. The touch pad and the casing surrounding the touch pad may both act as acoustic radiators with the casing acting as the primary acoustic radiator. If desired, at least one additional transducer may be mounted on the primary acoustic radiator. The touch pad assembly may replace a standard touch pad in any electronic equipment, e.g. a laptop or a personal data assistant. The resonant element may be active e.g. may be a piezoelectric transducer and may be in the form of a strip of piezoelectric material . Alternatively, the resonant element may be passive and the transducer may further comprise an active transducer, e.g. an inertial or grounded vibration transducer, actuator or exciter, e.g. moving coil transducer. The active transducer may be a bender or torsional transducer (e.g. of the type taught in WO00/13464) . Furthermore, the transducer may comprise combination of passive and active elements to form a hybrid transducer. A number of transducer, exciter or actuator mechanisms have been developed to apply a force to a structure, e.g. an acoustic radiator of a loudspeaker. There are various types of these transducer mechanisms, for example moving coil, moving magnet, piezoelectric or magnetostrictive types. Typically, electrodynamic speakers using coil and magnet type transducers lose 99% of their input energy to heat whereas a piezoelectric transducer may lose as little as 1%. Thus, piezoelectric transducers are popular because of their high efficiency. There are several problems with piezoelectric transducers, for example, they are inherently very stiff, for example comparable to brass foil, and are thus difficult to match to an acoustic radiator, especially to the air. Raising the stiffness of the transducer moves the fundamental resonant mode to a higher frequency. Thus such piezoelectric transducers may be considered to have two operating ranges. The first operating range is below the fundamental resonance of the transducer. This is the "stiffness controlled" range where velocity rises with frequency and the output response usually needs equalisation. This leads to a loss in available efficiency. The second range is the resonance range beyond the stiffness range, which is generally avoided because the resonances are rather fierce.

Moreover, general teaching is to suppress resonances in a transducer, and thus piezoelectric transducers are generally used only used in the frequency range below or at the fundamental resonance of the transducers . Where piezoelectric transducers are used above the fundamental resonance frequency it is necessary to apply damping to suppress resonance peaks. The problems associated with piezoelectric transducers similarly apply to transducers comprising other "smart" materials, i.e. magnetostrictive, electrostrictive, and electret type materials. Various piezoelectric transducers are also known, for example as described in EP 0993 231A of

Shinsei Corporation, EP 0881 856A of Shinsei Corporation, US

4,593,160 OF Murata Manufacturing Co. Limited, US 4,401,857 of Sanyo Electric Co Limited, US 4,481,663 of Altec

Corporation and UK patent application GB2,166,022A of Sawafuji. However, it is an object of the invention to employ an improved transducer.

The transducer used in the present invention may be considered to be an intendedly modal transducer. The coupling means may be attached to the resonant element at a position which is beneficial for coupling modal activity of the resonant element to the interface. The parameters, e.g. aspect ratio, bending stiffness, thickness and geometry, of the resonant element may be selected to enhance the distribution of modes in the resonant element in the operative frequency range. The bending stiffness and thickness of the resonant element may be selected to be isotropic or anisotropic. The variation of bending stiffness and/or thickness may be selected to enhance the distribution of modes in the resonant element. Analysis, e.g. computer simulation using FEA or modelling, may be used to select the parameters .

The distribution may be enhanced by ensuring a first mode of the active element is near to the lowest operating frequency of interest. The distribution may also be enhanced by ensuring a satisfactory, e.g. high, density of modes in the operative frequency range. The density of modes is preferably sufficient for the active element to provide an effective mean average force which is substantially constant with frequency. Good energy transfer may provide beneficial smoothing of modal resonances. Alternatively, or additionally, the distribution of modes may be enhanced by distributing the resonant bending wave modes substantially evenly in frequency, i.e. to smooth peaks in the frequency response caused by "bunching" or clustering of the modes. Such a transducer may thus be known as a distributed mode transducer or DMT.

Such an intendedly modal or distributed mode transducer is described in International patent application WO01/54450. The transducer may comprise a plurality of resonant elements each having a distribution of modes, the modes of the resonant elements being arranged to interleave in the operative frequency range and thus enhance the distribution of modes in the transducer as a whole device . The resonant elements may have different fundamental frequencies and thus, the parameters, e.g. loading, geometry or bending stiffness of the resonant elements may be different.

The resonant elements may be coupled together by connecting means in any convenient way, e.g. on generally stiff stubs, between the elements. The resonant elements are preferably coupled at coupling points which enhance the modality of the transducer and/or enhance the coupling at the site to which the force is to be applied. Parameters of the connecting means may be selected to enhance the modal distribution in the resonant element. The resonant elements may be arranged in a stack. The coupling points may be axially aligned.

The resonant element may be plate-like or may be curved out of planar. A plate-like resonant element may be formed with slots or discontinuities to form a multi-resonant system. The resonant element may be in the shape of a beam, trapezoidal, hyperelliptical or may be generally disc shaped. Alternatively, the resonant element may be rectangular and may be curved out of the plane of the rectangle about an axis along the short axis of symmetry.

The resonant element may be modal along two substantially normal axes, each axis having an associated fundamental frequency. The ratio of the two fundamental frequencies may be adjusted for best modal distribution, e.g. 9:7 (-1.286:1) . As examples, the arrangement of such modal transducer may be any of: a flat piezoelectric disc; a combination of at least two or preferably at least three flat piezoelectric discs; two coincident piezoelectric beams; a combination of multiple coincident piezoelectric beams; a curved piezoelectric plate; a combination of multiple curved piezoelectric plates or two coincident curved piezoelectric beams .

The interleaving of the distribution of the modes in each resonant element may be enhanced by optimising the frequency ratio of the resonant elements, namely the ratio of the frequencies of each fundamental resonance of each resonant element. Thus, the parameter of each resonant element relative to one another may be altered to enhance the overall modal distribution of the transducer.

When using two active resonant elements in the form of beams, the two beams may have a frequency ratio (i.e. ratio of fundamental frequency) of 1.27:1. For a transducer comprising three beams, the frequency ratio may be 1.315:1.147:1. For a transducer comprising two discs, the frequency ratio may be 1.1 +/- 0.02 to 1 to optimise high order modal density or may be 3.2 to 1 to optimise low order modal density. For a transducer comprising three discs, the frequency ratio may be 3.03:1.63:1 or may be 8.19:3.20:1. The parameters of the coupling means may be selected to enhance the distribution of modes in the resonant element in the operative frequency range. The coupling means may be vestigial, e.g. a controlled layer of adhesive. The coupling means may be positioned asymmetrically with respect to the panel so that the transducer is coupled asymmetrically. The asymmetry may be achieved in several ways, for example by adjusting the position or orientation of the transducer with respect to axes of symmetry in the panel or the transducer.

The coupling means may form a line of attachment. Alternatively, the coupling means may form a point or small local area of attachment where the area of attachment is small in relation to the size of the resonant element. The coupling means may be in the form of a stub and have a small diameter, e.g. 3 to 4 mm. The coupling means may be low mass.

The coupling means may comprise more than one coupling point and may comprise a combination of points and/or lines of attachment. For example, two points or small local areas of attachment may be used, one positioned near centre and one positioned at the edge of the active element. This may be useful for plate-like transducers which are generally stiff and have high natural resonance frequencies .

Alternatively only a single coupling point may be provided. This may provide the benefit, in the case of a multi-resonant element array, that the output of all the resonant elements is summed through the single coupling means so that it is not necessary for the output to be summed by the load. The coupling means may be chosen to be located at an anti-node on the resonant element and may be chosen to deliver a constant average force with frequency. The coupling means may be positioned away from the centre of the resonant element .

The position and/or the orientation of the line of attachment may be chosen to optimise the modal density of the resonant element . The line of attachment is preferably not coincident with a line of symmetry of the resonant element. For example, for a rectangular resonant element, the line of attachment may be offset from the short axis of symmetry (or centre line) of the resonant element. The line of attachment may have an orientation which is not parallel to a symmetry axis of the panel .

The shape of the resonant element may be selected to provide an off-centre line of attachment which is generally at the centre of mass of the resonant element. One advantage of this embodiment is that the transducer is attached at its centre of mass and thus there is no inertial imbalance. This may be achieved by an asymmetric shaped resonant element which may be in the shape of a trapezium or trapezoid. For a transducer comprising a beam-like or generally rectangular resonant element, the line of attachment may extend across the width of the resonant element . The area of the resonant element may be small relative to that of the acoustic radiator.

The acoustic radiator may be in the form of a panel . The panel may be flat and may be lightweight. The material of the acoustic radiator may be anisotropic or isotropic.

The acoustic radiator may have a distribution of resonant bending wave modes as taught in WO 97/09842 and the properties of the acoustic radiator may be chosen to distribute the resonant bending wave modes substantially evenly in frequency, i.e. to smooth peaks in the frequency response caused by "bunching" or clustering of the modes. In particular, the properties of the acoustic radiator may be chosen to distribute the lower frequency resonant bending wave modes substantially evenly in frequency. The lower frequency resonant bending wave modes are preferably the ten to twenty lowest frequency resonant bending wave modes of the acoustic radiator.

The transducer location may be chosen to couple substantially evenly to the resonant bending wave modes in the acoustic radiator, in particular to lower frequency resonant bending wave modes. In other words, the transducer may be mounted at a location where the number of vibrationally active resonance anti-nodes in the acoustic radiator is relatively high and conversely the number of resonance nodes is relatively low. Any such location may be used, but the most convenient locations are the near-central locations between 38% to 62% along each of the length and width axes of the acoustic radiator, but off-centre.

Specific or preferential locations are at 3/7,4/9 or 5/13 of the distance along the axes; a different ratio for the length axis and the width axis is preferred. Preferred is 4/9 length, 3/7 width of an isotropic panel having an aspect ratio of 1:1.13 or 1:1.41.

The operative frequency range may be over a relatively broad frequency range and may be in the audio range and/or ultrasonic range. There may also be applications for sonar and sound ranging and imaging where a wider bandwidth and/or higher possible power will be useful by virtue of distributed mode transducer operation. Thus, operation over a range greater than the range defined by a single dominant, natural resonance of the transducer may be achieved. The lowest frequency in the operative frequency range is preferably above a predetermined lower limit which is about the fundamental resonance of the transducer.

For example, for a beam-like active resonant element, the force may be taken from the centre of the beam, and may be matched to the mode shape in the acoustic radiator to which it is attached. In this way, the action and reaction may co-operate to give a constant output with frequency. By connecting the resonant element to the acoustic radiator at an anti-node of the resonant element, the first resonance of the resonant element may appear to be a low impedance. In this way, the acoustic radiator should not amplify the resonance of the resonant element .

According to a second aspect of the invention, there is provided a touch pad assembly for use in an electronic article, e.g. laptop or PDA, the assembly comprising a touch pad, coupling means for mechanically coupling the touch pad to casing of the electronic article and a transducer which is mounted on the coupling means or the touch pad to drive the casing as an acoustic radiator. The coupling means may be in the form of a frame surrounding the periphery of the touch pad. A transducer may be mounted on the touch pad or on the frame . The touch pad may be coupled to the casing by integral locking clips or separate locking components such as bolts, screws or bayonet fixings. Alternatively, the touch pad may be a friction fit on the casing.

BRIEF DESCRIPTION OF DRAWINGS The invention is diagrammatically illustrated, by way of example, in the accompanying drawings in which: Figures 1A shows a front perspective view of a disposable loudspeaker embodying the present invention;

Figures IB shows and a cross-sectional view along line AA of Figure 1A; Figures 2A and 2B show front perspective and side views of a loudspeaker component embodying the present invention;

Figure 3 shows a cross-section of a mouse or pointing device for a personal computer embodying the present invention; Figure 4 shows a cross-section of a loudspeaker embodying the present invention mounted in an enclosure;

Figure 5 shows a cross-section of a personal data assistant or other portable computer embodying the present invention; Figures 6A and 6B show side and plan views of a loudspeaker system embodying the present invention;

Figure 7 shows a perspective view of a laptop computer embodying the present invention;

Figure 8 shows a cross-section along line AA (shown in Figure 7) of a second laptop computer embodying the present invention;

Figures 9A and 9B show plan and cross-sectional views of a touch pad assembly; Figure 9C shows a partial cross-sectional view of the touch pad assembly of Figures 9A and 9B incorporated in electronic apparatus;

Figure 9D shows a partial perspective view of the inside of the casing of a laptop incorporating a touch pad assembly of Figures 9A and 9B;

Figures 10A and 10B show graphs of sound output versus frequency for a known laptop and a laptop using the touch pad assembly of Figure 9A, respectively; Figure IOC is a graph of spatial average transfer function in dB against frequence for a second laptop using a touch pad assembly of Figure 9B;

Figure 11A is a front view of a personal digital assistant (PDA) embodying the invention; Figure 11B is a cross-sectional side view of the PDA of Figure 11A;

Figure 12A is a perspective view of a visual display unit (VDU) embodying the present invention;

Figure 12B is a cross-sectional view of part of the VDU of Figure 12A;

Figure 13A is a front view of a credit card embodying the present invention;

Figure 13B is a cross-sectional side view of the credit card of Figure 13A and taken along the line A-A; Figure 13C is a cross-sectional view of the credit card of Figure 13A and taken along the line B-B;

Figure 14 is a perspective view of a greeting card embodying the present invention; Figure 15A is a front view of a personal digital assistant (PDA) embodying the present invention;

Figure 15B is a cross-sectional side view of the PDA of

Figure 15A;

Figures 16 to 22 are side views of alternative modal transducers which may be used in the present invention;

Figure 23 is a plan view of an alternative modal transducer which may be used in the present invention;

Figure 24a is a schematic plan view of a parameterised model of a transducer which may be used in the present invention;

Figure 24b is a section perpendicular to the line of attachment of the transducer of Figure 24a;

Figure 25a is a schematic plan view of a parameterised model of a transducer which may be used in the present invention and

Figure 25b is a schematic plan view of the transducer of

Figure 25a.

DETAILED DESCRIPTION

Figures 1A and IB show a disposable loudspeaker, for example, a polyphonic sounder, disposable buzzer, credit card or other novelty loudspeaker. The loudspeaker comprises a panel (60) which is capable of supporting bending wave vibration, preferably resonant bending wave vibration and a transducer (62) mounted to the panel (60) by a connecting stub ( 66) to excite bending wave vibration to produce an acoustic output. The transducer (62) is an intendedly modal transducer or distributed mode transducer as hereinbefore described and as described in WO01/54450. The transducer (62) comprises a piezoelectric plate element (64) . Two flexible wires in the form of connecting leads (68) provide electrical input to the element (64) .

Figure 2 shows a loudspeaker (58) similar to that of Figures 1A and IB and thus elements in common have the same reference number. The loudspeaker (58) is mounted by way of a flexible surround (86) onto a support frame (84) which extends around the loudspeaker periphery. The support frame (84) allows the loudspeaker to be easily mounted onto a surface or additional support .

Figure 3 shows a cross section of a mouse (88) which is used as a pointing device in a computer system (not shown) .

The mouse (88) comprises the standard components such as ball

(90) , lower case (92) and cover (94) . The cover (94) is designed to be capable of supporting bending wave vibration, preferably resonant bending wave vibration. An intendedly modal transducer (108) is mounted to the cover (94) by a connecting stub (66) to excite bending wave vibration to produce an acoustic output.

The transducer comprises upper and lower bimorph beams (112) and (110) , the upper beam (112) being connected to the cover (94) by the stub (66) which extends across the width of the beams. The stub may be 1-2 mm wide and high and may be made from hard plastics and/or metal with suitable insulating layers to prevent electrical short circuits . The upper beam (112) is longer than the lower beam (110) and the beams are connected by a centrally mounted stub (152) . Each beam consists of three layers, namely two outer layers of piezoelectric ceramic material, e.g. PZT 5H, sandwiching a central brass vane. The outer layers may be attached to the brass vane by adhesive layers which are typically 10 - 15 microns in thickness.

Figure 4 shows a panel (60) which is capable of supporting bending wave vibration, preferably resonant bending wave vibration. The panel (60) is mounted in a closed box (154) , by way of a flexible suspension (156) which extends around the periphery of the panel (60) . An intendedly modal transducer (108), similar to that used in Figure 3, is mounted to the panel (60) by a connecting stub (66) to excite bending wave vibration to produce an acoustic output. The closed box (154) prevents, by and large, sound radiated from the rear (155) of the panel (60) from interfering with sound radiated from the front (157) of the panel. The box (154) thus acts as a baffle to prevent acoustical cancellation. The box (154) may be filled with a suitable absorber.

Figure 5 shows a personal data assistant (PDA) (158) which comprises the normal components, namely a case (176) which supports keys (170) and a lid (180) which is hinged about a hinge (178) to the case (176) . The lid (180) supports a display (172) , which may be liquid crystal display

(LCD) or thin film transistor (TFT) and an optional front window (174) which may be fitted in front of the display

(172) . The lid (180) is designed to be capable of supporting bending wave vibration, preferably resonant bending wave vibration. An intendedly modal transducer (62) , such as the transducer (62) of Figures 1A and IB is mounted to the lid

(180) by a connecting stub (66) to ^'excite bending wave vibration to produce an acoustic output . The transducer (62) has a mechanical source impedance, which is matched to that of the lid (180) whereby maximum power transfer may be achieved. As an alternative or in addition, a transducer may be mounted to the case (176) .

Figures 6A and 6B shows a loudspeaker system comprising a panel (60) capable of supporting bending wave vibration, preferably resonant bending wave vibration and an intendedly modal transducer (62) , such as the transducer (62) of Figures

1A and IB. The transducer (62) is mounted to the panel (60) by a connecting stub (66) to excite bending wave vibration to produce an acoustic output .

The signal for the transducer (62) is provided by an amplifier (182) which is mounted on the panel (60) . The system further comprises a power source (184), e.g. a battery, solar cell or direct infrared link, which powers the amplifier. Thus, the loudspeaker system (186) is adapted for operation as a wireless device, which may be used in a wireless panel/personal PA, self-powered solar panel, cordless devices or portable radio/walkman. The system (186) may be fully remote powered - e.g. light/infrared power source

Figures 7 and 8 show a laptop computer (190) comprising the following standard components, namely a base (198) which supports keys (200) and a touch pad (202) , and a lid (194) which is hinged about hinge (196) to the base. A display screen (192) is fitted into the lid (194) . The keys (200) are located towards the screen (192) . The touch pad (202) which is used for pointing functions sits near to the centre of the edge (201) of the base (198) which is closest to the user.

In Figure 7, two modal transducers (62), such as those used in the Figure 1A and IB embodiment, are mounted by stubs (66) within the base to an inner upper surface of the base.

Alternatively, as shown in Figure 8, modal transducers (108) such as that used in Figure 3 may be used. The upper surface of the base is designed to have regions (204) which cover all or part of the base and which are capable of supporting bending wave vibration, preferably resonant bending wave vibration. In either embodiment, the transducers are mounted to two such regions to excite bending wave vibration to produce an acoustic output. The transducers may be designed to drive the local case mechanical impedance to achieve a high level of mechanical coupling efficiency.

Figures 9A and 9B show a touch pad assembly (203) which may be used to replace the touch pad of Figures 7 and 8, or the touch pad of other embodiments of the invention. The touch pad assembly (203) comprises a touch pad (202) , a frame (205) extending around the perimeter of the touch pad (202) and a transducer (207) mounted on the rame. The frame is grooved whereby the frame has a generally U-shaped cross- section. Both the touch pad (202) and the transducer (207) are mounted within the groove but are separated by a small air gap (213) .

The touch pad (202) is made from a glass fibre reinforced plastics circuit board material and has mechanical impedance of approximately 3.59Ns/m. The touch pad (202) is approximately 0.4mm thick and a plastics laminate which is 170 microns is adhered to a front surface of the touch pad

(202) . The plastics laminate provides a decorative or protective coating.

As shown in Figure 9C, the frame (205) of the touch pad assembly (203) is mounted to the casing (209) of electronic apparatus, e.g. a laptop or a personal data assistant. The transducer (207) drives bending wave vibration in the frame

(205) . The frame (205) is mechanically and acoustically coupled to the casing whereby vibration of the frame is transmitted to the casing (209) . The casing forms the primary acoustic radiator of the electronic apparatus.

The transducer (207) is chosen to match the impedance of the combined touch pad and wrist pad. The transducer is preferably a DMT but may alternatively be an inertial or a grounded vibration transducer, actuator or exciter, e.g. moving coil transducer, a piezoelectric transducer, a magneto-strictive exciter or a bender or torsional transducer

(e.g. of the type taught in WO 00/13464) .

Figure 9D shows a section of the inside of laptop in which the touch pad assembly is mounted. Figure 9D shows a view of the top of the casing facing downwards . The touch pad

(202) is supported in the casing (209) with areas (240) either side of the touch pad (202) forming wrist rests. The casing (209) also comprises an aperture (241) in which the keyboard is inserted with a moulded divider (242) separating the aperture from the wrist rest areas (240) . A strip (243) of 1.5mm thick polystyrene is attached down one edge of the wrist rests to help with the panel boundary conditions. A second strip (not shown) is attached to the wrist rest on the other side of the touch pad. The strips extend between a front wall (244) of the casing (209) and the moulded divider

(242) .

In each of the laptop embodiments, to avoid spurious rattles, small foam spacers may be fitted to any buttons on the casing and to the finned metal foil which connects the chassis of the central processing unit to a heat sink.

The benefit of such an arrangement is that the touch-pad and transducer are incorporated in a single integrated assembly. Furthermore, additional electrical connections (211) for the transducer may be easily added to the touch pad

(202) which already carries electrical connections for other purposes. The integrated assembly provides the possibility of reducing complexity, weight and cost as well as taking up less space, which is at a premium in compact portable electronic articles.

Figures 10A and 10B show the frequency response of a known DELL (TM) laptop with an existing microspeaker and a laptop in which the touch pad has been replaced with a touch pad assembly of Figures 9A to 9C. Measurements were taken at 25cms above the wrist pad with the laptop placed on a flat desk which was heavy enough to not contribute to the measured output. The laptop according to the present invention benefits from an improved level of treble.

Furthermore, the laptop according to the present invention has a speaker with a substantially capacitative impedance. In particular, the modulus of the impedance falls from over lOOOohms at 1kHz to lOOohms at 10kHz. Thus, generally the speaker has a falling power content as the frequency rises, especially for music. Figure IOC illustrates the performance of a Compaq (TM) laptop in which the touch pad has been replaced with an assembly similar to that shown in Figure 9A. The transducer is mounted to the touch pad and drives both the touch pad and the casing (wrist rest) either side of the touch pad. The panel boundary conditions are defined as in Figure 9D. The wrist rests on either side of the touch pad are adapted to have equal mechanical impedance . The impedance of the transducer was designed to match the overall impedance of the panel and wrist rests combined. Thus the transducer is a double beam transducer with wider than normal beams of length 42 mm and 39 mm respectively.

As shown in Figure 10C has a low frequency limit of approximately 400Hz. The acoustical output is dominated by the acoustical loading at the rear of the panel and if the small air space behind the panel is reduced, the bandwidth will be severely curtailed. For example, bandwidth extending down to 200Hz may be achieved with a suitable transducer in a

6mm air space but the low frequency limit rises to 600 Hz if the air space drops to 2 mm. Figure 11A shows a front view of a Personal Digital assistant (PDA) which often have a touch screen (214) , as well as buttons (216) for control and data input. A sectioned view in Figure 9 shows the PDA in more detail. The case (218) is usually made in two parts, which fit together to contain the display screen (220) and the electronics are fitted onto an internal printed circuit board (224) . The rear of the case (usually a plastics moulding) is used to radiate sound by attaching a transducer (108) via a stub (66) . The transducer (108) comprises a longer beam (112) driving the stub (66) with a second beam (110) connected by way of a second stub (152) . Although in this case the longer beam is close to the case (218) , the two beams could be exchanged without any detriment . Leads are provided for electrical input connections. Figure 12A shows a perspective view of a visual display unit (137) formed in any desired fashion, e.g. as a cathode ray tube or as a liquid crystal display. The unit (137) comprises a box-like housing (101) having a display screen

(37) mounted in a front face, a rear face and opposed sides (102) . As shown more clearly in Figure 12B, a generally rectangular panel (2) is defined by grooves (3) in each of the opposed sides (102) . Each panel (2) comprises a core (22) sandwiched between two skins (21) . A double beam transducer (108) as described above is attached to each panel (2) to launch/excite bending waves into the panels to cause them to resonate to produce an acoustic output.

The use of the intendly modal transducer (108) allows good mechanical coupling to be achieved by matching the mechanical impedance of the transducer to the side. Thus, although the panels (2) may be specifically designed to be optimised for acoustic performance, i.e. may be light and stiff, this is not essential. This avoids the need to substantially change the manufacturing requirements and methods utilised by the monitor/TV manufacturer. Figures 13A to 13C show a credit card (226) in which a single beam transducer (62) is mounted within a pocket (230) in the body (228) of the card (226) . The transducer (62) drives the card to radiate sound by way of a stub (66) which may be integrally moulded into the body (228) . The pocket (230) allows the ends of the transducer (62) to freely vibrate without touching any other parts of the card. The card is powered by an embedded electronic circuit (230) , which may comprise a power source, memory, signal processing and amplification and is connected to wires (236) linked to the transducer (62) . A thin cover (232) , which may be made from a suitable paper, plastics or metal, is provided to enclose the transducer (62) and electronic circuit (230) .

Figure 14 shows a greetings or similar card (81) in the form of a folded member having a front leaf (83) and a rear leaf (85) . A transducer (62) is attached to one of the leaves, preferably the rear leaf (146) , by way of a small stub (not shown) , to vibrate the leaf to cause it to resonate to produce an acoustic output. The transducer (62) is driven by a signal generator/amplifier battery unit (87) , which is actuated by a switch (89) concealed in the fold of the card so as to activate the signal generator when the card is opened.

Figured 15A and 15B show a PDA similar to that of Figures 11A and 11B and thus elements in common have the same reference number. The PDA differs in that it comprises a lid

(215) which protects the touch screen (214) and which is depicted in its open position, i.e. that of normal use. A double beam transducer (108) is attached to the lid (215) by a stub (66) to enable the lid (215) to be used as a loudspeaker. This stub (66) may be integrally moulded into the lid (215) . The transducer (108) comprises two beams (110,112) of different lengths connected together by a stub (152) . Electrical connections are made by wires to the drive circuitry within the body (218) of the PDA (not shown) . The remaining figures show alternative transducers which may be used in conjunction with the embedded loudspeakers embodied in Figures 1 to 15b. An intendedly modal transducer may be designed with reduced mass and depth compared to a moving coil/permanent magnet design. Accordingly, the use of such a transducer should reduce the overall weight of the loudspeaker and the transducer should be suitable for installations in which space is limited. Thus, the transducer is ideally suited to portable applications shown in Figures 1 to 15B.

Figure 16 shows a transducer (42) which comprises a first piezoelectric beam (43) on the back of which is mounted a second piezoelectric beam (51) by connecting means in the form of a stub (48) located at the centre of both beams. Each beam is a bi-morph. The first beam (43) comprises two layers

(44,46) of piezoelectric material and the second beam (51) comprises two layers (50,52). The poling directions of each layer of piezoelectric material are shown by arrows (49) .

Each layer (44, 50) has an opposite poling direction to the other layer (46, 52) in the bi-morph. The bimorph may also comprise a central conducting vane which allows a parallel electrical connection as well as adding a strengthening component to the ceramic piezoelectric layers. Each layer of each beam (44, 46) may be made of the same/different piezoelectric material. Each layer is generally of a different length.

The first piezoelectric beam (43) is mounted on a panel

(54) by coupling means in the form of a stub (56) located at the centre of the first beam. By mounting the first beam at its centre only the even order modes will produce output . By locating the second beam behind the first beam, and coupling both beams centrally by way of a stub they can both be considered to be driving the same axially aligned or co- incident position.

When elements are joined together, the resulting distribution of modes is not the sum of the separate sets of frequencies, because each element modifies the modes of the other. The two beams are designed so that their individual modal distributions are interleaved to enhance the overall modality of the transducer. The two beams add together to produce a useable output over a frequency range of interest . Local narrow dips occur because of the interaction between the piezoelectric beams at their individual even order modes. The second beam may be chosen by using the ratio of the fundamental resonance of the two beams. If the materials and thicknesses are identical, then the ratio of frequencies is just the square of the ratio of lengths. If the higher fO (fundamental frequency) is simply placed half way between fO and fl of the other, larger beam, f3 of the smaller beam and f4 of the lower beam coincide.

Plotting a graph of a cost function against ratio of frequency for two beams shows that the ideal ratio is 1.27:1, namely where the cost function is minimised at point. This ratio is equivalent to the "golden" aspect ratio (ratio of f02:f20) described in WO97/09482. The method of improving the modality of a transducer may be extended by using three piezoelectric beams in the transducer. The ideal ratio is 1.315:1.147:1.

The method of combining active elements, e.g. beams, may be extended to using piezoelectric discs. Using two discs, the ratio of sizes of the two discs depends upon how many modes are taken into consideration. For high order modal density, a ratio of fundamental frequencies of about 1.1 +/- 0.02 to 1 may give good results. For low order modal density (i.e. the first few or first five modes), a ratio of fundamental frequencies of about 3.2:1 is good. The first gap comes between the second and third modes of the larger disc. Since there is a large gap between the first and second radial modes in each disc, much better interleaving is achieved with three rather than with two discs. When adding a third disc to the double disc transducer, the obvious first target is to plug the gap between the second and third modes of the larger disc of the previous case. However, geometric progression shows that this is not the only solution. Using

fundamental frequencies of fO, α.fO and α².f0, and plotting

rms (α α²) there exist two principal optima for . The values are about 1.72 and 2.90, with the latter value corresponding to the obvious gap-filling method.

Using fundamental frequencies of fO, α.fO and β.fO so

that both scalings are free and using the above values of α as seed values, slightly better optima are achieved. The parameter pairs (α β are (1.63, 3.03) and (3.20, 8.19). These optima are quite shallow, meaning that variations of 10%, or even 20%, in the parameter values are acceptable.

An alternative approach for determining the different discs to be combined is to consider the cost as a function of the ratio of the radii of the three discs . The cost functions may be RSCD (ratio of sum of central differences) , SRCD (sum of the ratio of central differences) and SCR (sum of central ratios) . For a set of modal frequencies, f₀, fi, fn, ... tin, these functions are defined as: RSCD (R sum CD) :

¹ _T∑(Λ₊₁+Λ-₁-2Λ)²

N- «=1

SCRD (sum RCD) :

CR :

The optimum radii ratio, i.e. where the cost function is minimised, is 1.3 for all cost functions. Since the square of the radii ratio is equal to the frequency ratio, for these identical material and thickness discs, the results of 1.3*1.3=1.69 and the analytical result of 1.67 are in good agreement . Alternatively or additionally, passive elements may be incorporated into the transducer to improve its overall modality. The active and passive elements may be arranged in a cascade. Figure 17 shows a multiple disc transducer (70) comprising two active piezoelectric elements (72) stacked with two passive resonant elements (74), e.g. thin metal plates so that the modes of the active and passive elements are interleaved.

The elements are connected by connecting means in the form of stubs (78) located at the centre of each active and passive element. The elements are arranged concentrically. Each element has different dimensions with the smallest and largest discs located at the top and bottom of the stack, respectively. The transducer (70) is mounted on a load device (76), e.g. a panel, by coupling means in the form of a stub (78) located at the centre of the first passive device which is the largest disc. The method of improving the modality of a transducer may be extended to a transducer comprising two active elements in the form of piezoelectric plates. Two plates of dimensions (1 by α) and (α by α²) are coupled at (3/7, 4/9) . The frequency ratio is therefore about 1.3:1 (1.14 x 1.14 = 1.2996). As shown in Figure 18, small masses (104) may be mounted at the end of the piezoelectric transducer (106) having coupling means (105) . In Figure 19, the transducer (114) is an inertial electrodynamic moving coil exciter, e.g. as described in WO97/09842, having a voice coil forming an active element (115) and a passive resonant element in the form of a modal plate (118) . The active element (115) is mounted on the modal plate (118) and off-centre of the modal plate .

The modal plate (118) is mounted on the panel (116) by a coupler (120) . The coupler is aligned with the axis (119) of the active element but not with the axis (Z) normal to the plane of the panel (116) . Thus the transducer is not coincident with the panel axis (Z) . The active element is connected to an electrical signal input via electrical wires (122) . The modal plate (118) is perforate to reduce the acoustic radiation therefrom and the active element is located off-centre of the modal plate (118) , for example, at the optimum mounting position, i.e. (3/7, 4/9) .

Figure 20 shows a transducer (124) comprising an active piezoelectric resonant element which is mounted by coupling means (126) in the form of a stub to a panel (128) . Both the transducer (124) and panel (128) have ratios of width to length of 1:1.13. The coupling means (126) is not aligned with any axes (130, Z) of the transducer or the panel. Furthermore, the placement of the coupling means is located at the optimum position, i.e. off-centre with respect to both the transducer (124) and the panel (128) .

Figure 21 shows a transducer (132) in the form of active piezoelectric resonant element in the form of a beam. The transducer (132) is coupled to a panel (134) by two coupling means (136) in the form of stubs. One stub is located towards an end (138) of the beam and the other stub is located towards the centre of the beam.

Figure 22 shows a transducer (140) comprising two active resonant elements (142,143) coupled by connecting means (144) and an enclosure (148) which surrounds the connecting means

(144) and the resonant elements (142) . The transducer is thus made shock and impact resistant . The enclosure is made of a low mechanical impedance rubber or comparable polymer so as not to impede the transducer operation. If the polymer is water resistant, the transducer (140) may be made waterproof.

The upper resonant element (142) is larger than the lower resonant element (143) which is coupled to a panel (145) via a coupling means in the form of a stub. The stub is located at the centre of the lower resonant element (143) .

The power couplings (150) for each active element extend from the enclosure to allow good audio attachment to a load device

(not shown) . Figure 23 shows a transducer (160) in the form of a plate-like active resonant element. The resonant element is formed with slots (162) which define fingers (164) and thus form a multi-resonant system. The resonant element is mounted on a panel (168) by a coupling means in the form of a stub (166) .

In Figures 24A and 24B, the transducer (14) is rectangular with out-of-plane curvature and is a pre-stressed piezoelectric transducer of the type disclosed in US patent 5632841 (International patent application WO 96/31333) and produced by PAR Technologies Inc under the trade name NASDRIV. Thus the transducer (14) is an active resonant element . The transducer has width (W) and length (L) and the position (x) of the attachment point (16) .

The curvature of the transducer (14) means that the coupling means (16) is in the form of a line of attachment. When the transducer is mounted along a line of attachment along the short axis through the centre, the resonance frequencies of the two arms of the transducer are coincident .

The optimum suspension point may be modelled and is the line of attachment at 43% to 44% along the length of the resonant element. The cost function (or measure of "badness") is minimised at this value; this corresponds to an estimate for the attachment point at 4/9ths of the length. Furthermore, computer modelling showed this attachment point to be valid for a range of transducer widths. A second suspension point at 33% to 34% along the length of the resonant element also appears suitable.

By plotting a graph of cost (or rms central ratio) against aspect ratio (AR=W/2L) for a resonant element mounted at 44% along its length, the optimum aspect ratio may be determined to be 1.06 +/- 0.01 to 1 since the cost function is minimised at this value.

The optimum angle of attachment θ to the panel (12) may be determined using two "measures of badness" to find the optimum angle. For example, the standard deviation of the log (dB) magnitude of the response is a measure of "roughness". Such figures of merit/badness are discussed in International Application WO 99/41839, to the present applicants. For an optimised transducer, namely one with aspect ratio 1.06:1 and attachment point at 44% using modelling, rotation of the line of attachment (16) will have a marked effect since the attachment position is not symmetrical. There is a preference for an angle of about

270°, i.e. with the longer end facing left. Figures 25A and 25B show an asymmetrically shaped transducer (18) in the form of a resonant element having a trapezium shaped cross-section. The shape of a trapezium is controlled by two parameters, AR (aspect ratio) and TR (taper ratio) . AR and TR determine a third parameter, λ, such that some constraint is satisfied - for example, equal mass either side of the line.

The constraint equation for equal mass (or equal area) is as follows;

The above may readily be solved for either TR or λ as the dependent variable, to give:

1-22 . 1 + TR - l + TR² 1 TR TR = —₇ or 2 = »

22(1-2) 2TR 2 4

Equivalent expressions are readily obtained for equalising the moments of inertia, or for minimising the total moment of inertia . The constraint equation for equal moment of inertia (or equal 2^nd moment of area) is as follows; ^

The constraint equation for minimum total moment of inertia

TR = 3 - 6λ or 2 = --—

2 6

A cost function (measure of "badness") was plotted for the results of 40 FEA runs with AR ranging from 0.9 to 1.25, and TR ranging from 0.1 to 0.5, with λ constrained for equal mass . The transducer is thus mounted at the centre of mass .

The results are tabulated below and show that there is an optimum shape with AR=1 and TR=0.3, giving λ at close to 43%.

One advantage of a trapezoidal transducer is thus that the transducer may be mounted along a line of attachment which is at its centre of gravity/mass but is not a line of symmetry. Such a transducer would thus have the advantages of improved modal distribution, without being inertially unbalanced. The two methods of comparison used previously again select 270° to 300° as the optimum angle of orientation.

The transducer used in the present invention may be seen as the reciprocal of a distributed mode panel, e.g. as described in W097/09842, in that the transducer is designed to be a distributed mode object.

Claims

1. A touch pad assembly for use in an electronic article, the assembly comprising a touch pad, a coupler for mechanically coupling the touch pad to casing of the electronic article and a transducer which is mounted by coupling means on the coupler or the touch pad to drive the casing as an acoustic radiator.

2. A touch pad assembly according to claim 1, wherein the coupler is in the form of a frame surrounding the periphery of the touch pad.

3. A touch pad assembly according to claim 2 , wherein the transducer is mounted on the frame.

4. A touch pad assembly according to any one of claims 1 to 3, wherein the transducer has an intended operative frequency range and comprises a resonant element having a frequency distribution of modes in the operative frequency range.

5. A touch pad assembly according to claim 4, wherein the coupling means is mounted on the resonant element at a position which is beneficial for coupling modal activity of the resonant element to the coupler or the touch pad.

6. A touch pad assembly according to claim 4 or claim 5, wherein the parameters of the resonant element are selected to enhance the distribution of modes in the resonant element in the operative frequency range.

7, A touch pad assembly according to claim 6, wherein the distribution is enhanced by ensuring that a first mode of the resonant element is near to the lowest operating frequency of interest .

8. A touch pad assembly according to claim 6 or claim 7, wherein the distribution of modes in the resonant element is enhanced by ensuring the distribution has a density of modes which is sufficient for the active element to provide an effective mean average force which is substantially constant with frequency.

9. A touch pad assembly according to any one of claims 6 to

8, wherein the distribution of modes is enhanced by distributing the resonant bending wave modes substantially evenly in frequency.

10. A touch pad assembly according to any one of claims 6 to

9, wherein the resonant element is modal along two substantially normal axes, each axis having an associated fundamental frequency and the ratio of the two associated fundamental frequencies being adjusted for best modal distribution.

11. A touch pad assembly according to claim 10, wherein the ratio of the two fundamental frequencies is 9:7.

12. A touch pad assembly according to any one of claims 4 to 11, wherein the transducer comprises a plurality of resonant elements each having a distribution of modes, the modes of the resonant elements being arranged to interleave in the operative frequency range whereby the distribution of modes in the transducer as a whole device is enhanced.

13. A touch pad assembly according to claim 12, wherein the resonant elements are coupled together at coupling points which enhance the modality of the transducer.

14. A touch pad assembly according to claim 12 or claim 13, the distribution of the modes in each the transducer is enhanced by optimising a frequency ratio of the fundamental resonance frequency of each resonant element.

15. A touch pad assembly according to any one of claims 4 to

14, wherein the resonant element is plate-like.

16. A touch pad assembly according to any one of claims 4 to

15, wherein the shape of the resonant element is selected from the group consisting of beam-like, trapezoidal, hyperelliptical, generally disc shaped or rectangular.

17. A touch pad assembly according to any one of claims 4 to

16, wherein the transducer comprises at least one resonant element which is active.

18. A touch pad assembly according to any one of claims 4 to 16, wherein the transducer comprises at least one resonant element which is passive and an active element.

19. An electronic article comprising a body in or on which a touch pad assembly according to any one of the preceding claims is mounted.

20. An electronic article according to claim 19, wherein the transducer drives both the touch pad and the body surrounding the touch pad to produce an acoustic output with the body acting as the primary acoustic radiator.

21. An electronic article according to claim 20, comprising at least one additional transducer mounted on the body.

22. An electronic article comprising a body in or on which a bending wave loudspeaker is mounted, the loudspeaker comprising a bending wave acoustic radiator and an electromechanical force transducer mounted to the radiator to vibrate the radiator to produce an acoustic output characterised in that the transducer has an intended operative frequency range and comprises a resonant element having a frequency distribution of modes in the operative frequency range and coupling means for mounting the transducer to the radiator.

23. An electronic article according to claim 22, wherein the coupling means is mounted on the resonant element at a position which is beneficial for coupling modal activity of the resonant element to the radiator.

24. An electronic article according to claim 22 or claim 23, wherein the acoustic radiator is moulded integrally with the body.

25. An electronic article according to any one of claims 22 to 24, wherein the parameters of the resonant element are selected to enhance the distribution of modes in the resonant element in the operative frequency range.

26. An electronic article according to any one of claims 19 to 25, which is disposable.