GB2433174A

GB2433174A - Exciter for a bending wave distributed mode loudspeaker

Info

Publication number: GB2433174A
Application number: GB0524894A
Authority: GB
Inventors: Henry Azima
Original assignee: New Transducers Ltd
Current assignee: NVF Tech Ltd
Priority date: 2005-12-07
Filing date: 2005-12-07
Publication date: 2007-06-13
Also published as: GB0524894D0

Abstract

A bending vibration transducer 10 comprises a primary piezoelectric element 16 and a secondary piezoelectric element 18 having at least one end aligned with an end of the primary element 16. The aligned ends of the primary and secondary elements are attached using connecting means (14). The transducer is coupled to a load 12, e.g. a loudspeaker, using coupling means (20) which are attached only to the primary piezoelectric element 16. The secondary element 18 may lie below the primary element 16, or to the side (see figs 6,7a).

Description

TITLE: TRPNSDUCER

DESCRIPTION

TECHNICAL FIELD

The invention relates to transducers, actuators or exciters, particularly inertial bending vibration :.: * transducers. *S..

e BACKGROUND ART

Such bending inertial vibration transducers are - : 20 discussed in WO01/54450, incorporated herein by reference, and may employ a plate-like piezoelectric member that resonates in bending. A mass may be provided on the piezoelectric member. Coupling means, typically a stub, are provided for mounting the transducer to a site to which force is to be applied from or to the member. The member is free to bend and so generate a force via the inertia associated with accelerating and decelerating its own mass during vibration. The bending of the member can either be in response to an electrical signal, in which case the transducer acts as a vibration exciter, or can generate an electrical signal, in which case the transducer acts as a vibration sensor.

WOOl/54450 further discloses bending inertial vibration transducers having an intended operative frequency range and comprising a resonant element having a frequency distribution of modes in the operative frequency range and coupling means on the resonant element for mounting the transducer to a site to which force is to be applied or from which it is to be taken. Such transducers are known as distributed mode actuators or DMAs.

DISCLOSURE OF INVENTION

According to the invention, there is provided a bending inertial vibration transducer, e.g. for applying a force which excites an acoustic radiator to produce an acoustic output, the transducer comprising a primary piezoelectric element, a secondary piezoelectric element having an end aligned with an end of the primary element, connecting means attaching the aligned ends of the primary and secondary elements and coupling means attached only to the primary piezoelectric element whereby in use the transducer is coupled to a load, e.g. an acoustic device.

In this way, a lower first bending wave resonant mode fO may be achieved than either a transducer comprising separate, unconnected piezoelectric elements which are both coupled to a load or a transducer with individually formed but connected piezoelectric elements which are both coupled to a load. Accordingly, a transducer according to the invention may have an extended low-frequency bandwidth. The primary and secondary elements may be arranged to resemble a continuous piece of piezoelectric material which has been folded on itself.

There is no coupling between the secondary element and the load, e.g. acoustic device and thus the secondary element is inertial. The transducer as a whole may be inertial. Alternatively, the transducer may be grounded, i.e. supported on a frame.

There may be more than one secondary piezoelectric element. The primary and secondary elements may be arranged in a stack, i.e. in a vertical plane, whereby one element is above the other. Alternatively, the primary and secondary elements may be arranged side-by-side.

Each piezoelectric element may comprise two layers of : piezoelectric material between which is sandwiched a *S.S central metallic, e.g. brass vane to form a so-called *:. bimorph.

:: 20 The coupling means may be attached to the primary element over a small proportion of its length so as to leave the majority of the transducer free to vibrate in **1S bending. Each portion of the transducer may be rectangular, e.g. in the shape of a beam. The primary element may be longer than the secondary portion.

The coupling means may be in the form of a stub. The coupling means may be located at a non-central location on the transducer, e.g. at one end of the transducer. The coupling means may be located at the opposed end of the transducer to the aligned ends of the primary and secondary elements.

The transducer may be a distributed mode actuator as described in WO0l/54450.

According to a second aspect of the invention, there is provided an acoustic device comprising a bending wave member which is capable of sustaining bending wave vibration, and a transducer as described above mounted to the bending wave member via the coupling means to drive bending wave vibration in the bending wave member.

The enhanced, reduced exciter size form-factor indicates that the invention would be useful in driving compact bending wave acoustic panels for a superior low frequency output compared to that which would be possible

in this size with the prior art. In particular in

connection with display screens and transparent bending S * wave speaker panels as protective screen covers, small *:-. electronic articles, personal databanks and assistants, data and photostore, portable video players, games devices, and mobile telephone devices will benefit.

BRIEF DESCRIPTION OF DRAWINGS

S... ____________________________________________________ Embodiments of the invention will now be described by way of example with reference to the accompanying drawings in which: Figure 1 is a cross-sectional view of a transducer embodying the present invention; Figure 2a is a perspective view of the lowest mode of the transducer of Figure 1; Figure 2b is a perspective view of the second mode of the transducer of Figure 1; Figure 3 is a graph of blocked force response (N/V) against frequency (Hz) for the transducer of Figure 1 with the voltages in the two beams in-phase and out of phase; Figure 4 is a graph of blocked force response (N/V) against frequency (Hz) for the upper and lower beams when driven separately and when combined for the transducer of Figure 1; Figure 5 is a graph of drive point velocity (mm/s) against frequency (Hz) for variations of the transducer of Figure 1; Figure 6 is a plan view of a second embodiment of the invention; Figure 7a is a schematic plan view of a third embodiment of the invention, and * S..

S

Figure 7b is a schematic perspective view of the embodiment of Figure 7a.

DETAILED DESCRIPTION OF DRAWINGS

*:*:.* Figure 1 shows a transducer 10 comprising a primary piezoelectric beam element 16 and a secondary S...

piezoelectric beam element 18 connected to each other at one end by a connecting spacer 14 e.g. of plastics, glass-reinforced plastics (grp) or similar, or metal, e.g. aluminium or magnesium. The beams are arranged in a stack with the primary beam above the secondary beam. Coupling means in the form of a stub 20 is attached to the upper or primary beam 16 at the opposed end to the connecting spacer 14. The transducer 10 may be coupled to an acoustic device or other load 12 via the stub 20. There is no coupling between the lower or secondary beam and the acoustic device.

Each piezoelectric element 16, 18 comprises two layers of piezoelectric material between which is sandwiched a central brass vane. The upper piezoelectric element 16 has overall length 25mm and width 10mm and the lower piezoelectric beam 18 has length 20mm and width 10mm.

Figures 2a and 2b illustrate the first and second modes of the transducer of Figure 1 which occur at 253Hz and 406Hz respectively.

Figure 3 shows the blocked force response (N/V) against frequency (Hz) for the transducer of Figure 1.

Line 28 shows the blocked force response with the voltages * S. * in-phase in the upper and lower beams and line 29 with the voltages in the lower beam inverted. With the voltages in phase, the transducer has a useful output down to the lowest mode with a blocked force of 7mN/V.

5. The lowest mode fO of the transducer as a whole shows S...

as a peak in both lines. However, the second mode fl is not visible when the voltages in the beams are in phase. When the voltages are inverted, the second mode of the transducer as a whole is visible and provides extra output between fO and 500Hz. With the voltages inverted, the transducer provides useful output down to f 0, although below fO it is less than for in-phase voltages. Above 500Hz, the output drops to about 2.5mN/V and does not reach the in-phase output until about 3kHz.

Figure 4 shows the blocked force response (N/V) against frequency (Hz) for the transducer of Figure 1 (line 30), for the upper beam (length 25mm) when driven separately (line 31) and sum of the two beams driven separately (line 32) . For comparative purposes, the blocked force response for a transducer comprising a single 30mm piezoelectric beam is also plotted (line 33) . Such a transducer would have a lowest mode at 250Hz.

As is shown in Figure 4, a transducer according to the invention provides no more force than the upper beam on its own. Furthermore, a transducer according to the invention provides approximately the same bandwidth as the single 30mm piezoelectric beam but does so with a reduction in * S. * I S size. **.S

S

Figure 5 shows the results of a model which was used *. to check the response into a typical load, e.g. an acoustic device such as a screen for a telecommunication device *::.* which provides a load of 2OkN/m and 4kg/s. Symmetry ..* boundary conditions were used and thus the stub appears at S...

the centre of mass thereby eliminating any moments. No rotational effects were modelled here.

The resulting velocities are plotted for the transducer of Figure 1 (line with diamonds), for the upper beam (length 25mm) when driven separately (line with triangles), for the comparative 30mm piezoelectrjc beam (line with squares) and for the transducer when loaded with a mass (line with circles) . Below 1kHz, the 30mm beam and the mass-loaded transducer are equivalent. Above 1kHz, the mass-loaded transducer has lower output.

Figure 6 shows a transducer 110 according to a second embodiment of the invention. As in Figure 1, the transducer 110 comprises a primary piezoelectric beam element 116 and a secondary piezoelectric beam element 118 connected to each other at one end by a connecting spacer 114 e.g. of plastics, grp or similar, or metal, e.g. aluminium or magnesium. However, in this embodiment, the primary and secondary beams are arranged side-by-side in the same plane.

Coupling means in the form of a stub 120 e.g. of plastics, grp or similar, or metal, e.g. aluminium or magnesium, is attached to the primary beam 116 at the * *.

* opposed end to the connecting spacer 114. The transducer S...

S

is coupled to an acoustic device or other load 112 via *:. the stub 120. There is no coupling between the secondary beam and the acoustic device. Each piezoelectric element *::.* 116, 118 comprises two layers of piezoelectric material ". between which is sandwiched a central brass vane.

Figures 7a and 7b show a transducer 130 according to a third embodiment of the invention. The transducer 130 comprises a primary piezoelectric beam element 136 and two secondary piezoelectric beam elements 138,140 connected to each other at one end by a connecting spacer 144 e.g. of plastics, grp or similar, or metal, e.g. aluminium or magnesium. As in Figure 6, the primary and secondary beams are arranged side-by-side in the same plane.

Coupling means in the form of a stub 146 e.g. of plastics, grp or similar, or metal, e.g. aluminium or magnesium, is attached to the primary beam 136 at the opposed end to the connecting spacer 144. The transducer is coupled to an acoustic device or other load 132 via the stub 146. There is no coupling between either secondary beam and the acoustic device. * S. * S S.. SI.. * S

I * . S * S.

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55.5.. * S S 5 * . S * S. S * 5..

Claims

CLAIMS

1. A bending vibration transducer a primary piezoelectric element, a secondary piezoelectric element having at least one end aligned with an end of the primary element, connecting means attaching the aligned ends of the primary and secondary elements and coupling means attached only to the primary piezoelectric element whereby in use the transducer is coupled to a load, e.g. an acoustic device.

2. A transducer according to claim 1, wherein each piezoelectric element comprises two layers of piezoelectric material between which is sandwiched a central metallic vane.

3. A transducer according to claim 1 or claim 2, wherein the coupling means is attached to the primary element over a small proportion of its length so as to leave the majority of the transducer free to vibrate in bending. * * * *

4. A transducer according to any preceding claim, * **S * . wherein each element is rectangular.

5. A transducer according to claim 4, wherein each element is in the shape of a beam.

6. A transducer according to any preceding claim, ** wherein the coupling means is in the form of a stub. *Sa.

7. A transducer according to any preceding claim, wherein the stub is located at a non-central location on the transducer.

8. A transducer according to claim 7, wherein the stub is located at one end of the transducer.

9. A transducer according to claim 8, wherein the stub is located at the opposed end of the transducer to the aligned ends of the primary and secondary elements.

10. A transducer according to any preceding claim, comprising more than one secondary piezoelectric element, each having an end aligned with the primary piezoelectric element.

11. A transducer according to any preceding claim, wherein the primary and secondary elements are arranged in a stack.

12. A transducer according to any one of claims 1 to 10, wherein the primary and secondary elements are arranged side-by-side.

13. A transducer according to any preceding claim, wherein the transducer is a distributed mode actuator.

14. An acoustic device comprising a bending wave member which is capable of sustaining bending wave vibration, and * I. :.: a transducer according to any of the preceding claims S...

mounted to the bending wave member via the coupling means to drive bending wave vibration in the bending wave member. * . * S S * I. S... S * S...