Patent Publication Number: US-6342749-B1

Title: Vibration exciter

Description:
This application claims the benefit of provisional application No. 60/150,588, filed Aug. 26, 1999. 
    
    
     BACKGROUND 
     The invention relates to piezoelectric vibration exciters, e.g. of the kind which may be used to apply ending wave energy to resonant panels to form loudspeakers, and to panel-form loudspeakers employing such exciters. Our International patent application WO97/09842, and counterpart U.S. application Ser. No. 08/707,012, filed Sep. 3, 1996, describe such resonant panel loudspeakers and vibration exciters therefor. The latter application is incorporated herein by reference in its entirety. 
     Conventional piezoelectric actuators exhibit limited mechanical displacement. The output of conventional piezoelectric devices is limited by the materials&#39; basically low piezoelectric displacement constant. Thus conventional devices of reasonable thickness (i.e. of the order of a few millimetres) offer only micrometer-sized mechanical output motion. 
     Many attempts have been made to provide piezoelectric devices having increased mechanical output displacement. “Rainbow” actuators, “Moonies,”unimorphic and bimorphic piezoelectric actuators exhibit greater mechanical output motion. However, even the thinnest ceramic wafers, which exhibit the maximum observed output motion, provide a displacement limited to approximately 1 mm of motion in the z-axis direction for a device that is 3-4 cm long. Additionally, 0.25 mm thick ceramic devices are extremely brittle and fragile so that they are prone to breakage and require special handling. U.S. Pat. Nos. 5,632,841 and 5,802,195 and International application WO96/31333 describe high displacement ferro-electric devices. 
     SUMMARY OF THE INVENTION 
     It is an object of the invention to provide a piezoelectric device of enhanced output. 
     According to the invention there is provided a vibration exciter comprising a curved piezoelectric device having an attachment portion at which the device is to be attached to a substrate to be vibrated and at least one free portion remote from the attachment portion, and a mass load provided on at least one of the or each free portion. 
     The piezoelectric device may be a ferro-electric device and may be pre-stressed. Mass loading such pre-stressed ferroelectric devices may provide a surprisingly large increase in output, of around an order of magnitude, when the devices are used to drive resonant bending wave panels. 
     The device may be arcuate in shape or may be a generally circular dished device. 
     From another aspect the invention is a resonant panel-form loudspeaker driven in resonance by a vibration exciter as described above. 
    
    
     BRIEF DESCRIPTION OF THE DRAWING 
     Examples which embody the best mode for carrying out the invention are described in detail below and are diagrammatically illustrated in the accompanying drawing, in which: 
     FIG. 1 is a perspective view showing a first embodiment of a piezoelectric device according to the invention, 
     FIG. 2 is a perspective view showing a second embodiment of the invention, 
     FIG. 3 is a perspective view showing a third embodiment of the invention, and 
     FIG. 4 is a perspective view showing a fourth embodiment of the invention. 
    
    
     DETAILED DESCRIPTION 
     Referring to FIG. 1, a loudspeaker  1  comprises a plate  11  capable of supporting resonant bending wave modes, as set out in WO97/09842 and U.S. Ser. No. 08/707,012. A curved, composite piezoelectric device  13 , such as described in U.S. Pat. No. 5,632,841, has an attachment portion  12  centrally located on the piezoelectric device. The attachment portion  12  is mounted at a preferred, off-centre location on the panel  11 , preferably in accordance with the teachings of WO97/09842 and U.S. Ser. No. 08/707,012. The ends  14  of the device are free. At each end of the piezoelectric device  13  an elongated mass  15  is provided along the end of the device to mass-load the piezoelectric vibration exciter to increase the output of the exciter. Leads  17  provide an electrical signal to the exciter. 
     FIG. 2 shows a similar arrangement except that instead of a single mass  15 , a pair of shorter masses  19  are provided at each end of the piezoelectric vibration exciter  13 . 
     In FIG. 3, the piezoelectric vibration exciter  21  is genuinely circular and dish-shaped, attached to the panel at the centre of the dish, with a single elongated mass  23  provided on the free peripheral rim  25  of the exciter. The mass  23  mass-loads the piezoelectric vibration exciter to increase the output of the exciter. 
     FIG. 4 shows an alternative arrangement in which the continuous ring  23  of FIG. 3 is replaced by a plurality of discrete masses  27 , symmetrically arranged the rim  25  of the piezoelectric device. 
     The invention thus provides a simple way of increasing the output of an exciter.