Speakers, piezoelectric devices

More recently, the use of piezoelectric devices has grown dramatically as a consequence of increases in automation and consumer attraction to modern sophisticated gadgets. Piezoelectric devices are used in many of today s applications, including automotive—wheel balances, seat-belt buzzers, tread-wear indicators, keyless door entry, and airbag sensors computer/electronic—microphones, speakers, microactuators for hard disks and notebook transformers commercial/consumer—ink-jet printing heads, strain gauges, ultrasonic welders, and smoke detectors medical—insulin pumps, ultrasonic therapy, and ultrasonic cataract-removal devices. 

An electret film with the very high piezoelectric coefficient of 170 pCN is used in speakers, microphones and keyboards and related devices where mechanical and electrical signals are coupled. Calculate the change in thickness when 500 V are applied across a film 0.1 mm thick. 

For speakers or buzzers, audible by humans, devices with a rather low resonance frequency are used (kilohertz range). Examples are a bimorph consisting of two piezoceramic plates bonded together, and a piezoelectric fork consisting of a piezodevice and a metal fork. A piezoelectric buzzer is shown in Figure 4.1.12, which has merits such as high electric power efficiency, compact size and long life. 

AIN non-ferroelectric thin films are studied for their potential use as pressure transducers, speakers and SAW devices (Turner et al. 1994). Polyciystalhne AIN is an important substrate ceramic material due to its high thermal conductivity and dielectric breakdown strength however it does not exhibit piezoelectric activity in the bulk form. When properly oriented, AIN film shows piezoelectric activity up to the high temperatures. For some of the material coefficients of AIN (and also ZnO) films see Gualtieri et al. (1994). 

The brothers Jacques and Pierre Curie are credited with the discovery of piezoelectricity in a number of hemiedric crystals (Curie and Curie, 1880). Today, piezoelectrics are utiUzed in acousto-electronic devices and sensors based on bulk and surface acoustic waves, piezomechanical sensors to monitor pressure, power, and acceleration, as actuators for micropositioning devices, band pass filters with low insertion losses, as electro-optic devices for optical memories, displays for high-definition televisions, and possibly as transparent piezoelectric speaker membranes as well as miniaturized piezoelectric transformers and motors. As the classic piezoelectric material is a-quartz, the basic relationships are detailed below using it as a model structure. Further details on the piezoelectric properties of quartz, and of its history, discovery and utilization, are available elsewhere (Ballato, 2009). 

Piezoelectric transducers either convert an electrical signal to physical motion or vice versa. In the past, the sensor material was usually a ceramic based on titanates of barium or lead zirconium. Their rigidity made them especially useful for converting electrical energy to mechanical motion in products like audio speakers and signaling alarm devices. Unfortunately, ceramics are brittle and cannot be made with large surface areas or into complex shapes where plastics are also used. 

---