ABSTRACT
Transducers for the generation and detection of sound are one of the most important components of experimental apparatus; they are also one of the most problematic. The requirements that transducers must satisfy before they are useful in the present context are quite different to those commonly encountered in audio engineering. For the kind of measurements described in this book, very small power outputs are required (usually less than 1 μW) and the main applications can be broadly categorized into three classes. In the first, we have wide-bandwidth devices that present a high acoustic impedance to the medium. Typically, such transducers form part of the walls of a resonator and we require the acoustic impedance to be large so that the transducers do not perturb the properties of the cavity too. much. In the second class, we have narrow-bandwidth devices for use in fixed-frequency variable pathlength interferometers. These are typically operated at a mechanical resonance frequency where the intrinsic mechanical impedance is close to zero and the electrical input impedance is determined mainly by the impedance presented to the transducer by the acoustic load. Finally we have transducers for pulsed operation. These are also fixed-frequency devices but larger bandwidths are often required to achieve useful time-domain resolution.
