Microphones are electroacoustic transducers for converting acoustic energy into electric energy. They serve two principal purposes. First, they are used for converting music or speech into electric signals that are transmitted or processed in some manner and then reproduced. Second, they serve as measuring instruments, converting acoustic signals into electric currents that are processed and displayed. In some applications such as telephony, high electrical output, low cost, and durability are greater considerations than fidelity of reproduction. In other applications, small size and high fidelity are of greater importance than high sensitivity and low cost. In measurement applications we may be interested in determining the sound pressure or the particle velocity. In some applications the microphone must operate without appreciable change in characteristics regardless of major changes in temperature and barometric pressure.
For these different applications, a variety of microphones have been developed. For the purposes of discussion in this part they are divided into three broad classes, in each of which there are a number of alternative constructions. The classes are:
In this part we shall describe the distinguishing characteristics of these three types. In the next two parts we shall discuss in detail several examples of each type involving electromagnetic and electrostatic types of transduction. A brief summary of their characteristics is given in
Table 5.1, which will be explained in greater detail in this chapter.