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.