ABSTRACT
In the fi eld of acoustic measurements, the basic sensor is some kind of microphone (if the measurement is in a liquid, the sensor is usually called a hydrophone). Many measurements are related to the auditory response of human beings to the noise or sound, so the frequency range over which the sensor must measure accurately is taken as the audible range, nominally about 20-20,000 Hz. Most such microphones are pressure sensors designed to respond to the very small acoustic pressure variations around an average value which is the ambient atmospheric pressure of about 14.7 psia. The basic operation of such microphones is shown in a simplifi ed manner in Figure 7.1. The housing or cartridge has a thin diaphragm (typically a stretched nickel fi lm) attached, and a carefully designed leak (often a small capillary tube) serves to equalize the inside and outside pressures for pressure changes slower than about 10 Hz. For the higher frequencies of interest (the 20-20,000 Hz range), the leak is ineffective and the diaphragm feels a small pressure difference across it, which causes it to slightly defl ect. The equalizing leak protects the very sensitive diaphragm from slow atmospheric pressure changes that are however very much larger than the acoustic signals of interest and would damage the instrument if allowed to act directly on it. To obtain an electrical output signal proportional to the acoustic pressure, the defl ection of the diaphragm is measured with some type of displacement transducer, a capacitance method giving the highest accuracy. Further details of such microphones can be found in the literature.*
In this chapter, we discuss two special classes of microphones, infrasonic and ultrasonic, which are not used nearly as much as the “audio” instruments just described, but have important specialized applications. The names just given relate to the frequency range for which the devices are intended, infrasonic being the range below the lowest frequencies of human hearing and ultrasonic, the range above the highest.
