Transducer Characteristics

An ideal sound or vibration transducer should have the following characteristics [1]:

1. It should cause negligible diffraction of the sound field or structural vibration field (i.e. its dimensions should be small compared with the smallest sound or vibration wavelength of interest).
2. It should have a high acoustic or mechanical (driving point) impedance compared with the fluid medium or structure so that little energy is extracted from the field.
3. It should have low electrical noise.
4. Its output should be independent of temperature, humidity, magnetic fields, static pressure, and wind velocity, and it should be rugged and stable with time.
5. Its sensitivity should be independent of sound pressure or vibration level magnitudes.
6. Its frequency response should be flat.
7. It should introduce a zero phase shift between the sound pressure or structural vibration and the electrical output signal.

No transducer can meet all of the above criteria, and thus different types of transducers and vibration sensors are preferred for different measurements. The microphone is by far the most common form of acoustical transducer, and the piezoelectric accelerometer is the most widely used vibration transducer. Because of their importance the next section of this chapter concerns these devices. But it should be noted that other specialized noise and vibration transducers are used for measurements. For instance, sound intensity probes of different designs can be used for noise source identification and sound power measurements of a source in situ [2, 3], and several other types of vibration‐measuring transducers such as strain gauges and laser Doppler interferometer systems are also in use [4].