Transducers

The basis of all noise and vibration measurement systems is the transducer. The microphone is the main transducer used to measure sound, while the accelerometer is the main transducer used to measure vibration. Specialized transducer systems have been developed to measure sound intensity in air and vibration intensity of structural systems. In addition, measurement procedures have been formulated to determine the modes of vibration of structures. Special rooms and systems such as anechoic and reverberant rooms and impedance tubes [1] are now widely used for sound power and noise source identification measurements of machines and the measurement of the acoustical impedance of materials. With any measurement, calibration of the system is essential to obtain reliable results that can be compared with results obtained by others.

Transducers and their associated measuring systems generally suffer from two major shortcomings:

1. A transducer will normally respond to other variables in addition to its response to the variable of interest. For example, a microphone, although being most sensitive to sound pressure, may also be slightly sensitive to variations in temperature, humidity, magnetic fields, and vibration.
2. It is difficult, and in many cases not possible, to introduce a transducer into the measurement medium without disturbing the medium in some way. The transducer will extract some energy from the medium or structural system. In addition, other disturbances will be caused. An accelerometer will add mass to the system and alter the structure’s vibration. A pitot tube or hot‐wire anemometer will disturb the flow. A microphone will reflect, diffract, and refract the incident sound wave. Some noncontacting vibration transducers and systems will not directly interfere with the vibration of the system to be measured, but they will interfere with any associated sound field generated by the vibrating body.