The velocity transducer is one of the oldest types of vibration transducer, and even though it has many drawbacks, is still in fairly common use. It is a seismic transducer that contains either a moving coil of wire in a magnetic field, or a moving magnet inside a coil of wire.
The velocity transducer has moving parts, and is therefore subject to wear. It requires frequent calibration, and its response depends on temperature and the orientation of the transducer. Its frequency response extends from about 15 Hz to 1500 Hz, and its phase response can be erratic, especially at low frequencies.
A newer type of velocity transducer, called a "velometer", consists of a piezoelectric accelerometer with a built-in integrator to convert the signal to velocity, and it is much better in all respects than the traditional velocity transducer.
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