Calibration of vibration pickups at low ultrasonic frequencies

The requirements for and peculiarities of vibration measurements at low ultrasonic frequencies are discussed in the context of existing practice and needs. A method for calibrating vibration pickups in the frequency range 10-30 kHz is described and illustrated with examples. The method requires two types of measurements: the driving-point impedance of the vibration exciter loaded by the test pickup and a known mass, and the voltage ratio of pickup output to driver input. The calibration formula is based on the Moebius transformation of two-port impedances and the product relationship of cascaded systems. The sensitivity of a pickup, in both magnitude and phase, is determined from three sets of observations, using calibrated masses and a programmable impedance/gain analyzer