Absolute Measurement of Small Vibration Amplitudes

Abstract

This paper describes an acoustic method for the absolute measurement of small vibration amplitudes and which, as a by-product, results in a method of calibrating vibration pick-ups. Vibrations generated by an electrodynamic device are transmitted to a piston arranged to vibrate within a cavity. The diaphragm of a condenser microphone is coupled directly to the cavity, and the pressure change in the cavity resulting from the motion of the piston is detected by the microphone and transformed into an electrical output. One side of a parallel plate condenser is rigidly attached to the piston, forming a direct electrostatic pick-up. Transformation into electrical energy is accomplished in both cases by a low noise-level electronic circuit after Van Zelst, which provides a sensitive coupling device and eliminates the danger of stray fields. The output of the microphone circuit as a function of sound pressure is found independently by a reciprocity calibration, while the amplitude of motion of the piston is computed from the observed sound pressure. With the electrostatic device used as a pick-up, the method can be used to measure the absolute amplitude of vibration of other structures without loading them. Preliminary measurements of the order of magnitude of 10−9-cm displacement have been made. The frequency range of the instrument as presently constructed is 20 to 6000 cycles per second but could be readily extended.