Measurement of Vibration Amplitude at High Frequencies

Abstract

The pistonphone, previously devised as a method for calibrating condenser microphones up to 200 cps, is applied to the measurement of vibration amplitudes over an extended frequency range. Vibrations of a piston, projecting into a 19.5-cc cavity, are generated by a barium titanate driver. The pressure developed in the cavity is measured by a condenser microphone previously calibrated by a reciprocity technique. The amplitude of motion of the piston is then computed from the observed sound pressure. With the microphone removed from the cavity, a capacity type vibration pick-up probe is brought in proximity to the piston surface and is calibrated against the known motion of the piston. Mechanical coupling between the driver and the main supporting structure is minimized by the use of vibration mounts and the maintenance of a small, acoustically sealed clearance between the piston and the cavity walls. Using hydrogen in the cavity to limit wave motion, calibrations from 100 to 10 000 cps have been obtained with an estimated accuracy of 5 percent. The lower detection limit of a capacity probe designed for high sensitivity is 10−10 cm peak amplitude.