Modulated Photoelectric Measurement of Vibration

This paper describes an improvement in the method of setting vibration amplitudes for calibrating vibration pickups by the disappearance of an interferometric fringe pattern. In the usual method, one plate of an interferometer is stationary, the other vibrates with the pickup, and the fringe pattern disappears at zeros of the Bessel function J0[(4πξ)/λ], where ξ is the amplitude of vibration and λ is the wavelength of light used. In the improved method, the fringes are observed by a photomultiplier, the previously stationary plate is vibrated at a modulating frequency much lower than the calibrating frequency applied to the pickup, and the signal from the photomultiplier, filtered at the modulating frequency, has minima at the zeros of J0[(4πξ/λ]. Observation of the nulls on a meter allows faster calibration with greater precision and less observer fatigue.