A Theoretical Criterion for End Face Alignment in the Cylindrical Acoustic Interferometer

A more rational criterion for the end face alignment of the widely used cylindrical acoustic interferometer is sought to replace the usual intuitive requirement that the depth of any geometrical imperfection should be small compared to the acoustic wavelength. Whilst the latter criterion may be proved to be adequate for an instrument operated below its first cut-off frequency, it may very well be inadequate in the case of the ultrasonic instrument where the number of modes which are able to propagate is, for practical purposes, unlimited. Optical alignment of ultrasonic cavities is found to be adequate for many, but not all, ultrasonic interferometers. Where it is inadequate other acoustic alignment procedures should be possible.