The use of the sonogram in structural acoustics and an application to the vibrations of cylindrical shells

The time-varying spectrum, or sonogram, is a familiar tool in areas of acoustics concerned with perception—especially in speech studies. It can also be of great benefit in studies of wave propagation in structures, a fact which is less well known. Processing the impulse response of a structure at a distance from the driving point gives a direct measure of the group velocity as a function of frequency, a quantity not easy to measure by more conventional methods. The approach is illustrated by using simulated data from idealized models of a stretched string and a bending beam. These tests show how the features in the sonogram pictures vary with changes in the compromise between time resolution and frequency resolution: one can obtain a continuous change from a time-domain viewpoint to a frequency-domain one, learning on the way things not easily found from either extreme. The technique is applied to measurements of the different waveguide modes of a cylindrical shell, where it gives a rather stringent check against certain aspects of theory.