Acoustic relaxation spectrometers for liquids

In this tutorial article methods of acoustic relaxation measurements of liquids are reviewed. The present performance of various techniques is described and desirable trends in future developments are shown. An account is given on standards in the optical monitoring of acoustic waves (Brillouin scattering), on time domain (jump) techniques, as well as on frequency domain (continuous and pulse modulated ultrasonic wave) methods. The merits and limitations of the different techniques of measurement, covering the frequency range from 10 kHz up to nearly 10 GHz now, corresponding with relaxation times between about 0.1 ms and 100 ps, are discussed. Efforts to further enhance the experimental accuracy and to reduce the required sample volume are indicated.