ABSTRACT
Broadly, the transient methods to be described in this final chapter fall into two categories: pulse methods and reverberation techniques. In the former, the speed of sound is determined, often with very high precision, essentially from a measurement of a pulse propagation time over a known distance in the fluid. It is also possible to obtain the absorption coefficient from the attenuation of the pulse. Typically, pulse methods are used with carrier frequencies from about 1 MHz upwards. They function particularly well in dense fluids but can also be used in gases. The reverberation method is used to measure absorption of sound. In this method, one measures the logarithmic decrement for a freely oscillating normal mode of a resonator containing the fluid under study. Under favourable conditions this quantity is dominated by the absorption throughout the volume of the fluid and leads to a measurement of the bulk absorption coefficient. Reverberation techniques can be used at frequencies from the infrasonic to the ultrasonic, thus allowing relaxation mechanisms to be studied over many orders of magnitude in the time domain, and in either gases or liquids.
