Introduction

Abstract

First sound is the classical mechanism which allows us to hear, i.e. a disturbance of pressure (or density) which propagates through a continuous medium such as air or water. Second sound is a more recent phenomenon involving the propagation of heat as a temperature wave. The classical theory of heat propagation is via diffusion where a temperature field diffuses through a continuous body. However, experiments in the late 1960’s and early 1970’s showed that a thermal disturbance could travel as a wave and this has acted as an impulse to much subsequent theoretical work in this area.