The “Hadoh” group of societies for the study of thermoacoustics and thermoacoustical engineering, established in 1988, was adjourned in 1977. During the 9 years, members of these societies promoted great progress in thermoacoustics and thermoacoustical engineering, especially in the area of pulse-tube refrigeration. The thermoacoustical engineering supported by the Cryogenic Society of Japan is presently spreading into the Japanese Society of Mechanical Engineering and other societies. A wide variety of thermoacoustic phenomena has been studied, and the thermoacoustic theory has developed from “standing wave approximation” to a theory including “progressive waves” in order to understand thermoacoustic devices equipped with regenerators. While thermoacoustic theory succeeded in a qualitative understanding of thermoacoustic phenomena and has been used for the R&D of thermoacoustic refrigerators, there remain important problems; how to control the steady mass flow and how to understand and design heat exchangers of thermoacoustic devices. These are essential and urgent problems in developing practical devices of higher efficiency. A deep insight of thermoacoustics may be one of the keys to the future development of thermodymnamics that consider time.
The basic differences between thermoacoustic theory and traditional theory are reviewed. The first is a difference of image. Traditional theory discusses only enthalpy flow, while thermodynamic theory discusses not only enthalpy flow but also entropy flow and work flow in conjunction with the second law of thermodynamics. Definitions of these flows in the traditional theory are vague and sometimes conflict with both the first and second laws of thermodynamics, while the definitions in thermoacoustic theory are clear and consistent to thermodynamics. The concept of a state variable plays a key part of thermodynamic discussion regarding thermoacoustical phenomena. The last difference is related to transverse averages. Most people who follow traditional theory discuss the transverse averages of oscillating quantities at first, and after that, handle the product of oscillating quantities. Thermoacoustic theory discusses transverse distribution of the oscillating quantities at first and then handles the transverse averages of products of oscillating quantities. Since the product of averages and the average of a product are different in general, results of those following traditional theory are different from those obtained by thermoacoustic theory. This is one of the reasons why thermoacoustic theory succeeds in discussing a wide variety of thermoacoustic phenomena including intrinsically irreversible phenomena.