Thermoacoustics

Thermoacoustics has been an established and well-studied phenomenon in fluids -- whether as a gas or liquid. Applying heat to a fluid enclosed in a duct or cavity will cause the spontaneous generation of sound waves propagating in the fluid itself. This results in so-called singing pipes, or thermoacoustics machines. 
 
Thermoacoustics enables either waste heat or mechanical vibrations to be converted into other useful forms of energy. For refrigerators, sound waves generate a temperature gradient of hot and cold. The vibrating motion makes cold areas colder and hot areas hotter.