An experimental verification of Rayleigh's interpretation of the Sondhausstube

The interaction of a sound wave in a compressible fluid in contact with sol id boundaries produces thermoacoustic effects; A century ago, Lord Rayleigh gave a general qualitative explanation of the acoustic power production by unsteady heat transfer to compressible fluid in terms of the relative phas e of the pressure wave and the heat transferred to the fluid. In quantitati ve terms his interpretation is equivalent to saying that a wave is promoted if the absolute value of this relative phase is less than pi/2; otherwise the wave is attenuated. In this paper, an experimental investigation of the attenuation or promotion of an acoustic wave as a function of an axial tem perature gradient at the wall of the waveguide is presented. Using the tech niques previously developed [J. Acoust. Sec. Am. 103, 1532-1537 (1998)], ac oustic temperature and pressure oscillations measurements in the thermal bo undary layer of an acoustic standing wave in air were made. The results rep orted here experimentally confirm the validity of Rayleigh's interpretation of the power production for this case. This confirmation is more direct th an any previous evidence. (C) 1999 Acoustical Society of America. [S0001-49 66(99)02708-3].