The 1996 Trent-Crede Medal encomium by Barger states that Smith's favorite paper was ``Effect pf heat radiation on sound propagation in gases' (JASA, 1957). Smith stated this also in a private communication some time earlier to the present writer. The paper was prompted by Stokes and Rayleigh in which heat effects were modeled by Newton's law of cooling, which presumes that the heat radiation out from a limited region of heated matter is proportional to the difference of the local temperature and that of the surrounding medium. Stokes in 1851 constructed a theory for how this assumption leads to a prediction of the dependence of phase velocity and attenuation on frequency, and this theory was used by Rayleigh in the Theory of Sound to analyze whether acoustical fluctuations were more nearly isothermal or adiabatic. However, as Smith pointed out, apparently for the first time, neither Stokes or Rayleigh fully understood the relevant physics. When the modern theory of heat radiation in gases is taken into account, the effect of heat radiation (in contrast to the effect of thermal conduction) is negligible at all frequencies. For all frequencies for which the attenuation is small, the acoustic fluctuations are adiabatic.

This content is only available via PDF.