Molecular relaxation effects on sonic boom waveforms: A tutorial survey

Abstract

Although molecular relaxation has a dominant effect on the attenuation of sound in the audio frequency range, it was largely ignored in the early work on sonic boom propagation, until Hodgson and Johannesen circa 1971 brought the phenomena to the attention of sonic boom researchers. Although interest in the overall subject waned somewhat with the March 1971 decision to indefinitely postpone the SST program in the United States, the intriguing physics involved and its fundamental importance for atmospheric sound propagation in other contexts sustained the interest of researchers, especially Bass and Raspet, who sought a better quantitative understanding of relaxation effects on shock waves in the atmosphere. The present author and his students, spurred in more recent years by the revived interest in a possible US high-speed civilian transport, extracted a simple approximate formulation, which captured the essential physics and which predicted the detailed, albeit local, waveform shape at a shock in terms of peak overpressure and atmospheric humidity. The present paper discusses the basis of this theory, its predictions, its use to substantiate the claim that turbulence is typically more important than molecular relaxation, and the progress that has recently been made to understand its limitations. [Work supported by NASA−LRC and by the William E. Leonhard endowment to Penn State Univ.]