David Blackstock and the experimental explanation of the spikes on sonic boom waveforms

Abstract

The present paper discusses an extremely clever experiment perforrmed by David Blackstork and his student Bruce Davy at Rochester in the early 1970s. In the late 1960s people were puzzled by some anomalous features in the measured waveforms of sonic booms recorded at the ground during flyovers of supersonic aircraft. The waveforms were supposed to look like the letter N, but that was not always the case. Sometimes there were strange upward reaching spikes just behind the leading and trailing shocks and in other instances the wavefront was rounded. There were a lot of explanations kicking around, and the present author and David Blackstock came up with the idea that it was somehow caused by imperfect focusing of wavefonts after traveling through regions of higher sound velocity. During our discussions, Blackstock came up with the remark, ‘‘Let’s talk experiment.’’ What resulted was an experiment that would have been worthy of Lord Rayleigh. The paper that reported this, titled ‘‘Measurements of the Refraction and Diffraction of a Short N-Wave by a Gas-Filled Soap Bubble,’’appeared in J. Acoust. Soc. Am. in March 1971. Besides really nailing down the physical phenomena responsible for the spikes on sonic boom waveforms, it illustrated a wealth of physical concepts and experimental techniques. The present talk discusses the background of the paper, the physics that it used, and the influence it had on subsequent research.