David Blackstock and the physics of nonlinear sound beams

In the 1970s, David Blackstock and his students and colleagues examined nonlinear sound beams both theoretically and experimentally. Two noteworthy articles in JASA resulted from this research, one by Shooter, Muir and Blackstock (1974) and another by Lockwood, Muir and Blackstock (1973). Of particular interest was the generation of highly directive radiation at harmonics of the source frequency that at moderate source levels decreased in beamwidth with harmonic order. Additional sidelobes were also observed in the harmonics. The main lobe in both the fundamental and the harmonics eventually broadened with increasing source level as shock waves were formed. Eventually finite-amplitude absorption became significant, acoustic saturation set in, and the medium did not permit further increase in amplitude. David’s application of weak-shock theory to model this process was validated by underwater acoustic experiments at the Applied Research Laboratories Lake Travis Test Station involving transducers driven at high intensities, at several hundred kilohertz, with waveforms and beams measured at ranges of several hundred meters. The results provided guidance for many applications, ranging from audio frequency tools in ocean acoustics to ultrasonic biomedical instruments now used worldwide in diagnostic and therapeutic applications. [Work supported by ARL:UT.]