Weak sparks, N waves, and the calibration of microphones at ultrasonic frequencies

Abstract

A fortuitous situation involving the availability of wide-band condenser microphones, curiosity about acoustic waves generated by sparks, and his own development of nonlinear acoustic theory provided two significant contributions to David Blackstock’s continuing research. Theoretical understanding of the nonlinear distortion experienced by intense acoustic impulses in air, such as those generated by relatively weak sparks, led to a technique for calibration of microphones that have essentially flat frequency response up to almost 1 MHz. These sparks and calibrated microphones, in turn, provided the means to carry out several graduate student thesis projects that were supervised by David. Diffraction at edges, apertures, and discs was studied in the time domain. Model studies related to sonic boom phenomena were carried out, as was the propagation of N waves in waveguides. The most recent application has been a model study of focusing by an ellipsoidal reflector, such as is typically used in lithotripsy. In each case, the various contributions to the received signals were readily identified in the time-domain presentation.