A heat balance model to explain thresholds for thermoacoustic sound production in seawater using metal electrodes

Abstract

Experiments conducted with solid metal wire electrodes in seawater generated second harmonic sound waves thought to be thermoacoustic in origin. Using sine wave bursts of three to twenty-five cycles of 10 kHz voltage across the electrodes, the applied voltage threshold for thermoacoustic sound production was observed to decrease almost linearly for bursts up to ten cycles. At the time of these experiments, around 2014, we were unable to explain these sound production threshold values. A mass density continuity equation with a negative source term is used to explore the observed threshold values. The negative source term represents the decrease in fluid density with each half cycle of applied voltage due to the ohmic heating. This approach builds on an earlier heat balance model that successfully explains an observed delay in thermoacoustic sound production. Model results are compared with the experimental observations.