A heat balance model to explain delayed 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 five cycles of 10 kHz voltage across the electrodes, the thermoacoustic sound production was repeatedly observed to be delayed by about 240 μs from the initial applied voltage. At the time of these experiments around 2014, we were unable to account for this surprising delay in sound production. Recently, we built a heat balance model for the driven water volume that shows heat accumulating, during the applied voltage, faster than it can escape by heat conduction to the surrounding water. This results in a build up of heat in the driven water volume that increases the local temperature and density until a threshold level for sound production is reached. After the last cycle of the voltage burst, sound waves continue to be produced until enough heat has conducted away from the driven water volume for the temperature and density to fall below the sound production threshold. Although the model uses several simplifying assumptions, the results match the experimental observations reasonably well.