Metal electrode direct sound generation in seawater

Abstract

Thermoacoustic expansion of water due to Ohmic heating and volume changes in the very thin Helmholtz double layer that forms adjacent to metal electrodes are two mechanisms that have been used to explain sound generation by metal electrodes in seawater. Using 20 gauge solid sterling silver wire electrodes with only the circular cross section of the wire exposed, our experiments have demonstrated intense sound production in the form of acoustic tone bursts in seawater from a thermoacoustic mechanism at a second harmonic of 20 kHz of the electrical driving frequency at 10 kHz. Additionally, we observed a separate intense and transient acoustic tone burst generation from a mechanism we attribute to vesicle layer excitation where the vesicle is a highly charged layer of water associated with the boundary of droplets and bubbles. This vesicle layer excitation sound generation mechanism is distinct from any Helmholtz double layer mechanism since the double layer is always present while the acoustic tone burst we observe repeatedly occurs after about 4.4 ms of applied alternating voltage at 10 kHz and last only about 560 μs. We present our experimental setup and results. Moving seawater directly to generate sound may lead to more compact projectors.