Underwater sound absorption properties of polydimethylsiloxane/carbon nanotube composites with steel plate backing

Abstract Nanotechnology has been widely used for developing sound absorption materials in air, but studies for underwater applications remain limited. In this study, carboxyl functionalized multi-walled carbon nanotubes (MWCNT-COOH) were added into polydimethylsiloxane (PDMS) with dispersant to enhance the underwater acoustic properties. A water-filled impedance tube was built to determine the underwater sound absorption coefficient in the low frequency range from 1500 Hz to 7000 Hz and under variable hydrostatic pressures. Results demonstrate that pure PDMS is nearly acoustically transparent. With the addition of MWCNT-COOH and under normal pressure (0 MPa), the sound absorption coefficient was marginally improved to 0.3. The coefficient can nonetheless be further increased to above 0.75 with the introduction of both surfactant and MWCNT-COOH. It could be postulated that the trapped micro-voids and uniformly dispersed MWCNT-COOH contributed to the improvement when the pressure is at 0 MPa. Effects of MWCNT-COOH concentrations in a range of 0.1 wt% to 2 wt% were investigated. It was ascertained that the concentrations have only a marginal effect on the sound absorption performance. Tests were also conducted at variable hydrostatic pressure above 0 MPa. Results revealed that underwater sound absorption performance decreased with an increase in the hydrostatic pressure in a range of 0.5 MPa to 1.5 MPa. The drop in performance due the increase of hydrostatic pressure supported the claim that the presence of micro-voids within the material matrix structure contributed to the sound absorption.