Acoustic characterisation of panel materials under simulated ocean conditions

Viscoelastic materials are used for a variety of purposes in underwater acoustics, including encapsulation, housing and coating/lining. The acoustic properties of these materials can vary significantly with frequency, temperature and hydrostatic pressure. As a consequence, techniques of accurately characterising their acoustic properties are important when designing and selecting materials and when assessing their performance. Methods exist to determine the echo reduction and transmission loss of panels of materials, but these are typically undertaken in open laboratory tanks at atmospheric pressure. This limits the range of testing possible, since such tanks cannot provide the range of environmental conditions that exist during deployment in the sea. Sea-trials are themselves expensive and are limited by the prevailing temperature conditions in the sea where the measurements are carried out. An alternative approach has been investigated using the Acoustic Pressure Vessel (APV), recently commissioned at the UK's National Physical Laboratory (NPL), which is capable of simulating ocean depths down to 700 m and temperatures from 2 °C to 35 °C. In order to provide a wide frequency range and limit the effects of diffraction a parametric array was installed in the APV as an acoustic source, with the array being truncated by an acoustic filter in order to provide a source-free measurement region. This facility has been used to measure the transmission loss and echo reduction of materials over a frequency range from a few kHz to 50 kHz, for pressures up to 2.8 MPa and for a range of temperatures from 8.0 °C to 27 °C. The problems of establishing a parametric array in the closed, confined space of the APV are discussed, as are the effects of pressure on the acoustic filter. The potential of the technique is illustrated with experimental results for the measurement of echo reduction and transmission loss of a test panel