Acoustic characterisation of panel materials in simulated ocean conditions using the NPL acoustic pressure vessel

Esward, T., Humphrey, V.F., Evans, L., Beamiss, G. and Hayman, G. (2002) Acoustic characterisation of panel materials in simulated ocean conditions using the NPL acoustic pressure vessel. Proceedings of the Sixth European Conference on Underwater Acoustics, Gdansk, Poland. 23 - 26 Jun 2002. pp. 665-670 .

Abstract

Acoustic characterisation of panel materials in simulated ocean conditions using the NPL Acoustic Pressure Vessel.
A wide range of viscoelastic materials is used for encapsulation, housing and coating/lining in underwater acoustics. The acoustic properties of these materials can vary dramatically with frequency, temperature and depth, in particular when one is close to the glass-rubber transition for the material. Accurate characterisation of the acoustic properties is therefore of crucial importance when selecting materials and when assessing the performance of transducers. Techniques exist to determine properties such as echo reduction and transmission loss from measurements made on panels of the material, but these are typically undertaken in open laboratory tanks. This limits the range of testing of acoustic devices, since such tanks cannot provide the range of environmental conditions that exist during deployment in the sea. Sea-trials are themselves expensive and one is limited by the prevailing temperature conditions in the sea where the measurements are carried out. An alternative to sea-trials is now available using the Acoustic Pressure Vessel (APV), recently commissioned on site at the UK's National Physical Laboratory (NPL) to provide the capability to undertake acoustic measurements under simulated ocean conditions. The environmental conditions within the APV can be modified to simulate ocean depths down to 700 m (hydrostatic pressure from atmospheric to 7 MPa) and temperatures from 2 ^oC to 35 ^oC.
To meet the need for measurements of panel materials in ocean conditions, NPL is establishing a measurement facility in the APV, based on parametric array techniques, to allow the measurement of transmission loss, echo reduction and sound speed as functions of pressure, temperature and frequency.
The problems of establishing a parametric array in the closed, confined space of the APV are examined in detail. Apparently simple tasks, such as mounting acoustic filter materials, and supporting and aligning the test panels themselves, can present substantial engineering challenges. Finally, test results of measurements of transmission loss and echo reduction are analysed and compared with parametric array measurements in an open tank.

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Contributors

Author: V.F. Humphrey

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