National Physical Laboratory

Underwater acoustics panel measurements

A wide range of viscoelastic material is used in underwater acoustics for applications ranging from transducer encapsulation to use as acoustic absorbers. The acoustic properties of these materials can vary strongly with frequency, temperature and depth. Accurate characterisation of the acoustic properties is thus of crucial importance when designing and selecting materials and assessing their performance. Characterisation of these materials under ocean conditions may be undertaken during sea-trials, but these are expensive and limited by the prevailing environmental conditions in the sea during the trials.

To provide an alternative to sea-trials, NPL has established the capability to undertake the characterisation of these materials in the form of panels in the NPL Acoustic Pressure Vessel (APV). This work has formed part of a collaborative project with the Structures and Materials Centre at QinetiQ (Farnborough) and the Department of Physics at University of Bath. The project team has successfully established a parametric array facility in the APV, which enables the measurement of reflection and transmission loss for a test panel at frequencies greater than about 4 kHz. Measurements may be made at simulated depths of up to 700 m and water temperatures from 2 °C to 35 °C. Effort is now being concentrated on extending the range downward in frequency and on the study of the uncertainties in the method.

Results of measurements made using a hydrophone from a horizontal planar scan around a test panel

Results of measurements made using a hydrophone from a horizontal planar scan around a test panel, with the hydrophone signal colour coded according to amplitude. On the left, the incident acoustic pulse may be seen approaching the test panel; on the right is shown the reflected waves from front and back surfaces of the panel, the transmitted wave, and the diffracted waves from the panel edges. The scan was undertaken in the NPL Open Tank Facilities. The test panel was 300 x 300 mm and the acoustic pulse had a centre frequency of 40 kHz.

Results of measurements made using a hydrophone from a horizontal planar scan around a test panel

View/download a larger panel measurement movie (548 KB)

Graph of Reflection Loss Measurements

Results of reflection loss measurements undertaken in the APV on a 0.8 m by 0.9 m test panel
as a function of frequency for a series of applied hydrostatic pressures.

Graph of Insertion Loss Measurements

Results of insertion loss measurements undertaken in the APV on the same 0.8 m by 0.9 m test panel
as a function of frequency for a series of applied hydrostatic pressures.


Download Acoustical Measurements of Panel Materials under Simulated Ocean Conditions flyer Adobe pdf file

Contact

Customer Services tel: +44 20 8943 8631
E-mail: acoustics_enquiries@npl.co.uk

Last Updated: 25 Mar 2010
Created: 6 Jun 2007

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