NPL is pleased to announce a new tile for lining underwater acoustic test tanks or for coating hydrophone and transducer mounts. The tiles are made from a special polyurethane-based material, the acoustic properties of which have been optimised to match those of water. The result is a material with excellent echo-reduction properties and high transmission loss, making the tile suitable for use at acoustic frequencies above 10 kHz.
Scientists at the National Physical Laboratory (NPL), the UK’s National Standards Laboratory, in conjunction with Acoustic Polymers Limited, have developed the new material for use by the maritime, oceanographic and hydrocarbon exploration industries as a sound absorbing material.
Testing of sonar transducers and hydrophones is commonly carried out in large water tanks and important measurements can be prevented, or their accuracy severely compromised, through acoustic echoes or reflections from the tank walls, fixtures used to hold the devices, as well as the water surface. The new tiles are made of a polyurethane rubber based on a polytetramethylene ether glycol that has been cross-linked through the addition of chemical species to vary the acoustic properties such as the speed of sound.
Tile properties have been optimised through the addition of small micro-balloons to boost sound absorption and high density fillers that enable the acoustic properties of the tile to be matched to that of water. A structured front surface of conical pyramids is additionally used to break up or scatter incident sound. Crucial to the optimisation has been the specialist measurement facilities at NPL that have been applied to characterise key reflection and transmission properties of the tiles over the frequency range 20 kHz to 200 kHz. They are now commercially available although research continues to develop novel sonar materials for application at frequencies below 10 kHz.