A slightly different but relevant technology is underwater noise control. A dream of many submarine designers is to make `invisible’ or `quiet’ submarines. Although highly desirable, in practice such systems are difficult to realise.
Instead, methods are being developed to reduce the sonar signature of a vessel and to reduce the amount of engine noise transmitted through the hull. Again, this can be carried out using passive absorbing materials, or an ANR system, or by a combination of the two. To control the reflection and transmission of sound on an underwater structure, it is envisaged that the outside of the structure would be covered with a conformable `smart’ material structure; thus the weight (and cost) of the material is important. In reality, at least in the short term, it is likely that a noise reduction system would be applied to selected `noisy’ areas of a vessel.
GMMT has been developing an underwater ANR system, in collaboration with the DERA. This is based on 0-3 composites (a composite comprising a piezoelectric ceramic in a polymer matrix) which is conformable and can be produced in large areas. It is also much lighter in weight than a piezoelectric ceramic. The system utilises the hydrostatic modes of the materials (voltage coefficients for the sensor and strain coefficients for the actuator), which are optimised by using lead titanate-based composites.
The system comprises two layers; the underwater sound is detected by the sensing layer and the signal is used to drive the actuator layer. It acts to modify the acoustic impedance mismatch at the material/water interface. If the surface of the actuator moves in the same direction as the acting force on the sensor, the actuator has a very high compliance (low acoustic impedance mismatch) and the reflected signal is reduced. Alternatively, when it moves in the opposite direction, the actuator has a high stiffness (high acoustic impedance mismatch) and the reflected signal is enhanced. Thus, the acoustic signature of a vessel can be reduced or enhanced. The GMMT design (comprising layered 0-3 composite actuators and sensors, and an analogue feedback circuit) was found to reduce the acoustic energy by up to 79%, or enhance it by 24%. The reduction in acoustic energy leads a reflected pulse 6dB lower than that obtained without ANR.
Future work will aim at combining the active noise reduction with passive noise-reducing materials, and to produce a large area demonstrator.