Experiments with coated surfaces to generate sound waves in water

Sources for underwater sound have a wide range of applications, such as active sonar, navigation, and underwater communication. Particularly, sources with large dimensions l, such as a coating on a boat structure, can be suitable for generating low-frequency underwater sound. In this context, low-frequency refers to the range of 0.15 ≤ He ≤ 4.5 with Helmholtz number He = kl. In this study, a sample of an active surface for generating underwater sound is created by attaching an array of 18 circular piezoelectric actuators to a glass-fiber reinforced plastic (GRP) plate as substrate for the coating. The array is coated with an impedance matching layer. The radiation characteristics of the active surface are investigated in an underwater test range with nearly free field conditions. The setup shows suitability for generating low frequency sound, although the vibroacoustic behaviour of the plate has a significant influence on the generated sound pressure. Depending on the frequency range, the sound pressure is either exponentially or linearly dependent on He. The active coating on the GRP-plate proved to be working as expected. For further investigations and improvements, the deviations between the mathematical model and the real behavior of the array must be investigated more intensively.