The effect of polyurethane encapsulant on the response of PVDF hydrophones in the frequency range from 30 kHz to 100 kHz

PVDF has many properties that make it attractive for use as an underwater sensor and a number of groups have reported on such work. In order to use such sensors it is necessary to encapsulate the piezoelectric material in polyurethane for waterproofing. In terms of ceramic sensors the ceramic itself acts as the acoustic pickup and it is desirable to have a polyurethane encapsulant with an acoustic impedance close to that of water. PVDF however has an acoustic impedance similar to water and consequently the coupling of sound into the PVDF is via the encapsulantor the supporting substrate. In order to fully understand and model the PVDF sensor performance it is therefore necessary to have a detailed knowledge of the physical properties of the polyurethane encapsulant. However, in our experience, obtaining such information for various commercially available polyurethanes has proven very difficult. This paper reports on the fabrication and characterisation of thin film and coaxial PVDF encapsulated in a Scorpion polyurethane. The effect of the polyurethane on the sensitivity and directivity of various sensors will be discussed.