In most
cases, piezoelectric hydrophones are used for such measurements.
In particular, polyvinylidene fluoride (PVDF) related
piezoelectric polymers are widely used as the sensing elements
of hydrophones, since they possess flexibility and low
acoustic impedance, which facilitates impedance-matching with
water. However, piezoelectric polymers such as this have insuf-
ficient sensitivity when the sensing area of their active elements
becomes smaller in size