A sensor for sensing a predetermined parameter having a known relationship with respect to the transit time of an energy wave through a medium, includes a body of soft elastomeric material having high transmissivity and low attenuation properties with respect to the energy waves; and a transmitter and receiver carried by the body in spaced relation to each other such that the energy waves received by the receiver are those transmitted by the transmitter after having traversed at least a portion of the body of soft elastomeric material. The transit time of the energy wave through the elastomeric body is measured to produce a measurement of the predetermined parameter. In the described preferred embodiments, the energy wave is a sonic wave, such that the body of soft elastomeric material serves as an acoustical channel between the transmitter and receiver.