the other end the transducer fixed a large non radiating backing mass 20 is provided for this purpose the mass is affixed to the housing by means of adhesive 22 which may be similar to adhesive 14 an annular support 30 is affixed to the inside of the housing and contains an annular recess containing coil 32 the face of the coil is directed rearwardly a second annular support 34 is carried by the radiating mass 12 and also carries a coil 36 in a recess facing forward this arrangement of coils is designed to partially provide a pressure release system as described in u s pat no 3 790 928 the present invention may be used to supplement this coil system or used without these coils a volume of liquid 44 is contained between the backing mass and the housing a heating element 40 is present in the liquid and connected to a source of electrical power through leads 42 above the surface of the liquid is a volume 46 filled with vapor from the liquid passages 48 carry vapor from the volume 46 to the spaces on the other side of the backing mass while the liquid is shown as being in contact with the backing mass it is actually preferable that a compartment separate from the mass be used.