the second chamber plus a restoring force generated in the photoelastic material to the acoustic force applied to the rigid diaphragm utilizing the well known principle that the force applied to a given surface is equal to the pressure applied thereto times the area of the surface it can be shown that the pressure p p on the photoelastic material is where p a is the pressure applied to the rigid diaphragm p o is the back pressure applied by the oil and m is the ratio of the area of the rigid diaphragm 35 to the area of the photoelastic material 16 a ratio that is much greater than one it is readily apparent from equation 6 that the pressure applied to the photoelastic materials 16 due to an instanteous acoustic force applied to the flexible diaphragm 36 is maximized when the oil back pressure p o is minimized those skilled in the art will recognize that the back pressure generated by the oil in the second chamber and the pressure applied to the photoelastic material 16 are functions of the bulk modulus of the oil b o young 39 s modulus for the photoelastic material y p the length of the second chamber l o the length of the photoelastic material l p and the compression length u03b4l of the oil and the photoelastic material these factors being related by equ5 thus it is readily seen that the oil back pressure may be reduced with the lengthening of the second chamber 34 consequently the second chamber 34 is of a length that is very much longer than the length of the first chamber 33 since it is not possible to eliminate the oil back pressure a loss of sensitivity u the ratio of the distributed.