The sensitivity of a hydrophone is often specified as the end-of-cable open-circuit sensitivity. This is the sensitivity of the hydrophone at the end of its cable when not connected to an electrical load. When a specific electrical load, such as an oscilloscope, an amplifier or extra cable is used at the output of the hydrophone, the end-of-cable loaded sensitivity of the hydrophone may be related to the open-circuit sensitivity in the following way.

Corrections using complex impedance

Interactive Version

Consider the general case in which the hydrophone is considered as a two-terminal network of complex impedance Z_H connected to an electrical load of complex impedance Z_L. The end-of-cable loaded sensitivity of the hydrophone, M_L, when connected to the specified load is related to the end-of-cable open-circuit sensitivity, M_O, by

where Re and Im denote the real and imaginary parts of the relevant complex impedance.

Often, the electrical load can be assumed to be a parallel combination of a resistance R_L and capacitance C_L. In this case, Re(Z_L) and Im(Z_L) are given by

and

where is the angular frequency.

Corrections using only capacitance values

Interactive Version

A further simplification is possible if the impedances of both the hydrophone and the load can be assumed to be purely capacitive. This is often a valid assumption for a hydrophone at frequencies much less than the resonance frequency and for loads such as extension cables. In this case, if C_H is the end-of-cable capacitance of the hydrophone including any integral cable and connector, and C_L is the capacitance of the load (eg the extension cable), the above equation reduces to:

For further information, contact Stephen Robinson.