1. In a hydroacoustic filter and electrolytic detector system of the charer described, means for providing a housing for a unitary assembly of an electrolytic detector and a hydro-acoustic filter circuit of selectable frequency attenuation characteristic to provide a plurality of band pass ranges for said system, which comprises an electrolytic detector cell, a first acoustic capacitance means connected in series with a first side of the input of said circuit, a second series connected acoustic capacitance connected in a second signal input side of said circuit, switching means providing a first plurality of hydraulic inductance-resistance tubes, with a preselected one of said plurality of tubes being normally disposed in a position to provide a shunting of the remainder of the hydroacoustic filter and electrolytic detector system, said switching means further including means providing a second plurality of hydraulic inductance-resistance tube members disposed for selective connection of a predetermined one thereof in series with said first named input capacitance and contemparaneously with a predetermining one of said first tubes, means for selecting one each of said plurality of series and shunt tubes to provide in conjunction with the other filter elements preselected signal attenuation in a desired frequency range, means providing a third acoustic capacitance in series with said series inductance-resistance tube, a fourth capacitance means providing a shunting of the signal output of said third capacitance around said detector cell, means providing a fluid connection of said detector cell, and means for deriving an output of predetermined frequency characteristics from said detector.