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there was a patent in 1998 that is exactly this technology...a sphere of different fluids

Concentric fluid acoustic transponder US 5822272 A

This is claim 1 of the current PCT application:

A method of identifying and locating an underwater acoustic reflector having a shell surrounding a core wherein an incident acoustic wave may be partially reflected by the front of the reflector and partially pass into the core to be reflected from the shell at the rear of the reflector by acoustically interrogating an underwater search area in which such an acoustic reflector may be present characterised in that it includes one or more of the steps of: identifying objects that apparently reflect the acoustic interrogation with a double echo, measuring the separation of the two echoes and comparing those measurements with anticipated measurements for underwater acoustic reflectors that may be present in the search area, and accepting or rejecting the reflected acoustic wave as potentially including the echo from the front of such an acoustic reflector and the echo from the shell at the rear of the reflector; measuring the target strength of the echoes in a double echo received from an object and rejecting the object as one of interest if the target strength in the first echo of a double echo exceeds that of the second echo; measuring the total target strength of an echo from an object, the object being rejected as being a potential underwater acoustic reflector of interest if the target strength is less than a predetermined minimum; measuring the acoustic width of an object and comparing that with the acoustic width of known acoustic reflectors, the object being rejected as being potentially an underwater acoustic reflector of interest if the acoustic width is not that of a known underwater acoustic reflector; measuring the frequency response of an object, the object being accepted or rejected as being potentially an acoustic reflector of interest if the frequency response corresponds to the known frequency response of such an underwater reflector; and measuring the phase response of an object, the object being accepted or rejected as potentially an underwater acoustic reflector of interest if the phase response corresponds to the known phase response of such an underwater acoustic reflector.

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Not my field, but they look very different.The older document uses concentric spheres filled with liquids having different audio propagation characterizes to make an audio lens. In extreme contrast to that the newer document has a shell (no complete sphere needed) as its target.

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