Abstract: A pressure-balanced electromechanical converter is described including a structure that converts displacement into electrical energy or electrical energy into displacement, said structure designed to separate an enclosed volume for an outside pressure wave channel, wherein said enclosed volume has a filtering pressure transparent connection to said outside pressure wave channel with said filtering connection be pressure transparent to static pressure or low frequency pressure waves and filtering pressure waves at higher frequencies.

Abstract: An acoustic telemetry apparatus and method for communicating encoded digital data from a down-hole location through a borehole to the surface is described including an acoustic channel terminated at a down-hole end by a reflecting terminal (133, 134), an acoustic wave generator (140) located at the surface and providing an acoustic wave carrier signal through said acoustic, channel, a modulator (162, 163) located down-hole to modulate amplitude and/or phase of said carrier wave in response to an encoded digital signal and one or more sensors (150) located at the surface adapted to detect amplitude and/or phase related information of acoustic waves traveling within said acoustic channel to determine the encoded digital data.