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Piezoelectric properties of polymer electrets consisting of cellular polypropylene (PP) or of porous polytetrafluoroethylene (PTFE) with thicknesses of 50 to 100 μm are studied. In addition, bilayer or multilayer structures composed of one of these polymers in solid or voided form plus an additional air layer are investigated. In particular, the quasistatic and the dynamic piezoelectric d33 coefficients are determined with electro-mechanical, optical, and acoustic methods. The quasistatic coefficients are of the order of 100 to 350 pC/N for the cellular PP and much lower for the porous PTFE. With increasing frequency up to about 50 kHz the coefficient of cellular PP decreases where it starts to rise toward the resonance at approximately 300 kHz. The bilayer or multilayer structures show considerably higher quasistatic coefficients which reach up to 20,000 pC/N for certain combinations. As for the single-layer systems, a decrease toward higher frequencies is observed. The effect is linear as a function of load pressure at relatively low pressures, ranging up to 100 Pa for some air-gap systems and up to several kPa for the cellular PP.