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The build-up of the effective polarization in ferroelectrets under suitable voltage waveforms was studied by means of acoustical measurements and dielectric resonance spectroscopy. The radiated sound from ferroelectrets under external voltage signals has contributions not only from the effective charge density, but also from the Maxwell stress. Therefore, a "butterfly" curve was obtained for the received microphone signal during charging of a ferroelectret under linearly increasing or decreasing voltages. The microphone-signal measurements were converted to piezoelectric d33 coefficients by comparing the zero-charging-field microphone signals with the absolute d33 values determined by means of dielectric resonance spectroscopy immediately after charging. A polarization-voltage (P-V) hysteresis loop was obtained by analyzing the data in the light of an existing model for the piezoelectric d33 coefficient of ferroelectrets.