Piezoelectret-microphones with coiled film stacks

Piezoelectret-microphones are characterized by low harmonic distortion, high cut-off frequencies and high sensitivities [1,2]. The high sensitivities are obtained by using film stacks, in which an electrical series connection of the individual films is realized and therefore a linear increase of the open-circuit sensitivity with the number of films is expected. Since the capacitance of the film stack decreases inversely proportional to the number of films and since a capacitive voltage division at the FET input stage is unavoidable, highest microphone output signals are achieved when each ferroelectret film has both high sensitivity and high capacitance. Naturally, small microphones are desirable and therefore various microphone designs with coiled film stacks and integrated FET were constructed and characterized by free-field measurements.