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Study and application of free-form electret actuators

In this paper, electret based actuators were developed to create a novel free-form loudspeaker. The flexible electret actuator sheet can be easily mass produced in roll or sheet form for good cost efficiency. Compared to the rigid structure of traditional electret actuators, this novel loudspeaker structure is not only thin, flexible, and lightweight at 685 grams/m2, but also it can be cut into any shape to fit various applications and requirements. The free-form electret loudspeaker is composed of centimeter-scale cell electret actuators which possess a bandwidth from 550 Hz to 55 kHz. To demonstrate the benefits of this new technology, a large area loudspeaker was created by joining by 350 pieces of B4-sized electret actuator sheets to create a projection screen 9 m (L) × 3.5 m (W) × 1.1 mm (H). The frequency response of the loudspeaker was measured to have a nearly flat response between 500 Hz and 4 kHz with less than 6% total harmonic distortion (THD) between 400 Hz and 6 kHz. The capacitance value (1.1 μF) of the loudspeaker was limited only by the current capability of the amplifier in the high frequency range. Another creation was a loudspeaker made of over 200 leaf-shaped electret actuator sheets. This unique flexible loudspeaker design was displayed and demonstrated at a year-long public exhibition. The sound level generated by these free form electret speakers were measured and are discussed in this paper.