A device for the conversion of acoustical or mechanical energy into electrical energy, and vice versa, which utilizes a quasipermanently charged or polarized dielectric material (electret). Examples are certain microphones, headphones, hydrophones, and ultrasonic devices. Depending on the electret material and the usage, either the external electric field of real-charge electrets or the piezoelectric activity of dipolar electrets is used. In the simplest implementation, a transducer based on real-charge electrets consists of a metal backplate (first electrode) covered by a mechanically tensioned diaphragm. The diaphragm is a film electret carrying a metal coating (second electrode) on the side facing away from the backplate. Provisions are made to maintain a shallow air gap between electret and backplate. The air gap is occupied by an electrostatic field originating from the electret charges. Upon acoustical or mechanical deflection of the diaphragm, such a device generates an electrical output signal between its two electrodes; similarly, application of an electrical signal results in diaphragm deflections. Such electret devices are therefore self-biased electrostatic or condenser transducers. They thus exhibit all the advantages of this transducer class, such as wide dynamic range and flat response over a frequency range of several decades, without requiring the external bias necessary in conventional transducers of this kind. On the other hand, a transducer based on the piezoelectric activity of a dipolar electret may consist of just the electret film covered by two electrodes. Compression or bending of the film causes again an electrical output signal between the electrodes, while the application of a voltage to the electrodes results in mechanical action.