Ferroelectrets microphone

To calculate the dynamic piezoelectric d- j, coefficient from the microphone sensitivity, the pressure sensitivity of the ferroelectret microphones is required. This sensitivity is defined as Mp = V/p, where p is the actual sound pressure p in front of the microphone after it is positioned in the sound field. This sound pressure differs at higher frequencies from the free-field value pf, due to diffraction effects. The value ofp can be determined from p hy means of the free-field correction factor (Xiaoqing Zhang et al. 2014 Zhang et al. 2012). From such evaluations Mp was obtained, and the dynamic piezoelectric 33 coefficient of the ferroelectret is obtained by means of (Zhang et al. 2007)... 

Electret materials are meanwhile used in a large number of modern high-tech applications including microphones, acoustic sensors, transducers, radiation and pollution dosimeters, power generators, filters, and many more. Additionally, electret technology is of great interest in the field of biomaterials, for instance in callus formation and wound healing [10, 11], When used in cellular or in multilayer sandwich structures, polymer electrets can exhibit piezoelectricity. Such materials are ferroelectrets, as they show typical features of ferroelectric materials such as piezo-and pyroelectricity [12-17],... 

Rychkov D, Altaflm RAP, Gerhard R (2014) Unipolar ferroelectrets - following the example of the electret microphone more closely. In Annual Report, IEEE Conference on Electrical Insulation and Dielectric Phenomena. IEEE, New York, pp 860-862. doi 10.1109/CEIDP.2014.6995907... 

Since measurements are made such that the same sound pressure acts on all exposed points of the ferroelectret film, this evaluation of 33 yields an average value over the microphone area. 

Polymer electret and ferroelectrets have unique characteristics such as electrostatic transduction wifliout external voltage, light weight, flexibility, and so on. Their most successful apphcations are microphones and air Alters, but various other types of devices have also been proposed. In this chapter, after giving overview of their apphcations, developments of acoustic devices and power generators/energy harvesters arc discussed. 

After reducing the complex voided stmcture to a simplified three-layer system, the electret microphone theory can be applied. To demonstrate the relation between this theory and the ferroelectret model, the electric charged layered system, as shown in Fig. 2b, was considered. The eharged model represents the charges trapped on the inner void surfaces and the charges induced on the proximity of the electrodes, after the stmcture has been submitted to an external high electric field and dielectric barrier discharges had occiured inside the air cavities. 

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