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P1I-6 A New Electrostatic Transducer Incorporating Fluidic Amplification

A methodology for improving the performance of electrostatic transducers that comprise a thin membrane positioned above a conducting backplate is presented. The concept has its foundations in the normally tubular sections used for amplification of sound in musical instruments, where the resonant frequency of a fluid tilled conduit is determined by the conduit length. Finite element modelling was used to evaluate a range of potential configurations, including conduit aspect ratio and the membrane - cavity structure immediately above the conduit (ie membrane dimensions, cavity width and depth). Results are presented that reveal the potential variation in amplification and these demonstrate that under certain conditions, a very significant improvement can be obtained. For example, at a centre frequency of 215 kHz, a conduit with an aspect ratio of 10:1 will produce a tenfold increase in displacement at the membrane end of the system. Although this is achievable over a wide frequency range, the best overall performance in terms of absolute sound pressure level was obtained when the length of the conduit was matched to the centre frequency of the vibrating membrane. Proof of concept devices were manufactured and tested experimentally, for both quarter wavelength and half wavelength conditions. Laser vibrometry was used to measure the displacement of the membrane, while a calibrated PVDF hydrophone and electret microphone were employed to measure the acoustic output at the end of the conduit. The presence of the vibrating air conduits was observed to increase the measured acoustic output by a factor greater than 20 dB. The results confirm that the prototype devices can operate efficiently at both ends of the system and the membrane can be excited effectively by the air column. Such transducers are ideally matched to air and also offer the potential to be used with other load media