5H-5 Broadband Omnidirectional Ultrasonic Transducer for Air Ultrasound based on EMFi

Narrow-band ultrasonic transducers with highly-directive radiating patterns are commonly used in ranging, obstacle detection and localization applications. Consequently, problems such as mutual-interferences, limited accuracy in ranging estimation and poor coverage of regions of interest are typical. Some important efforts have been done in order to develop non resonant omnidirectional broadband transducers useful in industrial applications, such as PVDF cylindrical film transducers, electrostatic transducers, multi-frequency piezoelectric composites, etc. However, the problem is still open because of the trade-off among directivity, bandwidth and sensitivity. Recently, a new multipurpose polypropylene electret film called EMFi has been developed by VTT (Technical Research Centre of Finland). EMFi´s advantage over other materials, including polymer electrets, is based on its flexibility due to the voided internal structure combined with strong permanent charge and high sensitivity. These particular features of EMFi make it suitable for fabricating non-resonant transducers with almost arbitrary directivity pattern. In this work, characterization results of flat and cylindrical ultrasonic transmitters based on EMFi are presented, i.e. actual frequency response, radiating emitting pattern and transmitting sensitivity. In the cylindrical case, an omnidirectional polar response has been obtained at frequencies ranging from 30 to 100 kHz and a sound pressure level from 64 to 76 dB in a distance of 30 cm and at 220 Vpp of excitation level. A comparative analysis of both types of developed prototypes responses is carried out. Moreover, obtained results are compared with reported in literature for cylindrical omnidirectional PVDF film transmitters. This paper shows that EMFi film proves to be a well-suited material for developing true omnidirectional broadband ultrasonic transducers