Air-coupled flexural electrodynamic acoustic transducers

Flexural transducers use the bending modes in a plate or membrane to produce sound in low acoustic impedance media. Traditionally, piezoelectrically actuated flexural transducers have been used to generate ultrasound with large amplitude for a relatively low excitation voltage. In this work, the use of electrodynamic forces generated by a current carrying coil is investigated, as an alternative method for generating ultrasound by flexural vibrations. Using a coil instead of a piezoelectric element makes the transducer easier to manufacture, and able to operate at high temperatures. The analytical theory of vibrating plates as well as finite element modelling was used to predict transducer behaviour, i.e. mode frequencies and shapes of the vibrating front face. Prototype transducers were made from aluminium with a pancake copper coil at the back for generation. A Polytec laser vibrometer was used to measure the front face displacement of these prototype transducers. The displacement measurements revealed a frequency spectrum with narrowband (~3 kHz full width half maximum) modal frequency peaks, and a dominant fundamental mode at ~50 kHz. The spectrum is in good agreement with calculated frequency values, and the experimental mode shapes are similar to those predicted by theory.