A Low Frequency Broadband Flexural Mode Ultrasonic Transducer for Immersion Applications

Ultrasonic transducers for broadband watercoupled operation in the low ultrasonic 30-100 kHz frequency range find their application in different industrial and biomedical areas, such as non-destructive testing and therapy. In this paper we propose the design and the fabrication process of a multi-cell, piezoelectrically-actuated, flexural-mode transducer structure, characterized by a low mechanical impedance leading to wideband and high sensitivity immersion operation and by a high flexibility in the definition of the active area shape. After investigating the use of different materials for the different components, we report on the design of a fabrication process flow for the precise assembly of the transducer, based respectively on epoxy resin and wire-bonding for mechanical and electrical interconnection of the individual parts. A reduced-thickness (<10 mm) and weight (350 g) transducer prototype, featuring 121 circular cells placed in a hexagonal tiling arrangement covering a quasi-circular area of 80-mm diameter, is designed, fabricated, and characterized. A transmit sensitivity of 5 kPa/V at 100 kHz resulted from hydrophone measurements performed at 150 mm while the pulse-echo response from a planar surface placed at the same distance showed a 125% -6dB fractional bandwidth centered at 53 kHz.