Abstract
Three designs for electrodynamic flexural transducers (EDFT) for air-coupled ultrasonics are presented and compared. An all-metal housing was used for robustness, which makes the designs more suitable for industrial applications. The housing is designed such that there is a thin metal plate at the front, with a fundamental flexural vibration mode at ∼50 kHz. By using a flexural resonance mode, good coupling to the load medium was achieved without the use of matching layers. The front radiating plate is actuated electrodynamically by a spiral coil inside the transducer, which produces an induced magnetic field when an AC current is applied to it. The transducers operate without the use of piezoelectric materials, which can simplify manufacturing and prolong the lifetime of the transducers, as well as open up possibilities for high-temperature applications. The results show that different designs perform best for the generation and reception of ultrasound. All three designs produced large acoustic pressure outputs, with a recorded sound pressure level (SPL) above 120 dB at a 40 cm distance from the highest output transducer. The sensitivity of the transducers was low, however, with single shot signal-to-noise ratio dB in transmit–receive mode, with transmitter and receiver 40 cm apart.
Highlights
Transducer B produced the greatest pressure amplitudes around 50 Pa or sound pressure level (SPL) = 128 dB. This is about an order of magnitude greater than the pressure amplitudes from the other two designs, which is largely due to the overall greater surface area of the front face of transducer B
The advantage compared to standard flexural transducers is the independence from piezoelectric elements, which enables the transducer to operate at higher temperatures, reduces the number of bonds that can fail over time, and introduces new possibilities in terms of aperture design
Of the different designs outlined and tested in this work, it has been demonstrated that the design of transducer B offered the greatest design flexibility, as it allowed a larger transducer aperture for a given operational frequency, and a greater pressure output
Summary
Air-coupled ultrasonics is of great interest for many applications, including wireless communication [1], contactless nondestructive evaluation (NDE) [2,3], and materials characterization [4].One of the major challenges for air-coupled transducers is the large acoustic impedance mismatch between the transducer element and air, resulting in an inefficient power transmission.Different solutions to this problem have been developed, and often make use of matching layers [5] to gradually reduce the impedance between the transducer and the propagation medium.More recently, the use of wedges with power-law taper profiles making use of the acoustic black hole effect [6] to smoothly reduce the acoustic impedance have been investigated [7,8]. One of the major challenges for air-coupled transducers is the large acoustic impedance mismatch between the transducer element and air, resulting in an inefficient power transmission. Other available air-coupled transducers include capacitive micromachined ultrasound transducers (CMUTs) [9]) and piezoelectric micromachined ultrasound transducers (PMUTs) [10], which have excellent coupling to air and improved bandwidths. Both transduction techniques rely on cells with thin-stretched membranes, which reduces robustness and makes them unsuitable for some industrial applications
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