Air-coupled generation and detection of ultrasonic bulk waves in metals using micromachined capacitance transducers

Abstract

Micromachined capacitance transducers have been used for the first time to generate and detect ultrasonic longitudinal waves in a 5 mm thick aluminum plate in a completely air-coupled mode (i.e. without any contact). The source and receiver were placed on opposite sides of the aluminum plate in a normal-incidence through-thickness arrangement, leaving air gaps between the transducers and sample. The source was excited by a toneburst voltage whose frequency could be swept through the sample's through-thickness longitudinal resonance; whereas the reception circuitry employed a superheterodyned receiver. The measured resonant peak is consistent with that expected for a 5 mm thick aluminum plate, thus confirming generation and detection of bulk longitudinal waves in a metal by a capacitive air-coupled transducer. This work, when combined with previous studies by the author, shows that the entire range of solid materials (i.e. materials with acoustic impedances ranging from that of polymers and paper all the way to that of metals) can now in principle be investigated using micromachined air-coupled capacitance transducers and any of the available ultrasonic wave-modes.