AIR-COUPLED GENERATION AND DETECTION OF ULTRASONIC BULK WAVES IN METALS USING MICROMACHINED CAPACITANCE TRANSDUCERS

Micromachined capacitance transducers have been used for the first tim e to generate and detect ultrasonic longitudinal waves in a 5 mm thick aluminum plate in a completely air-coupled mode (i.e, without any con tact). The source and receiver were placed on opposite sides of the al uminum plate in a normal-incidence through-thickness arrangement, leav ing air gaps between the transducers and sample. The source was excite d by a toneburst voltage whose frequency could be swept through the sa mple'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 com bined with previous studies by the author, shows that the entire range of solid materials (i.e. materials with acoustic impedances ranging f rom that of polymers and paper all the way to that of metals) can now in principle be investigated using micromachined air-coupled capacitan ce transducers and any of the available ultrasonic wave-modes. (C) 199 7 Elsevier Science B.V.