13412 Air Coupled Ultrasonics

The air-coupled ultrasonic method [47-49] is a relatively new method since 1995. The method has become feasible primarily through advances in piezo-electrics and digital signal processing. A typical air-coupled, through-transmission system consists of a traditional x-y-z positioning system with two matched aircoupled transducers in a coaxial transmission geometry (Fig. 13.27). The yoke assembly on which the transducers are mounted is connected to x-y scan stepper motors that are controlled by the host computer. The sample is mounted on an adjustable support so that the focal point of the transducers is within the thickness of the sample. A C-scan image of the sample is built up with a nominal 0.8-mm step size in both x and y directions. Tone bursts of acoustic energy at 0.4 MHz are incident on the sample from the transmitter side, and these ultrasonic

FIGURE 13.27 Schematic diagram of an air-coupled ultrasound system.

waves propagate through the sample and emerge to be detected by the receiving transducer. The detected signal is preamplified by a low-noise preamplified amp attached directly to the receiving transducer connected to a tuned amplifier. The digital value of the peak-transmitted signal is displayed and stored. The resulting image consists of a large number of pixels, whose gray level depends on the transmitted amplitude. As acoustic waves propagate through the material, they are scattered by defects. Areas with defects (such as pores and delaminations) appear with different gray-scale values on the image.

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