Through-transmission imaging of solids in air using ultrasonic gas-jet waveguides

An ultrasonic waveguide has been produced in air by using a gas jet. This uses the fact that a lower acoustic velocity can be produced within the jet, relative to the air surrounding it. The lower velocity is achieved by mixing carbon dioxide with air within the jet at a concentration that is a compromise between lower acoustic velocity and increasing attenuation. Using a capacitance transducer placed within the flowing gas, it is shown that improvements in the beam width can result when the gas jet is used. Air-coupled images of solid samples have been produced in through transmission, which demonstrate that an improved lateral resolution can result when a comparison is made to images from conventional air-coupled testing.