There is considerable interest in using non-contact ultrasonic techniques on engineering materials. Some of the practical difficulties associated with air-coupled techniques include:-

• The large acoustic impedance mismatch between most solid materials and air leads to an extremely small transmitted signal
• Attenuation in air becomes excessive above 3MHz.
• Air movement and temperature variations in the propagation path causing additional refraction and scattering effects.

Because of these problems a new approach is being investigated, using a gas-jet to provide a stable a stable enviroment for ultrasonic propogation and could also act act as an ultrasonic waveguide.

  Transducer arrangement within a gas-jet used as a waveguide.

By using different gases it is possible to have acoustic velocity, density and attenuation properties within the gas jet that are substantially differnt from those of air. Results of some preliminary studies using a capacitive transducer with a micromachined silicon backplate and metallised mylar film membrane as the source are shown below. It can be seen that the directionality of the emerging ultrasonic beam can be changed substantially, by using different gas jet velocities and gas mixtures. The first figure shows that increasing the air jet velocity causes the radiated field to diverge.

  Cooling the gas jet causes a waveguide to be created, as the acoustic velocity within the jet is now lower than that in the air suurounding it.           The addition of CO₂ to the gas jet mixture also causes a waveguide to be created, as the acoustic velocity within the jet is now again slower than the ambient air surrounding it.  

---

Related Papers
D.A. Hutchins, C.S. McIntyre, D.R. Billson, D.W. Choi and T.J. Robertson, "Ultrasonic propagation in a gas jet",J. Acoust. Soc. Am. 110, 2964-2973 (2001).
D.W.Choi, C.McIntyre, D.A.Hutchins and D.R.Billson, "Gas jet as a waveguide for air-coupled ultrasound'.", Ultrasonics, 40, 145-152 (2002).