Volume 110, Issue 6, December 2001
Index of content:
- ULTRASONICS, QUANTUM ACOUSTICS, AND PHYSICAL EFFECTS OF SOUND 
110(2001); http://dx.doi.org/10.1121/1.1413998View Description Hide Description
A study is presented in which the directional characteristics of an ultrasonic signal have been modified due to propagation within an axial jet. The radiated ultrasonic field from a transducer positioned within the air jet has been studied at frequencies above 100 kHz for the first time. The effects of nozzle shape, nozzle diameter, and variations in air jet velocity and temperature have been investigated. At high air flow velocities, divergence of the ultrasonic beam was observed. This was attributed to the increased acoustic velocities in the direction of the flow. An effective waveguide was also demonstrated by cooling the air jet to below-ambient temperatures, so that the acoustic velocity in the air jet was lower than that in the surrounding ambient atmosphere. The result is likely to be of use in air-coupled ultrasonic materials inspection.
110(2001); http://dx.doi.org/10.1121/1.1413999View Description Hide Description
Acoustic attenuation in a mixture of gases results from the combined effects of molecular relaxation and the classical mechanisms of viscosity and heat conduction. Consequently, the attenuation depends on the composition of the gas mixture, acoustic frequency, temperature, and pressure. A model of the relaxational attenuation that permits the calculation of acoustic attenuation is used to predict the effect of composition, frequency, temperature, and pressure on the acoustic attenuation in a three-component gas mixture of nitrogen, methane, and water vapor. The attenuation spectrum is dependent upon the composition through the appearance of peaks in the spectrum related to the relaxation frequencies of the particular components and their relaxing complexes. The relaxation peak related to methane dominates except at low methane concentrations, where the nitrogen peak, which is dependent upon the water vapor and methane concentration, is evident. Temperature and pressure significantly alter the relaxation frequency and the degree of attenuation, but water vapor plays little role in the attenuation.