Volume 115, Issue 4, April 2004
Index of content:
- ULTRASONICS, QUANTUM ACOUSTICS, AND PHYSICAL EFFECTS OF SOUND 
115(2004); http://dx.doi.org/10.1121/1.1687733View Description Hide Description
Experimental study of the propagation of high-frequency acoustic waves in grid-generated turbulence in laboratory conditions by means of ultrasound technique is considered. The travel time and log-amplitude variance of acoustic waves are investigated as functions of travel distance and mean velocity. Experimental data were interpreted using theoretical analysis based on ray acoustics and Rytov approximation. The travel time shift in acoustic wave propagation in turbulent and undisturbed media is observed.
Acoustic waves generated by a laser line pulse in cylinders; Application to the elastic constants measurement115(2004); http://dx.doi.org/10.1121/1.1651191View Description Hide Description
A model is proposed to predict acoustic waves generated in a transversely isotropic cylinder by a laser line pulse extended in beamwidth and time duration, and an application to elastic constants measurement is presented. Documented good agreements are observed in the comparison of experimental and theoretical normal displacements for aluminum cylinders under either ablation or thermoelastic generation. Bulk waves are identified and processed for the elastic constants measurement. The effects of source beamwidth and time duration on wave forms and on the elastic constants measurement are predicted by numerical simulations. For nondestructive evaluation applications using bulk waves, a radius of 0.3 mm appears as a minimum limit for the sample size using a laser source of 0.1 mm beamwidth and 20 ns time duration. Elastic constants of aluminum rods are experimentally measured with very good accuracy.
115(2004); http://dx.doi.org/10.1121/1.1646406View Description Hide Description
A new formulation is presented for the prediction of the signals transmitted by a rectangular acoustic source, and detected by a finite-sized rectangular receiver. This problem is encountered in measurements where the receiver is of a finite size compared to the shortest wavelength emitted by the source. The paper examines this problem theoretically, and a new formulation is presented in which the geometrical aspects are considered in some detail. The result is a simplified approach, in which the overall impulse response can be obtained from a combination of certain contributions defined geometrically. Theoretical field predictions are compared to experimental measurements for ultrasonic signals radiated into air, to demonstrate the usefulness of the approach.
Air-coupled ultrasonic sensing of grass-covered vibrating surfaces; qualitative comparisons with laser Doppler vibrometry115(2004); http://dx.doi.org/10.1121/1.1650329View Description Hide Description
The paper addresses several sensitive issues concerning the use of air-coupled ultrasound to probe small vibrations of surfaces covered with low-lying vegetation such as grass. The operation of the ultrasonic sensor is compared to that of a laser Doppler vibrometer, in various contexts. It is shown that ambient air motion affects either system, albeit differently. As air speed increases, the acoustic sensor detects a progressively richer turbulent spectrum, which reduces its sensitivity. In turn, optical sensors are prone to tremendous signal losses when probing moving vegetation, due to randomly varying speckle patterns. The work was supported by the Office of Naval Research.
115(2004); http://dx.doi.org/10.1121/1.1649942View Description Hide Description
The time-of-flight (TOF) method is an ultrasonicnondestructive testing (NDT) technique. The TOF of an ultrasonic wave can be correlated to weld penetration depth, and hence weld quality. Changes in material properties due to temperature gradients will cause ultrasonic speed to vary during welding, which causes a curved propagation path. A ray tracing algorithm is required in order to study how ultrasound propagates within a weld sample. In this paper, a three-dimensional (3-D) ray tracing algorithm based on Fermat’s principle is presented. First, ray equations are derived using the calculus of variation. Then, a numerical algorithm is developed to solve the derived ray equations and obtain the curved propagation path. This algorithm includes finite element analysis(FEA) to obtain the transient temperature distribution during the welding and shooting method to solve the boundary value problem. After the curved ray path is obtained, the TOF can be found by integrating the time variable along the ray path. An analytical relationship between the TOF and penetration depth can be established by repeating the ray tracing algorithm for different penetration depths. Experimental measurements of TOF have been performed, and this data is to be used to validate the numerical results.