Volume 6, Issue 2, April 2010
Index of content:
6(2010); http://dx.doi.org/10.1121/1.3467645View Description Hide Description
In this article, we focus on one aspect of acoustics—the application of acoustic resonances to probe the properties of materials and objects. Such an application has a significant impact on us all as it connects to science, engineering, quality control, public safety and protection (including military applications), and more. The idea of using resonances to study the elastic response of a material goes back to the 1960's. Resonant Ultrasound Spectroscopy (RUS), in its present form, was developed in 1988 by Migliori and Visscher, who—with the advantage of modern computing power—developed sophisticated codes and turned RUS into a powerful method for measuring elastic constants of materials with a variety of shapes and symmetry.
6(2010); http://dx.doi.org/10.1121/1.3467643View Description Hide Description
On 27 August 1883, the island of Krakatoa was virtually destroyed by an immense volcanic explosion. The resulting pressure wave was recorded worldwide for days afterward and is one of the most frequently‐cited acoustical events of global proportion. Observations of the pressure wave from the explosion were not restricted to one or two locations; more than 50 weather stations around the world recorded the wave's passage. Several stations recorded as many as seven passages as the wave orbited the globe. Photographic and pen‐and‐ink recordings of barometric pressure were forwarded to London and the initial analyses were reported in a pair of papers, one by R. H. Scott and one by Lt. Gen. R. Strachey, in December, 1883.
6(2010); http://dx.doi.org/10.1121/1.3467644View Description Hide Description
Parametric array systems are a promising tool for multifrequency acoustical tomography techniques for monitoring range dependent temperatures and current distributions in a complex ocean environment.The parametric array (PA) is a nonlinear transduction process that can generate a narrow beam of low frequency sound in a medium, through the interaction of co‐linear, intense, high frequency sound waves, called pump waves. The unique characteristic of a parametric array is found in its extremely narrow directivity pattern (1°–3° angular resolution) for low frequency acoustical signals. The effective width of the directivity pattern remains practically constant over a wide range of signal frequencies. The parametric array has become essentially a virtual acoustic end‐fire array that has been formed in the medium (water) by the non‐linear interaction of the two high frequency waves at their sum and difference frequencies. As a result, it radiates a sharp, low‐frequency, directional signal at the interaction frequency of its pump waves that propagates independently of the pump waves.