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Jan 2013

Volume 9, Issue 1, pp. 8-36

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On the Fascinating Phenomenon of Diffraction by Periodic Structures

Nico Felicien Declercq

Acou. Today Volume 9, Issue 1, pp. 8-13 (2013); (6 pages)

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Although physically very complex, under a relatively wide range of conditions the diffraction of sound by a corrugated structure can be described and understood under the principle of a plane wave expansion. A plane wave expansion was first considered by Lord Rayleigh and was later applied by the team of Oswald Leroy while incorporating the mandatory mechanical coupling conditions between the two media separated by the corrugation. In essence the plane wave expansion (i.e., expansion into diffraction orders) is based on a Fourier series expansion with incorporation of propagation properties given by the dispersion relation and based on the acoustic wave equation. The fundamental connection between the periodic structure and the decomposition into different diffraction orders is the grating equation as also used in optics. In this article, a brief overview of a number of selected phenomena studied in the recent past with concise explanation and historical context will be presented.

Microbubbles as Ultrasound Contrast Agents

Thomas J. Matula and Hong Chen

Acou. Today Volume 9, Issue 1, pp. 14-20 (2013); (7 pages)

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The future of healthcare is bubbles. That may be an overstatement, but micron‐sized bubbles (called microbubbles) play an important role in diagnostic imaging. Current research is exploring how microbubbles can be used for molecular imaging and targeted drug delivery. The bubbles act as very good ultrasound scatterers, and because they oscillate upon ultrasound exposure, they can also do (therapeutic) work on the surrounding tissue (e.g., breaking blood clots or opening up the blood‐brain barrier). Current research with microbubbles has focused on two main areas—developing new ultrasound pulse sequences to improve the contrast/noise ratio, and developing specialized microbubbles for molecular imaging and therapy. This article discusses aspects of microbubbles and their dynamics in actual blood vessels.

A Simple Electronically‐Phased Acoustic Array

Martin L. Smith, Michael R. Roddewig, Kurt M. Strovink, and John A. Scales

Acou. Today Volume 9, Issue 1, pp. 22-29 (2013); (8 pages)

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Phased source arrays are physically‐distributed arrays of radiators (source transducers) driven by time‐delayed copies of a common signal. The time delays are chosen so that the net antenna pattern of the array is focused on either a particular target or lies in a particular direction. A simple example is an array of broadcast antennae with an individual element's time delay determined by the length of its feed cable. By switching among sets of feed cables the composite antenna may be made to have major lobes in different directions, in terms of both elevation and azimuth, as needed to meet the varying requirements of daytime and nighttime operation. These arrays have either fixed delays or a few, selectable sets of fixed delays. Phased receiver arrays are based on exactly the same principles as their source counterparts—a correspondence that is spelled out in a little more detail in the implementation section of this article. In all other respects, the term phased array will mean phased source array. We are interested here in electronically‐steerable phased arrays, that is, phased arrays with electronically‐controlled time delays. These systems are capable of rapid shifts over many antenna patterns which make it practical to quickly scan the major lobe of the outgoing signal over a range of azimuths and elevations. One application of this technology is military target‐tracking radar systems, which may have to (nearly) simultaneously track many potential threats much more rapidly than would be feasible with a physically steered antenna. Perhaps the most active area of the application of phased‐array technology is in bio‐medical devices which image the body with ultrasound. This field is driven by the needs of medical diagnosis and treatment; it is fortunate that the human body represents a relatively homogeneous environment of acoustic speeds which rewards the development of sophisticated imaging methods.

Report of ISO/TC 43, Acoustics, and ISO/TC 43/SC1, Noise, Standards Meetings in Florianopolis, Brazil, November 2012

Robert D. Hellweg, Jr., Jeff G. Schmitt, and Laura Ann Wilber

Acou. Today Volume 9, Issue 1, pp. 30-36 (2013); (7 pages)

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Having received ASA Robert W. Young Travel Awards for the Development of International Standards, the authors were able to attend the ISO technical committee 43 (TC 43) (Acoustics) and the ISO/TC43 Subcommittee 1 (SC 1) (Noise) meetings in Florianopolis, Brazil. There were representatives from thirteen countries: Brazil, Canada, Denmark, France, Germany, Japan, Republic of Korea, Luxembourg, Norway, South Africa, Sweden, United Kingdom, and the United States of America. It is important that the United States have effective representation at ISO working group meetings since, in practice, the responses to comments on draft standards are determined during those meetings.
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