November 2014

Sound’s too good to be true

They say silence is golden, and much like the precious metal it’s rather rare. Sound is everywhere, from the symphony of life in a city centre to the hum of nature in the countryside. Even when completely isolated you hear your own breathing and your beating heart.Almost everything makes a noise and hearing plays a vital role in how we perceive the world. Yet sound isn’t just an information source; when directed and controlled it becomes a powerful tool with numerous applications from cleaning to navigation, and following the recent developments at Lawrence Berkeley National Laboratory sound might just make you levitate.

How the sound coming from the array (2D acoustic source) propagates in a straight line at first then curves out and then back in returning to its original form (the blue ‘bottle’). In the centre is a null region where there is no acoustic pressure. (Image by Xiang Zhang group / UC Berkeley).

Okay, full disclosure it may have been an exaggeration to say that the Acoustic Bottle could make you levitate, unless you’re a Borrower, but the Berkeley Lab project can levitate 3D objects millimetres big such as water droplets[1]. The ‘bottle’ beam technique developed by Xiang Zhang and his group does, however, potentially open doors of development in numerous applications.The technique claims to be able to accurately redirect sound waves, which travel in straight lines, in open air without the need for mechanical aid or waveguides. The sound wave formed by the technique is also able to flow around objects in its path without distorting its shape. The benefit of this is that sound waves will be able to reach hard to reach objects that may be hidden behind obstacles, improving accuracy in acoustic imaging and improving the effectiveness of therapeutic ultrasound. This reaches beyond determining the sex of a baby; acoustic imaging is used industrially to diagnose flaws in materials including aerospace composites, and medically, for example finding a tendon lesion. Ultrasound is also used to kill cancerous cells within the body, improved control over the wave could allow for better targeting of these cells [2]. The technology can also be used for acoustic cloaking, as sound waves could be redirected around an object and reformed to their original state on the other side, rendering that object invisible to detection methods like Sonar.

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