Bottle Wall

Researchers from Lawrence Berkeley National Laboratory, California have successfully created ‘acoustic bottles’ in open air that can bend the path of sound waves along prescribed convex trajectories.

The research group created an acoustic bottle that features a three-dimensional curved shell, in which a wall of high acoustic pressure surrounds a null pressure region in the middle. Sound waves forming around the bottle are concentrated into a beam that travels through the high pressure wall of its curved shell. The sound waves are generated by an array of loud speakers, 1.5 centimetres in diameter and spaces 2.5 centimetres apart, operating at a frequency of 10 kilo Hertz (kHz) and can be launched along a designated trajectory by precisely adjusting the phase profile of the speaker array. Since the principle of adjusted phased arrays is well-established and now being used in ultrasound imaging, researchers can directly apply the acoustic bottle beam technique to current acoustic systems.

Because the high pressure wall of the acoustic bottle exerts a pulling force, there are no sound waves passing through the null pressure interior of the bottle and therefore the bottle can be used for acoustic trapping. Additionally, the beam is not influenced by any obstacle placed inside the bottle and can even restore itself when an obstacle blocks its path. Researchers demonstrated the same by placing a steel rod to cut the waves, but the waves quickly restored as soon as the rod was removed. Presently, artificial nanoconstructs known as "metamaterials" are used to bend sound waves sufficiently but the nature of these materials places limits on their applications, especially for biological purposes.

The technology has a wide range of applications from ultrasonic imaging to acoustic cloaking to avoiding leakage of sound and levitation and particle manipulation. 

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