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To vocalize, animals create sound waves in their pipe-like vocal tracts. Shorter pipes produce higher-frequency sounds, so small animals like the cuddly koala should have high voices. [Koala sound.] Or not.

While many hotel rooms, recording studios, and even some homes are built with materials to help absorb or reflect sound, mechanisms to truly control the direction of sound waves are still in their infancy. However, researchers at the California Institute of Technology (Caltech) have now created the first tunable acoustic diode-a device that allows acoustic information to travel only in one direction, at controllable frequencies.

In some cases, doctors have learned how to remove non-cancerous tumors with sound waves instead of invasive surgery

(PhysOrg.com) -- Physicist Stefano Lepri of the Italian National Research Council and his partner Giulio Casati of the University of Insubria, have published a paper in Physical Review Letters, where they demonstrate through mathematical theory that it should be possible to create asymmetric materials that allow most light or sound waves to pass through in one direction, while mostly preventing them from doing so when going the opposite way.

Scientists at the Vienna University of Technology (TU Vienna) have developed a method to steer waves on precisely defined trajectories, without any loss. This way, sound waves could be sent directly to a target, avoiding possible eavesdroppers.