Porpoises twist laws of physics to aim their focused sonar beams

By Sam Wong

PORPOISES have the combination of acoustic controls built into their heads to thank for their ability to focus a directed beam of sonar on prey. The bone, air and tissues in their skulls behave like a metamaterial, a material designed to defy the normal laws of physics. These sea mammals can convert non-directional sound waves into a narrow laser of sound.

Like dolphins, porpoises use echolocation to detect prey under water up to 30 metres away. To do this, they emit high frequency clicks in a focused beam in front of their faces, controlling the direction of the beam without moving their heads. They can also widen the beam as they approach their target, helping them catch fish that try to escape.

How they focus the beam is something of a mystery, particularly as the structures that produce the sound – called phonic lips – are smaller than the wavelength of the clicks they produce. This should result in the waveform being spread out instead of targeted. A large fatty organ in the front of the head, called the melon, appears to be important, but the details of the role it plays have been unclear.

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