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For acoustic waves, in general the source must be much larger than the wavelength to produce directional waves, yet porpoises have efficient biosonar systems that break this size rule. How? This study reveals that the porpoise's forehead is a natural gradient-index material, and key to production and control of directional beams. The whale's compression of the multiphase structure of its forehead effectively manipulates the view angle of the beam. Aside from advancing our knowledge of cetaceans, these results could inspire the development of human-made metamaterials for our own subwavelength applications.
Zhang et al. (Fri,) studied this question.