Sound waves can squeeze through holes smaller than their wavelength in a specially fabricated wall.

Metamaterials are artificial materials precisely engineered from metals or plastics to have structures that manipulate sound or light waves. Such materials have already been used to direct light waves through seemingly impassible openings. To accomplish such 'extraordinary transmission' with sound, Sam Lee at Yonsei University in Seoul, South Korea, and his colleagues stretched pieces of plastic film across tiny perforations in a thin metal plate. Incoming sound waves of a selected frequency resonated with the film, causing air to flow as if it were massless and funnel as much as 97% of the sound through the holes. The sound was intensified within the holes by a factor of up to 5,700, and thus could be used in sensitive detectors, the researchers suggest.

Phys. Rev. Lett. 110, 244302 (2013)