An acoustically invisible, rigid wall

You can’t have a conversation across a rigid wall—all the sound impinging on it is reflected. Moreover, drilling small holes in the wall won’t do much to facilitate communication: Just as heat energy is poorly conducted through a thin wire, acoustic energy is poorly transmitted through small holes. But, reports a collaboration led by Sam Hyeon Lee of Yonsei University in South Korea and Oliver Wright of Hokkaido University in Japan, covering the holes with taut plastic film can make a world of difference. When excited at its resonance frequency, a membrane oscillates like a tiny loudspeaker and creates pressure waves that pass through the wall. The figure shows the results of a plane wave launched from the left and normally incident on a rigid, 5-mm-thick acrylic plate (white dashed line) perforated by four small holes. Red represents compression; blue, rarefaction; and yellow and green, near-zero excess pressure. Not much sound got through the uncovered holes, but the membrane-covered holes transmitted 80% of the acoustic energy. Follow-up experiments with different angles of incidence and cylindrical waves also found excellent film-enhanced transmission. Because the passageways are so tiny, the incident acoustic energy density becomes greatly concentrated in those conduits—by a factor of 5700 in one experiment. Moreover, the individual holes in the experimental trials had radii much smaller than the wavelength of the impinging sound. Those two features, say the study’s authors, could lead to sensitive acoustic detectors that achieve subwavelength resolution. (J. J. Park et al., Phys. Rev. Lett., in press.)