Soda cans focus sound to subwavelength spots
DOI: 10.1063/PT.3.1248
Soda cans focus sound to subwavelength spots. Beating the diffraction limit requires accessing a field’s evanescent waves, which harbor its subwavelength spatial details but don’t propagate—they are stuck to the field’s source. The scanning probe of a near-field microscope can directly access those evanescent waves (see July 2011, page 47 ). But it’s also possible to directly convert the standing waves to propagating ones by scattering them with a metalens—a cluster of strongly coupled subwavelength resonators—an approach introduced last year by ESPCI ParisTech researchers led by Geoffroy Lerosey. The Paris group has now demonstrated that broadband, audible sounds can be manipulated and focused onto spots as small as 1/25th of their wavelength λ. In a refreshingly low-tech experiment, various tones near 400 Hz from eight loudspeakers surrounding an array of empty soda cans were used to excite resonant modes with spatial variations as small as the diameter of a can. Using the principle of time reversal (see March 1997, page 34 ), the researchers then focused the modes by adding them coherently at a specific time and location and incoherently at other times and positions. Microphone-recorded maps of the acoustic pressure upon time reversal before and after cans are introduced demonstrate the subwavelength focusing. (Videos also show the dynamic acoustic intensity as the modes are focused.) The authors say the approach may lead to exotic new designs of smart sensors and actuators. (
