2014/11/30 by A A Maznev, A. A. Maznev, Gen Gu +5
Engineering · Materials Science · Physics and Astronomy · #Acoustic Wave Phenomena Research #Acoustic resonance #Diffraction #Evanescent wave #Helmholtz free energy #Metamaterial #Metamaterials and Metasurfaces Applications #Near and far field #Resonance (particle physics) #Ultrasonics and Acoustic Wave Propagation #Wavelength #physics.class-ph
paper · pdf · doi:10.1088/1367-2630/17/4/042001
published as New J. Phys. 17, 042001 (2015)
openalex publication_date 2015/04/02 · arxiv created 2015/04/18 · arxiv updated 2015/04/21 · openalex created_date 2016/06/24 · openalex updated_date 2026/08/06
Recently, Lemoult et al (2011 Phys. Rev. Lett. 107 064301 ) used time reversal to focus sound above an array of soda cans into a spot much smaller than the acoustic wavelength in air. In this study, we show that equally sharp focusing can be achieved without time reversal, by arranging transducers around a nearly circular array of soda cans. The size of the focal spot at the center of the array is made progressively smaller as the frequency approaches the Helmholtz resonance frequency of a can from below, and, near the resonance, becomes smaller than the size of a single can. We show that the locally resonant metamaterial formed by soda cans supports a guided wave at frequencies below the Helmholtz resonance frequency. The small focal spot results from a small wavelength of this guided wave near the resonance in combination with a near field effect making the acoustic field concentrate at the opening of a can. The focusing is achieved with propagating rather than evanescent waves. No sub-diffraction-limited focusing is observed if the diffraction limit is defined with respect to the wavelength of the guided mode in the metamaterial medium rather than the wavelength of the bulk wave in air.