Acoustic focusing of sub-wavelength scale achieved by multiple Fabry-Perot resonance effect

It is demonstrated that a novel design of a multi-branch structure can converge plane acoustic waves to a focal region of sub-wavelength scale. The structure consists of multiple sub-wavelength apertures; each has length equal to the integer times that of the shortest aperture. The periodicity length of the inlet array is selected equal to the length of the structure, inducing a cooperation between the surface resonances and the Fabry-Perot (FP) resonances in apertures. The design allows FP resonances to exhibit in all apertures simultaneously, hence a multiple FP resonance is achieved. The generated evanescent waves are amplified effectively, and the acoustic focusing of sub-wavelength scale is realized within a wide frequency range, with an approximate maximum transmission gain of 7 dB. (C) 2014 AIP Publishing LLC.