Enhancement of sound in chirped sonic crystals

We propose and experimentally demonstrate a mechanism of sound wave concentration based on soft reflections in chirped sonic crystals. The reported controlled field enhancement occurs at around particular (bright) planes in the crystal and is related to a progressive slowing down of the sound wave as it propagates along the material. At these bright planes, a substantial concentration of the energy (with a local increase up to 20 times) was obtained for a linear chirp and for frequencies around the first band gap. A simple couple mode theory is proposed that interprets and estimates the observed effects. Wave concentration energy can be applied to increase the efficiency of detectors and absorbers.The work was supported by Spanish Ministry of Science and Innovation and European Union FEDER through projects FIS2011-29734-C02-01 and -02 and GVA/2011/055. V. R.-G. is grateful for the support of post-doctoral contracts of the UPV CEI-01-11. K. S. acknowledges the grant of UPV PAID-02-01. 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