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Summary form only given. Photonic crystal fibers (PCFs) are all-silica fibers in which the cladding consists of a two-dimensional array of microscopic air holes running along their length. The central hole of the structure is missing, leaving a solid silica core to guide the light. The periodic nature of the cladding makes also possible the existence of acoustic band gaps. Moreover, the core can also act as a defect of the phononic crystal and so elastic defect modes can be confined in this region. We present the first experimental demonstration of an acoustic defect mode trapped at the solid core in a square-lattice PCF preform.