SOUND-ATTENUATION BY A 2-DIMENSIONAL ARRAY OF RIGID CYLINDERS

In this Letter we present an experimental analysis of the acoustic tra nsmission of a two-dimensional periodic array of rigid cylinders in ai r with two different geometrical configurations: square and triangular . In both configurations, and above a certain filling fraction, we obs erve an overlap, in the range of the audible frequencies, between the attenuation peaks measured along the two high-symmetry directions of t he Brillouin zone. This effect is considered as the fingerprint of the existence of a full acoustic gap. Nevertheless, the comparison with o ur calculation of band structures shows that the triangular lattice ha s band states in that frequency range. We calf them deaf bands. This c ontradictory result is explained by looking at the symmetry of the dea f bands; they cannot be excited by experiments of sound transmission.