Experimental evidence of band gaps in periodic structures

The appearance of band gaps in periodic media is a well known phenomenon. In the past 15 years an increasing attention was payed to sonic crystals, as the properties of such arrangements could lead to the design of innovative noise barriers. In fact, sonic crystal barriers exhibit good insulation properties at some frequencies and could be tuned in order to cause stop bands in a frequency range centred at 1 kHz, classic for road traffic noise pollution. In this paper measurements are described that were conducted over an array of cylinders arranged in a square lattice, whose Bragg band gap was centred at about 900 Hz. Measurements were conducted in accordance with EN 1793-6. Together with measurements, FE simulations were carried out in order to address and verify the procedure. In the simulations, cylinders were assumed to be infinitely rigid and the system was investigated in a 2d domain. The first hypothesis is consistent with the high contrast between the acoustic properties of air and PVC. The second one is consistent with the measurement method, as the windowing of the IRs leads to an evaluation of the attenuation due only to the BG without comprising any effect from ground reflections. Simulation and measurements are compared and discussed over a variety of cases.