Excess attenuation by reactive obstacles at a noise barrier edge

Abstract

The excess attenuation by small obstacles with a reactive surface installed at the noise barrier edge was studied. So far, the absorbing obstacle has been used to obtain large attenuation, keeping the barrier height constant. It was found that an obstacle with a soft surface at the barrier edge can suppress noise more effectively than the absorbing one does. To realize this soft surface an arrangement of one-quarter wavelength acoustic wells was used. Although these acoustic wells behaved as the soft surface in a certain frequency range, at a barely lower frequency range the sound pressure level behind the barrier was amplified. To overcome this negative effect a new arrangement of wells tuned to several different resonance frequencies is studied. As a result, the excess attenuation by a small round obstacle of 53 cm width and 33 cm height installed at the edge of a 3-m barrier is about 4.7 dB (OA) at the ground 25 m behind the barrier. When the barrier has a T shape and is of 2 m width and 5 m height, the excess attenuation is 7 dB (OA). Details of the cross sections of these barriers will be presented.