Improving the noise reduction of picket barriers

A barrier diffraction theory incorporating the effect of partial sound transmission through a barrier of finite size is developed in this article. The insertion loss of picket barriers predicted by this theory on the basis of an optical model is shown to be significantly better at low frequencies than the measured insertion loss of real picket barriers. This is shown to be due to the actual acoustical boundary conditions at the barrier surface differing substantially from the classical boundary conditions assumed by the theoretical model. This article demonstrates two methods by which the classical boundary conditions can be satisfied and the insertion loss improved as a consequence. This improvement is in the frequency region where the sound transmitted through the gaps interferes destructively with the sound diffracted across the barrier top, resulting in enhanced noise reduction over that of an equivalent solid barrier. The performance of the picket barrier can be improved further by the introduction of sound absorbent into the gaps, yielding high-frequency performance approaching that of a solid barrier while retaining the better low-frequency performance of a picket barrier.