Quenching of acoustic bandgaps by flow noise

Abstract

We report an experimental study of acoustic effects produced by wind impinging on noise barriers based on two-dimensional sonic crystals with square symmetry. We found that the attenuation strength of sonic-crystal bandgaps decreases for increasing values of flow speed. A quenching of the acoustic bandgap appears at a certain speed value that depends of the barrier filling ratio. For increasing values of flow speed, the data indicate that the barrier becomes a sound source because of its interaction with the wind. We conclude that flow noise should be taken into account in designing acoustic barriers based on sonic crystals. © 2009 American Institute of Physics. ͓DOI: 10.1063/1.3111797͔A sonic crystal ͑SC͒ is a periodic array of sound scatterers in air. SCs based on two-dimensional ͑2D͒ distributions of cylindrical scatterers have been mainly studied in view of their possible applications as acoustic barriers.1-9 Hexagonal and square configurations of cylinders are those widely studied, but other combinations made of them have been also analyzed.3,4 For the case in which the scatterers are made by solid materials, such as aluminum or concrete, the physical mechanism giving the transmission loss through the barrier is controlled by Bragg reflection between successive planes of scatterers in the barrier. The phenomenon of Bragg reflection creates frequency bands ͑bandgaps͒ at which the sound propagation is forbidden. Cylindrical scatterers with circular section are mainly studied although sections like square, hexagonal, and many others have been also considered looking for stronger and wider complete bandgaps. 8,9 More recently, additional mechanisms of sound reduction have been essayed in order to improve the barrier's performance in the range of frequencies not covered by the bandgaps. For example, the absorption mechanism associated to a porous covering of the scatterers would produce additional sound reduction in a wide range of frequencies.10 Another possibility is using periodic arrays of trees, which also produce a reduction in the transmitted sound by natural means.11 Sound attenuation by outdoor barriers based on 2D SCs have already been demonstrated at realistic size. 6 In this letter we report an experimental study showing that wind impinging on a noise barrier based on a 2D SC produces effects changing its attenuation performance. We analyze here the quenching of SC bandgaps by flow noise, a problem that has not been previously tackled, although, as it is shown below, it has a paramount importance. It is demonstrated that flow noise can completely deteriorate the barrier performance if the speed flow is large enough. We also demonstrate that for barriers too dense ͑i.e., when the distance between cylinders is too short͒ the barrier even becomes in a source of noise for large flow speeds.In order to quantify the attenuation performance of SC barriers under the effect of impinging wind, a series of measurements are developed in a wind tunnel consisting of a duct 6 m long that has been constructed in the...

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