Sound Wave Diffraction at the Edge of a Sound Barrier

Abstract

The diffraction phenomenon is described by the Huygens-Fresnel principle. The review of physical laws ruling the bending of sound waves at the edge of the screen allows the effective selection of both acoustical and geometrical parameters of the screen. Sound wave diffraction theories have been developed on the basis of wave optics, when wavelength is small in comparison to the size of the obstacles, which can be also used in acoustics with the same assumptions about geometry of the system. Diffraction can be seen as a result of the interference of waves reaching the point of observation in accordance with the laws of geometrical optics and wave disturbances arising as a result of interaction with the edge of an obstacle. The paper describes a test method using maximum length sequences for determining the intrinsic characteristics of sound diffraction in situ during testing of roadside noise barriers. A scale model experiment has been performed in an anechoic room. Also, a real noise reducing device was tested in free field conditions.

Investigation of noise barriers enhancement efficiency for attenuation of low frequency traffic noise

“…One of most popular and effective noise barrier enhancement solutions is to reduce diffraction of sound wave at the top of noise barrier by setting absorptive octagonal or T-shape [31][32][33][34]. To simulate acoustic field transformation 0.8 m inner diameter octagonal and 1.5 x 0.1 m T-shape tops, with properties identical to noise barrier properties (4.3 section), were chosen.…”

Sound Diffraction Over Noise Barriers with Added Devices Installed on the Top Edge

“…where H dk is the impulse response for a wave diffracted over the top edge of the barrier, H i -the impulse response for a wave in a free field, d dk -distance between the loudspeaker and the observation point for the diffracted wave, and d i -distance between the loudspeaker and the observation point for the direct wave. The diffraction index differences ∆DI [15] have been determined for various shapes of the added devices installed on the top edge of the noise barriers using procedure described in [2,15]. In one series of measurements, the reference impulse responses have been determined for free field transmission and two arrangements of the loudspeaker-microphone system.…”

“…The performance of new elements, based on modified diffraction of the acoustic wave, has been the subject of experimental studies described in the present work. The paper is related to the author's previous work [2,3] in which determination of the diffraction modification evaluation index has been described for an octagonal added device.…”

“…The acoustic wave diffraction phenomena taking place on the top edge of the barrier are described e.g. in [2], where results of effectiveness studies are also shown for octagonal device mounted on the barrier edge. The determined diffraction index difference (∆DI = 0), has indicated that there was no improvement in the acoustic properties of the barrier when comparing the barrier with and without the added device.…”

“…In the discussion it has been shown that for the barriers with reflecting surface panels, the barrier effectiveness can be described using the propagation path differences, and in majority of environmental conditions, the best acoustic parameters are achieved with the 'T' profile. The distributions of acoustic field parameters in the area near the sound barrier can be studied by sound pressure [2,5] or sound intensity methods [6]. It is of particular importance when verification of numerical modeling is required.…”