Attenuation of noise by diffraction at a barrier edge

Abstract

Traffic noise attenuated by a barrier alongside a highway was measured with microphones positioned above an asphalt surface 15 and 30 m behind the barrier. The sound levels were analyzed to obtain the average sound pressure levels per ⅓‐octave band. An idealized experiment was also performed using a barrier 2.5 m high and 17 m long erected on a flat asphalt surface. Sound pressure levels from a point source were measured for various source and receiver positions. Certain features of these measurements were presented at the last meeting in Los Angeles where the effects of atmospheric turbulence were discussed. The same measurements will now be compared with two versions of classical Kirchhoff‐Fresnel diffraction theory which is currently used to predict the attenuation of noise by barriers. This comparison shows that the sound levels behind the barrier are higher than widely used diffraction theories by 1 to 3 dB at all frequencies—and, surprisingly, by 2 to 10 dB when compared with a theory that should be more precise. The largest discrepancies occur at the shorter source to receiver distances. An alternative diffraction theory as well as an empirical correction to Kirchhoff‐Fresnel theory are offered which show good agreement with all the results. [Work supported by NSERC grant to Carleton University.]