Estimation of attenuation of A-weighted traffic noise level due to air absorption

Abstract

Current versions of the U.S. DOT Federal Highway Administration (FHWA) traffic noise model assume 500 Hz as a representative sound frequency and apply an air absorption attenuation rate of 0.54 dB per 1000 ft for the distance associated with the maximum noise level. According to the American National Standards Institute, the air attenuation rate at 500 Hz is approximately 1 dB per 1000 ft for a wide temperature and relative humidity range. However, the rate of decrease of the composite A-weighted traffic noise level depends on the spectral distribution of car and truck noise, and varies with distance. A more representative air attenuation for traffic noise is obtained by including this effect during integration as source distance varies to obtain the attenuated sound exposure level (SEL) since the traffic noise level is determined from single event SEL and number of events. In many cases, an attenuation due to air absorption of 0.5 to 3.0 dB may be warranted, as compared to less than 0.5 dB assigned by the traffic noise model. These considerations are appropriate only for the ‘‘hard site’’ case since the ‘‘soft site’’ attenuation factor assumed by the model includes extra attenuation due to shielding and air absorption. Air absorption and roadway truncation by terrain shielding may account for most of the traffic noise level attenuation now attributed to ‘‘ground absorption’’ in some cases.