抄録
The time sequential variations of sound pressure levels and frequency characteristics of the traffic noise of different origin were measured on the sidewalks; regarding each traffic noise as a point source and applying equally-spaced vehicle model, we obtained theoretical formulas for variation of traffic noises as functions of time sequence, distance from lane and traffic volume. The results were as follows. 1) By frequency characteristics the traffic noise could be classified into three groups. The first group included the noises of autobicycles, motor scooters and tricycle trucks, whose octave band levels showed a peak value in 300-600 cps band(i. e. middle-frequency noise); the second group included the noises of automobiles, motor busses and trucks, whose octave band levels showed almost constant value up to 75-600 cps octave bands and slowly decreased as the frequency became higher; the third group represented the noise of streetcars whose octave band levels showed a slow peak in 300-600 cps band. 2) The noises radiated from a single car under running conditions could be regarded to have transmission characteristics of a point source. 3) The sound pressure level of tricycle trucks and passenger cars increased proportionally with the increase of car speed. The sound pressure level of autobicycle noise showed the same tendency up to 30km/hr, and increased slowly beyond 30km/hr. 4) We found theoretical formulas which express the variations of sound pressure level of street noise caused by traffic. It was ascertained that the observed data agreed well with the calculated values. By the increase of the distance from lane, the decrease of maximum sound pressure level (SPLmax) can be expected from the formula, but the decrease of mean, median, and minimum sound pressure level (SPLmean, SPLmed, and SPLmin) cannot be expected. Provided that the distance from observation point to lane is sufficiently long as compared with vehicle spacing, SPLmean increases by 3 dB when the traffic volume is doubled.
