Survey of Motor Vehicle Noise

Transportation noise, and motor‐vehicle noise in particular, account for the steady or slowly varying ambient noise level, particularly in urban areas. Being so numerous, motor vehicles can be treated statistically, and this establishes consistent noise emission characteristics. Vehicles are categorized as: passenger cars [including light delivery trucks up to 6000 lb gross vehicle weight (GVW)], light, medium, and heavy trucks with GVW ranges of 6000–15 000, 15 000–30 000, and over 30 000 lb, respectively, tractor trailers, buses, cement‐mixer trucks, and motorcycles. Speed ranges are 20–29, 30–39, 40–49, 50–59, and 60–69 mph. The sound level of the average vehicle of a given category increases consistently with speed and weight. The average vehicle is defined as the hypothetical vehicle having a sound level (A, B, or C weighting) and band‐pressure levels equal to the weighted‐mean decibels in the statistical distributions. The rate of increase with speed is such that in going from 30–39 mph to 60–69 mph, the sound level (A weighting) increases 8.5 dB for passenger cars, 9 dB for trucks and buses, 7 dB for tractor trailers, and 12 dB for motorcycles. For an estimated doubling of weight (motorcycles not included), sound levels increase 3.5 dB. For motorcycles, maximum noise occurs for full throttle setting, independent of road speed, gear, or engine loading. The octave‐band spectrum of the average vehicle has a shape which is characteristic of each category and shows progressive change in level and shape with increasing speed. The octave‐band spectra of four motorcycles, as examples, indicate dependence of level and shape on such parameters as type and size of engine, muffler configuration, and throttle setting. The sound level for acceleration is equivalent to about 40–49 mph cruising speed for tractor trailers and heavy trucks, and 30–39 mph for cement‐mixers, but the octave band spectra for the two modes of operation exhibit consistent differences. Sound levels recorded continuously for 24‐h periods at five locations, and analyzed for percentile distributions in each 1‐h interval, have a diurnal cycle that follows a consistent pattern in response to motor‐vehicle traffic.