Transportation noise
Transportation noise is the main source of environmental pollution, including; road traffic, railway traffic, air traffic, sea traffic. As a general rule, larger and heavier vehicles emit more noise than smaller and lighter vehicles. Exceptions would include: helicopters and 2/3 wheeled motorized road vehicles.
Road traffic noise
The noise of the road vehicles is mainly generated from the engine and from frictional contact between the vehicle and the ground and air In general, road contact noise exceeds engine noise at speeds higher than 60 km/h. The physical principle responsible for generating noise from tire-road contact is less well understood.
The sound pressure level from traffic can be predicted from the traffic flow rate, the speed of the vehicles, the proportion of heavy vehicles, and the nature of the road surface. Special problems can arise where the traffic movements involve a change in engine speed and power, such as traffic lights, hills, and intersecting roads; or where topography, meteorological conditions and low background levels are unfavorable (for example mountain areas).
Railway noise
Railway noise depend primary on the speed of the train, but variations are present depending upon the type of engine, wagons, and rails and their foundations as well as the roughness of wheels and rails. Small radius curves in the track, such as may occur for urban trains, can lead to very high levels of high-frequency sound referred to as wheal squeal. Noise can be generated in stations because of running engines, whistles and loudspeakers and in marshalling yards because of shunting operations.
The introduction of high-speed trains has created special noise problems with sudden, but not impulsive rises in noise. At speeds greater than 250 km/h, the proportion of high frequency sound energy increases and the sound can be perceived as similar to that of jet aircraft. Special problems can arise in areas close to tunnels, in valleys or in areas where the ground conditions help generate vibrations. The long distance propagation of noise from high speed trains will constitute a problem in the future if otherwise environment-friendly railways are expanded.
Aircraft noise
Aircraft operation generates substantial noise in the vicinity of both commercial and military airports. Aircraft takeoffs are known to produce intense noise, including vibration and rattle. The landings produce substantial noise in long low-altitude flight corridors. The noise is produced by the landing gear and automatic power regulation, and also when reverse thrust is applied, all for safety reasons. In general larger and heavier aircraft produce more noise than lighter aircraft.
The main mechanism of noise generation in the early turbojet-powered aircraft was the turbulence created by the jet exhaust mixing with the surrounding air. The noise source has been significantly reduced in modern high by-pass ratio turbo-fan engines that surround the high velocity jet exhaust with lower velocity airflow generated by the fan. The fan itself can be a significant noise source, particularly during landing and taxiing operations.
The sound pressure level from aircraft is typically predicted from the number of aircraft, the types of airplanes, their flight paths, the proportions of takeoffs and landings and the atmospheric conditions. Severe noise problems may arise at airports hosting many helicopters or smaller aircraft used for private business, flying training and leisure purposes. Special noise problems may also arise inside airplanes because of vibration. The noise emission from future super jets is unknown.
Noise from military airfields may present particular problems compared to civil airports, for example, when used for night time flying, for training interrupted landings and takeoffs (so called touch and go), or for low altitude flying. In certain instances, including wars, specific military activities introduce other intense noise pollution from heavy vehicles, helicopters and small and large fire-alarms.
Sea traffic noise
Even it does not widely take place in literature; the noise from sea traffic is not negligible. Ships, ferryboats, jet skis, sea motors cause different types of noise most of all emitting high noise levels.
