Outside sounds are, of course, carried in the direction of the wind, and they may be blocked by buildings, but urban sounds generally radiate in all directions. The time distribution of sound sources are categorized as steady-state, fluctuating, or impulsive. Steady-state noises, such as air conditioners, are from fixed locations and maintain a constant magnitude over a period of hours. Fluctuating sounds include motor traffic and airplane landings or takeoffs. Impulsive noises come from sources such as jackhammers, explosions, backfiring autos, or sonic booms.
Impulsive noises are more annoying than fluctuating noises because of their unexpectedness.
Quality of noise varies with the operating conditions of its mechanical source, if it has one. A car engine that is racing is louder than one that is idling, and aircraft noise varies with the thrust, jet velocity, fan or compressor speeds, aircraft position, and speed. Copyright 1999-2000 Encyclop dia Britannica, Inc.