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Sound that is generated by the unsteady motion of a gas and its interaction with surrounding surfaces. Aerodynamic sound or noise may be pleasant, such as the sound generated by a flute, or unpleasant, such as the noise of an aircraft on landing or takeoff, or the impulsive noise of a helicopter in descending or forward flight.
Sources of aerodynamic sound may be classified according to their multipole order. Sources associated with unsteady mass addition to the gas are called monopoles. These could be caused by the unsteady mass flux in a jet exhaust or the pulsation of a body. The sound power radiated by monopoles scales with the fourth power of a characteristic source velocity. Sources related to unsteady forces acting on the gas are called dipoles. The singing in telephone wires is related to the nearly periodic lift variations caused by vortex shedding from the wires. Such sources, called dipoles, generate a sound power that scales with the sixth power of the characteristic source velocity. See Kármán vortex street
A turbulent fluid undergoes local compression and extension as well as shearing. These events are nearly random on the smallest scales of turbulent motion but may have more organization at the largest scales. The earliest theories of aerodynamic noise, called acoustic analogies, related these unsteady stresses in the fluid to the noise that they would generate in a uniform ambient medium with a constant speed of sound. Such sources are called quadrupoles. The sound power that they radiate scales with the eighth power of the characteristic source velocity. See Turbulent flow
Subsequent extensions of these theories allowed for the motion of these sources relative to a listener. This may result in a Doppler shift in frequency and a convective amplification of the sound if the sources are moving toward the listener. In addition, if the sources are embedded in a sheared flow, such as the exhaust plume of a jet engine, the sound is refracted away from the jet downstream axis. As sound propagates away from the source region, it experiences attenuation due to spherical spreading and real-gas and relaxational effects. The latter effects are usually important only for high-amplitude sound or sound propagation over large distances. See Atmospheric acoustics, Doppler effect, Sound absorption