The pressure fluctuation (rms) of wall pressure is related directly to the area enlargement, where values of d/D greater than 0.3 (d/D > 0.3) tend to produce more severe pressure fluctuations within the fluid and resulting external noise, especially close by the disturbance. As a rule of thumb, within four pipe diameters of an area change, the sound level will be dominated by the turbulence produced by the area change. After just four pipe diameters, sound will be dominated by planar sound waves in most cases. Area expansions tend to cause more internal noise than area contractions.