Air turbulence

[extracted from L108 "Controlling noise at work" p 57]

When any rotating part such as a fan blade or a woodworking cutter block passes close to a stationary part of the machine, noise is produced. If the distance between the rotating part and the stationary part is increased, the noise level will be reduced. Also if cutter blocks are fitted which have helical blades, the smooth transition of the curved cutting edge next to the stationary table instead of the abrupt impact of a normal blade will reduce the noise considerably.

CAUTION: Gaps between stationary and rotating parts of machinery are dangerous. You should not alter gaps without ensuring that the machinery can be used without risks to safety.

When air flows past an object or over sharp edges, turbulence is caused which produces noise. Also when air flows over cavities or voids a noise tone can be produced (similar to blowing over a milk bottle). Making edges as smooth as possible and removing voids or rounding the edges can reduce the level of noise created. Similarly, air flowing smoothly through ducts and pipes will produce less noise.