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Just to confuse matters, a noise source can also be characterised by the acoustic power that it produces. This is expressed in decibels as a sound power level relative to 1 picowatt, which is a measure of the total sound radiated by the source in all directions and is independent of the measuring environment.

Machine and equipment manufacturers are often required to quote the sound power levels of their products, which have to be calculated according to internationally agreed standards from a large number of noise measurements in special test chambers. If the sound power level of a source is known, then the sound pressure level at any given distance can usually be calculated, provided information is known about its directional properties.

The use of the decibel to describe both sound pressure level and sound power level can and does cause confusion since they are both expressed in the same ‘units’ but do not mean the same thing. Fortunately, sound power levels are now often given in bels rather than decibels (which is one tenth of a bel) to avoid confusion. For example, if a hard disk manufacturer quotes a sound power level of say 3.1 bels for one of its drives, this would be the same as 31 decibels.

Unfortunately, equipment manufacturers don’t always provide a statement about the noise characteristics of their products. Even those that do, don’t always explain how the noise tests were carried out, or make it clear whether they are quoting sound power levels or sound pressure levels. This makes it practically impossible to use manufacturers’ noise data to compare products before buying.

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