Sound power and pressure level, what's the use?
The sound power level indicates the total acoustic energy that a machine, or piece of equipment, radiates to its environment. The sound pressure level is a measure for the effect of the energy of an acoustic source (or a collection of sources) and depends on the distance to the source(s) and acoustic properties of the surroundings of the source.
Given a well defined operation condition, the sound power level of a machine is a fixed value, were the sound pressure level always depends on position and environment.
In relation to the machine directive we speak about the "emission sound pressure level". This represents the sound pressure level an operator or bystander is exposed to, in a free field above a reflecting plane.