How does sound power level differ from sound pressure level?
Sound power is related to the amount of acoustical energy produced by a sound source. If an object is rated at some sound power level, it means that that is the amount of power it is capable of radiating. Sound power level is based on the specific object and does not take into account its surroundings.
On the other hand, sound pressure level does take into account the surroundings. A piece of machinery producing a sound power level of X dB will be much quieter 50 feet away than at 5 feet away. Hence, the measured sound pressure level will be lower at a distance of 50 feet than at a distance of 5 feet although the sound power level of that piece of machinery has not changed.