The difference between sound power and sound pressure levels is similar to that between the power of a heater and the temperature that it produces. If you stand close to a heater in a large room you will be able to feel heat from it; if you are at the far end of the room you will probably not feel anything. The temperature that the heater produces depends upon how far you are from it and the characteristics of where it is located. The heater will produce a higher temperature in a well insulated room than in a draughty, uninsulated one.
Sound power level is the acoustic energy emitted by a source which produces a sound pressure level at some distance. While the sound power level of a source is fixed, the sound pressure level depends upon the distance from the source and the acoustic characteristics of the area in which it is located.
A sound source will produce a higher sound pressure level in a small reflective (reverberant) room than in a large acoustically dead (absorptive) room or an open space. The units in which the power of a heater is measured may be Watts while the temperature may be measured in degrees(e.g. Centigrade). Similarly, the energy emitted by a sound source is in Watts and the sound pressure this produces can be measured in Pascals.
For acoustic purposes, sound power and sound pressure levels are quantified in decibels which are a logarithmic ratio. In the case of a sound power level the reference value is 1012 Watts whereas the reference value for sound pressure level is 2.10-5 pascals. This means that although the sound power and sound pressure levels equate to Watts and Pascals, respectively, they are both measured in decibels (relative to the appropriate reference unit). This tends to cause considerable confusion because a sound source producing a sound power level of 100dB, for example, will produce a sound pressure level that varies, as noted above, with distance and other factors such that it may be 90 dB relatively close to the source and 80 dB at a somewhat greater distance.
It is best to quantify a source in terms of the sound power level emitted because this does not change, whereas the resultant sound pressure level will depend upon how and where it is measured, which means that quantifying a source by the sound pressure level produced may leave considerable uncertainty in the actual sound energy of the source. Sound is measured in decibels (dB [capital B]) where 0dB is around the threshold of hearing for somebody with good hearing and every 10dBA increase equates to an approximate doubling of loudness. This means that a level of 40dB sounds about twice as loud as 30dB whereas as 50dB sounds approximately four times as loud.