23 Sep 10 14:30
The short answer to your question is Yes.
You need to make/determine a few parameters.
1) Distance to the sound pressure level point, (usually 3 feet or 1 metre).
2) Is the measurement point indoors or outdoors. Most contracts just assume outdoor conditions or "free field conditions". i.e. Q=2.
If the source is indoors and your guaranteed pressure level is indoors, then you will need complete room acoustics information which is covered in a typical acoustics book.
3) Or you can do a very simple estimate/conversion by subtracting 12 dB from the sound power level to get a ballpark sound pressure level (free field assumed).
4) One last thing to note is that the conversion is frequency dependant so the best conversion is done with octave or third octave band levels. This is not very significant if the distance is short such as 3 feet but can be very significant if the distance starts to approach two or three hundred feet or more.
I don't know the specifics of your contract but if your guarantee is 90 dBA power level and the equipment supplier has guaranteed 90 dBA pressure level, you have a much tighter limit to meet. (90 dBA power level in free field is about 80 dBA pressure level).