db(spl) = decibel sound PRESSURE level. It is sound output expressed purely as a pressure (force/area ). It is used primarily by car stereo competitions to see whose can generate the "loudest"(more pressure) sound in closed car interior. They are measuring sound PRESSURE. A sound man for a rock concert measures db(watt) = db(w) = db(swl), he wants more sound WATTAGE so the sound is still loud very far away.

Say they both generate an enormous 1000 acoustic watts = 150 db(w). In a car stereo this is a VERY violent and painful level of about 165 db(p). This same 1000 acoustic watts in a large stadium it is "diluted" to a nice but loud level of 100 db(p). So depending on the environment the various sound db levels change "BY THEMSELVES".

Sound can be a Pressure; db(p) or a Watt db(w) or a Velocity db(v). Conveniently, they are very closely equal and interchangeable below 120 db. Above 120 db it should be expressed as a pressure db(p) or a sound wattage db(w). 95% of all sound meters measure PRESSURE. Special Computer controlled sound meters or programs may give you a db(watt) level they can be found at: http://www.makeitlouder.com/Computer%20Software.html

For a detailed explanation visit and read near the 80% bottom of the page; http://www.makeitlouder.com/Calibration.html

For more read appendix in the middle 55% of this page; http://www.makeitlouder.com/Decibel%20Level%20Chart.txt