There are lots of devices that purport to measure the absolute value of sound pressure levels. Here is an example, here's another, there's also this iPhone App. Putting such devices side-by-side in a sound-filled environment usually yields a discrepancy between the decibel readings of each device. This inevitably leads to the question "Which reading is correct?"
If we were similarly measuring temperature with a collection of thermometers, there are a variety of naturally-occurring physical references against which to verify or calibrate the thermometers. For example, you could use the triple point of water or the critical point of another substance.
So, for thermometers, we have a naturally occurring phenomenon with a known temperature to which we can compare our thermometer's reading.
Is there a naturally occurring phenomenon with a known sound pressure level to which we can compare a sound pressure level meter's reading?