That's messy. The dB numbers in Audacity represent absolute audio signal numbers where they "run out" at 0. All sound signals are lower than that and measured in negative numbers. -20dB is the volume of the test tone at the beginning of broadcast television video tapes.
There is no connection between those numbers and the loudness of the sound at the TV station in the control room. That sound goes up and down with the volume knobs on the console. So that's the basic disconnect between the signals and physical events.
You are trying to measure dBSPL. dB Sound Pressure Level. That's the one you use to measure actual physical objects. Jets taking off, printing press pounding, casual conversation, rock music, etc.
You can cross calibrate an existing microphone by exposing the microphone to specific sounds or noises and compare Audacity to a calibrated SPL meter like the Radio Shack Sound Meter.http://www.radioshack.com/product/index ... d=12680845
The meter is perfectly accurate but doesn't record anything. Audacity will record for hours, but isn't accurate without a lot of work.
The catch when you cross calibrate Audacity and a microphone with the Radio Shack meter is you can't change anything after that. You can't use the computer for anything else and it's best if you never turned any of it off.
So that's the problem.