How fix vertical scale on Spectrum plot for different sounds


I am looking at several sound via the spectrum graph.

The vertical axis scales changes between sounds, is it possible to fix the scale?
Is there a way to display the sound level on a positive dB scale instead of negative scale.

Any suggestion on how to calibrate the system in terms of sound level with dB scale. I was able to create frequency spectrum with known several input frequency and the plots look as expected(general display, not the amplitude).

Windows 7
Audacity 2.1.0
USB microphone Samson Meteorite -

Unfortunately it is not currently possible to do that. The next version of Audacity allows the scale to be adjusted, but not fixed. A future version may allow the scale to be fixed.

The dB scale is a “relative” scale. That is, the number of dB is relative to some defined amount. When measuring signals, the reference point that is almost always used is “full-scale”. That is, a signal that has an amplitude that touches the top or bottom of the track is said to be “0 dB”. Thus normal signals are almost always negative numbers.

That is different from the case of measuring “sounds” (“sound pressure level” / SPL). For SPL measurements the 0 dB reference point is usually taken to be “the threshold of hearing” (the threshold between sounds that are too quiet to be heard and sounds that are just audible). Thus SPL measurements are almost always positive numbers.

The important thing with dB measurements is that it is a “ratio”, not an “absolute measure”. Thus signals that are measured against the same reference point may be directly compared with each other - if one signal is double the amplitude of another, then it is +6 dB greater than the other (the “other” being 6 dB lower than the first).

“Plot Spectrum” is already calibrated.
The calibration is such that a 0 dB (full-scale) sine wave produces a peak level in Plot Spectrum of 0 dB (though it may not display as exactly 0 dB due to limitations in the way that it is measured and displayed).