measuring tire / tyre noise

Hello I am new to Audacity and am attempting to measure and compare recordings of tyre noise inside a vehicle. We are currently taking our recordings into Audacity and trying to compare different before and after treatments in aid of reducing tyre noise. We are currently using a plot spectrum however find we always get a different -dB range meaning we have to re scale our charts and over lay them in graphics software.
Q1. Is there a way to control the dB range on plot so we always have a consistent parameter?
Q2. Can we adjust the dB value so our base is not always -90dB? Our superior would like to see dB range as something they can compare in the real world regardless of this simply being a delta

Possible there is another way of graphing our comparisons or another for of software which is better suited, any help would be appreciated

A SPAN is a very good free spectrogram-plugin which works* in Audacity : frequency & dB scales are adjustable …

[ * Only The 32-bit VST (not VST3) version works in Audacity, even if your computer is 64 bit ].

The “graph” in Plot Spectrum is really just intended to give a quick visual representation of the spectrum. For detailed “scientific” analysis, you can export the data to a text file, which can then be used in most spreadsheet / graphing applications. See: