If you have an SPL meter it’s easy with pure tones and you can use peak or RMS. You just subtract. If 80dB SPL reads -6dBFS, then 75dB SPL will read -11dBFS. An xdB change is an xdB change in everything!
But with real-word sounds it gets complicated. Real world sound changes moment-to-moment and SPL is usually A-Weighted to account for the fact that our ears are most-sensitive to mid-frequencies, so mid frequencies sound louder (and measure louder when A-weighted) that low & high frequencies. SPL meters also have short-term averaging.
A-weighting is not “perfect” but it’s the standard for industrial noise measurements. Some SPL meters have selectable weighting.
For digital files there LUFS which also takes frequency into account. Audacity doesn’t have LUFS measurement built-in, but it it’s an option for Loudness Normalization so you can adjust your file to an LUFS target level.
Try it! It’s giving you a ratio or factor, not an absolute value.
A negative dB value will give you less than 1, and a positive value greater than 1.
-6dB gives you (about) 0.5. +6dB gives you 2. Both are factor of 2. If 1 Volt is 0dB then +6dB is 2V. If 0dB is 1mV, +6dB is 2mV.