You can measure noise floor (room tone) in Audacity by recording your environment without extrinsic noises. When recording noise floor make sure there are no unusual background noises and keep your audio recording levels at the usual settings.
Here’s how you measure your noise floor in Audacity step by step:
- Record in a quiet room for ~5s
- Play the recording back
- Observe playback level
- The highest point on the playback level is your noise floor
I am using Audacity in this example, but any DAW will be able to measure your noise floor and the same principles apply.
Knowing your noise floor will be important when applying such effects as noise gate or noise reduction.