noise level
background level

If that level is greater than the background level, it can be heard.

Levels of cesium-137 were found to be three to ten times higher than background levels.

It's so widespread that what we do can't be noticed against the background levels.

The testing found that radiation is not present above natural background levels.

The meter dropped back to background levels almost immediately, surprising her.

Once it was switched on, however, the signal dropped to background levels.

He found a very strong field on and around the stone, much greater than background levels.

The effects everywhere else would be way below the natural background level that exists anyway.

The issue needs to be addressed as to why those background levels are there in the first place.

The level drops back to background levels a few feet from the source.