Further to my previous post here, having just written a note about the different types of noise in hifi in this thread How Much Have You Invested In Cables
I think it worth copying here:
There are three basic categories of noise relevant when playing music on a hifi system.
- Environment noise - the background noise in the room. This will be the green line in your chart. Anything from the speakers at your listening position below that level will be inaudible.
- System noise. This is due typically to electronics and electromagnetic effects, from hiss to hum to superimposed radio pickup etc, and in this consideration that means noise when the system fully powered up, source connected but not playing, and volume control at the playing setting. System noise can be continuous all the time (blue line, not varying), or could vary between nothing above background, up to the peak level (red line), the average level being blue. BUT significantly for this discussion, any time the signal is above background (anywhere above green), it will be audible, whether constant around the blue level, fluctuating however slowly or rapidly (near steady is most common). However if the system noise is inaudible at all times then it is below the green line and cannot mask detail in the signal.
- Noise in the recording. Actually there are two factors here:
3a) Noise embedded in the recording itself, e.g. studio background noise picked up by microphones, electronic noise, tape hiss, etc.
3b) Noise added by the replay medium - and here vinyl is the obvious example: disk surface noise, scratches, rumble, etc.
if any of these exceed the background noise level in the room (green line) they will mask otherwise audible parts of the music, and will be audible directly during sections of music falling below the noise level