Originally Posted by JohnFerrier
I can follow your numbers. You will more easily hear the louder signal. Noise is not easier to hear when competing with another signal. If you wish to believe so, then that is fine. What you write makes sense otherwise.
Noise is easier to hear when signal is weaker. S/N ratio is poorer when the signal is weak. Because amplifier self-noise is constant in voltage, not affected by the incoming signal. It maybe -107 dB relative to the loudest full signal on CD, but only -47 dB to a softer passage in the CD.
But a noise inaudible by itself can be made audible by a (weak) signal. This is the beauty of our sensitve hearing. No, you don't really sense this small (inaudible when alone) noise as a separate entity. It is just that the distortion to the signal is audible (if you remove the noise the music sounds better), and hence we say the noise is audible.
Vice versa, a signal inaudible by itself can be made audible by adding noise. But this is unimportant in hi-fi, and only relevant if you try to receive/intercept minute signals. This is well proven in all kinds of sensory perception studies, even in fish. When you hear something interesting on TV, the signal is already audible. Hence, you don't need your friend to make noise. If they make noise the S/N will decrease, and you will be pissed. I can give you a real world example of noise making inaudible signal audible. Ever used an analog radio with tuning dials? There are some very distant stations whose signals are too weak to be audible. You tune into that frequency and you hear lots of noise but there seems to be a faint voice speaking underneath that noise, like eavesdropping on Martians. If you remove all that noise in the radio you may find that the signal is too weak to be heard (keeping the volume knob in the same position).