sound volume - sound engineering

[jim sims]

Perhaps a sound engineering or physics expert might provide me with some insight into my perception of how players, playing sound, work - what the dynamics of sound are.

When I listen to one player playing a sound at 100% and then listen to two players playing that same sound but having the playLoudness of both set to 70%,  to my untrained ears that it seems to have the same volume as the one player set to 100%.

IOW - it seems (to my old compromised ears), that two players playing the same sound/signal at 70% seem as loud as one player set to 100%.

I cannot discern any difference until I get the two players into the 50% range.

It's as if there is  some sort of threshold, where the change in volume becomes obvious if two players are involved. When I play one player and change the playLoudness I can near slighter changes in playLoudness percentage.

I'm having a difficult time explaining this... 

Is there some known effect that matches up with what I'm trying to explain?

Is there some non-linear multiplier effect? Some formula for this?

sims