

So a signal of -40 dBFS is quite audible, since it’s all relative to 0 dBFS (full scale).
At -40, it doesn’t make any difference what kind of dB you are talking about.
(Well, if you are looking for superstrings evidence or even quantum gravity, it does, but I guess it’s not your case.)


Relevant line:
That means the US definition you can’t hear anything below -24 dBFS. It’s the largest value there, it’s -18 for the EU.