[William Sommerwerck]

"Arny Krueger" wrote in message
. ..
"William Sommerwerck" wrote in
message

Assuming they're identical and near each other, three
times the noise. That's about 5dB more.

True only if they are phase-locked. If they are AC powered,
there's a pretty good chance of that. If they are DC powered,
then they will be turning at similar but not identical speeds.

If the fans can be modelled as statistically independent acoustical
sources, then you add their power using the square root of the sum
of the squares.

I think you need to re-think this.

I was assuming random noise (primarily from air motion). In such cases, the
powers simply add.