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SPEEDRE
03-14-2011, 01:54 PM
Please, someone answer this, maybe obvious, question. What makes, a shop vac noisy? Is it the speed, or the size of the impeller?. I'm talking about the standard everyday shop vac one can purchase at any box store. I'd really like an answer this time.

Geof
03-14-2011, 01:58 PM
The air flow past the fan combined with the speed. The noise is mostly due to turbulence in the air stream caused by the fan blades and sharp corners and direction changes in the air flow.

Al_The_Man
03-14-2011, 02:11 PM
As Geoff said, shop vac et al are Universal series motors and they operate in a run away condition, this is the reason for the high rpm, the only thing restricting the top rpm is friction and windage.
You may notice it speeds up if you block the hose, (take the load off).
A series motor run on DC tends to be quieter, incidentally this is the type of motor you car starter is, so take care if testing it on the bench with no load.
Al.

SPEEDRE
03-15-2011, 10:57 AM
Thanks, I guess that the motor is the culprit. If it runs at runaway speeds, wow, I never would have guessed that. O.K. on to make it a little quieter.