Quote Originally Posted by JDGJr View Post
I hooked my new vac box to my table last weekend and ran it briefly to make sure it worked, and thought I liked the sound level.

Today, I finally got around to cutting with the vacuum on, and it has a very annoying variable vibration, with a frequency around .7-.9 hertz. I can feel the vibration when touching the box, and the pulsing sound gets annoying fast. This is with 2 of the 4 motors running.

Has anyone had this, and corrected it? I may disconnect all but 1 motor and see if that makes a difference. Any other ideas?

thanks,
John
It sounds like harmonic beats. This is what happens when you have tones that are very slightly out of tune. If you run two vacs at the same time, this will frequently happen. When tuning a musical instrument, you listen to the beats and adjust your tuning to make them go away. You are getting these beats because of minute differences in the two systems. It could be a slightly different shape to one vac impeller or one zone is not sealed quite the same as the other. If you wish to learn more about this, you can look up constructive and destructive interference.