powered roof vent noise

[rieferman]

Hello,
I installed a powered roof vent per the manufacturer's instructions in the roof of my barn. Ceiling in the space is finished with sheetrock in a vaulted fashion up until the collar ties where the ceiling is flat (tray) creating a mini-attic cavity near the peak. Baffles installed under the insulation allow air to travel from the vented soffits on both eaves to that cavity. The roof vent is then installed centrally close to the ridge to draw the hot or moist air from that cavity (the roof vent has a humidistat as well as thermostat).

The good news is that it's working. When it's hot or humid, the fan kicks on, runs for awhile and then kicks off. Repeat as conditions require.

The bad news is that is sounds like a helicopter is flying far overhead. It's not so annoying as to drive me absolutely crazy, but it's noticeable for sure. I can hear this through the insulation and the glued/screwed sheetrock. The vent is at 12 feet above where I'm listening from.

My hypothesis is as follows: The barn roof rafters are 24" apart with 1/2" plywood over ancient lathe strips. The 14" fan cut out is centered in the 24" space, and I think it's basically turning the plywood/lathe into a drum. That drumming is what I hear (and sounds like a helicopter).

My proposed solution: From the underside (through the hatch I thankfully had the sense to leave myself) stiffen the area supporting the fan so that the vibrations is absorbed. I was thinking something like a 3/4" layer of rigid foam against the roof deck (glued) followed by a 3/4" layer of plywood (also glued). Block those both in place with 2x4 framing between the existing rafters.

What do you all think?

Thanks in advance!

 

[tylernt]

Vibration can be dealt with in two ways: isolation and damping.

Isolating lets the vibrating thing go ahead and vibrate without transferring to attached structure. The motor mounts on your car's engine, for example.

Damping simply increases the mass, which lowers the resonant frequency of a structure. The goal is to lower the resonance to below whatever is creating the vibrations.

The foam should be quite good for blocking direct acoustic noise, but I'm not optimistic about how much of a vibration-isolating effect it will have. Usually I think of rubber for isolation, but anything flexible will in theory work (to some degree). I've also seen things like compressors mounted on steel springs, for example. You may be able to find industrial isolation mounts used in factories etc for machinery to mount your fan to.

As for mass, throw as much wood at it as you can afford / fit in the space. I'd start with framing it in and keep adding more wood until things settle down.

You could also deal with the nosie/vibration at the source... replace your direct-drive fan with a belt-drive. Belt-drive fans can turn at a slower RPM to move the same amount of air, so they put you way ahead of the noise/vibration game.