Installation Methods Reduce Fan Noise
To get good indoor ventilation, the typical 80 cfm fan should run at least eight hours a day. Research indicates that occupants seldom allow this much operation time because of fan noise. The best way to reduce fan noise is to mount the fan away from the living space in an attic or crawlspace. One or more ducts draw stale air from the house. These remote fans tend to be more expensive than the typical surface mounted fan, so they haven't been widely used.
Given a surface-mounted fan, what can be done to reduce the noise? Arthur Noxon, PE, an acoustic engineer in Eugene, Oregon, put some thought into the problem. He offers these thoughts about fan noise and several potential solutions.
A fan generates several types of noise. Rushing air hisses. The fan motor hums. If you take the fan out of its box and plug it in, it wouldn't seem loud. After installing the fan, the difference is striking. It's very similar to a guitar string.
"If you hold a guitar string up in the air and pluck it, you won't hear anything," says Art. "But attach the string to a resonating box, and now you have sound."
That's what happens with surface mounted fans--a vibrating object is attached to a resonating surface. The motor operating at 60Hz experiences 120 pulses each second (two for each cycle). The force of each pulse pushes the blower motor forward. That pulse also pushes the fan housing backward. The housing pulses against the wood framing and drywall, causing a low frequency tone that permeates the building and annoys everyone.
"To reduce the tone, you have to isolate the vibration where it connects to the building," says Art. Three connections to the fan need to be isolated: the framing, the drywall and the duct.
After pondering the problem, Art came up with ideas for each of these connections. The drawings on these two pages suggest ways to install surface-mounted fans to reduce noise.
Your first thought might be to mount the fan through some kind of rubber material? That would be easy, and somewhat effective. However, rubber bushings, grommets and other types of gaskets have two problems. First, the installer is likely to over-tighten the fastener to get solid mounting. Second, the weight of the fan would be pressing down on the "gasket" placing it in compression. Held tightly between the surfaces, the gasket will pass along most of the vibration to the framing.
Art suggests resilient channel (RC) that's normally used to hang drywall. It's strong and easy to find. Short strips of RC should suspend the fan housing so that it rocks slightly. The channel should be installed so that it runs parallel to the shaft of the blower motor (see drawings).
The 120 pulse per second vibration of a fan motor creates a sound wave about 9 ft. long. By placing the center of the fan 2 ft. 4 in. from two adjacent walls, the reflected sound wave will partially cancel itself. Better yet, mount the fan on the wall 2 ft. 4 in. from the ceiling and 2 ft. 4 in. from the other wall. Moving the fan off the ceiling makes it a less effective sound generator. If it's an inside wall you wouldn't have to worry about air sealing between the housing and the drywall.
Mass for the Box
Any freely vibrating surface generates sound. Adding mass to the fan housing will reduce the sheet metal vibration. A relatively easy way to do this is to attach small scraps of drywall to the flat surfaces on the outside of the housing. Simply apply a good bead of construction adhesive to the housing and push the drywall into it. A screw in each corner will make a tighter connection, but probably isn't necessary. If you use screws, be sure they don't damage parts inside the housing.
Fans with Hanger Bars
First, note the direction that the fan rotates. Attach the hanger bars to blocks, then attach the blocks to the framing with resilient channel (RC). Here's a possible construction sequence: Cut a block several inches longer than the distance between the hanger bars. Attach a short piece of RC to the end of the block. Position the block back from the angle, where the RC is more flexible. Make a block for the other side. Attach the hanger bars to the blocks. Place a scrap under the block when hammering so you don't mash the RC. Lift the fan with the blocks attached into position. Nail the RC to the joist. (You may need a helper or a power nailer for this last step.)
Fans with Brackets
If necessary, move the brackets so they run the same direction as the shaft of the blower motor. Fans with brackets often allow attachment on any of the four sides. Attach RC to the joist or blocking that will support the fan. Adjust the brackets, so the fan will be flush with the ceiling drywall, accounting for the thickness of the resilient channel. Set the fan in place and drive screws through the RC. The screws must touch only the RC not the wood.
Rigid duct reduces resistance to air flow, making it much better for fan performance. But rigid duct also creates a bridge that allows vibrations to move from the fan to the framing. You can use a short section of flexible, plastic duct to isolate the fan housing from the rigid duct. Be sure to check with local codes for acceptable duct materials. Vinyl dryer hose isn't a good choice, because it doesn't meet requirements for fire resistance. Heating contractors have access to insulated, plastic flex duct in smaller diameters (down to 4 in.) with proper fire ratings.
Drywall is like a drum head that amplifies small vibrations. To isolate the fan housing from the drywall, cut the drywall about 1Ú4 in. bigger than the housing. In some cases, you can just cover the gap with the grille. If the fan is in an outside ceiling, it must be sealed against air leakage. Don't caulk between the housing and drywall. Caulk is too stiff, so much of the vibration will pass through. Find a material that is soft but not porous. One possibility is polyethylene backer rod (see the October 1992 issue). It's sold by masonry materials suppliers for filling large cracks before applying sealant. Closed cell backer rod provides a good air seal by itself.
This article appeared in Energy Source Builder #29 October 1993