Air return cover

I couldn't think of a better group to ask this question to so...
It's basically related to quieting air circulation in a room.
The room is a classroom in a school. It's an L-shaped, fairly large room that underwent a renovation years ago and so the air ducts are a bit screwy right now.
There is one air duct pulling in fresh air and two ducts pulling out the return air. For some reason the two return air ducts in this room are very noisy. In other classrooms you can't hear them at all.
I'm trying to find a way to silence or greatly reduce the noise level coming from the ducts for just one period per day (80 minutes). This would be a temporary fix (just for a few months). Going through the 'proper channels' is not an option. I have to do this one myself.
For the air return at the front of the class I took a piece of cardboard, cut it to shape and size and then just covered the duct completely, then stapled the bottom part to the wall. This worked well. It is now completely silent.
The tricky part with these ducts is that they are at least ten feet high on the wall, making it difficult to access.
I could use some advice on what to do with the *second duct*. If I covered it up completely like the first I'd need a way to access it quickly at the start of the period (covering it up then) and then removing the cover at the end of the period to let the air flow properly again until the next day at the same time.
I'm thinking about some mechanism whereby I can use a pole and reach up to open/close a cover. Would a porous cover be better than a non-porous one? How about some curtain material? Or a type of blinds? A pourous material would allow some air flow but would also be noisier. Whatever it is, I should be able to access it fairly readily at the start and end of the class.
Thanks in advance!
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you could try a magnet first, then old magnetic signs as used on a vehicle. but instead, maybe just get the school janitor to have a look and see if maybe a couple of sheet metal screws or duct tape in a loose duct might cure the actual vibration or whistling problem. maybe the other cold air returns are blocked or insufficient and the higher air speed is noticed there. you are locating the noise but follow thru and pinpoint it to let them fix it for best air circulation and heat and air conditioning. remember how many air changes are required by the building code in a school. find out which pta kid's parent is a handyman the parents will give you a helping hand on this one.
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buffalobill wrote:

Thanks BB for these tips!

The interesting thing here is that the school just had a couple of air duct workers in to work on all the air ducts in the school. When they were there I mentioned the problem to them and they said that depending on where the ducts are in the chain, the sound/noise will be louder/quieter, accordingly. Apparently my room is at the beginning of the duct chain. I was teaching in the room a couple of years ago (when the ducts were probably very clogged) and you couldn't hear any air flow in the room!
I talked to a couple of janitors and it was actually they who first gave me the idea of using cardboard. They just said to make sure I keep quiet about it.
Magnets wouldn't even be necessary as the pull from these return ducts is so strong. It just sucked up the cardboard in an instant.

There is no vibration or whistling sound at all. This is pure air suction.
Eric
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I don't know about mounting covers. But what kind of noise is it?
Is it the metal duct vibrating?
Is it the sound of the air whooshing in?
Is it the sound of the fan coming through the duct from the furnace?
Is it other sounds from the furnace room coming through the duct?
If it is the first, just putting a spring loaded curtain rod or something similar in the duct from one side to the other might stop it.
If it is the second, just removing the grill might stop it.
On 23 Feb 2006 17:37:07 -0800, snipped-for-privacy@yahoo.com wrote:

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mm wrote:

Thanks MM for your reply! It's the second. It's the sound of air whooshing. Whooshing out to be exact. I hadn't thought of removing the grill. I'll first try adjusting some of the slats and if that doesn't work I'll try removing the grill. Thanks again.
Eric
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The noisy ducts. Is the air going from the room to the duct, or from the duct into the room?
If the grille has slats (like the old trailer jalousie windows) maybe you can use a screw driver to bend a bunch of the slats. Make them straight out, instead of angled down.
Just a thought, might or might not work.
--

Christopher A. Young
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Stormin Mormon wrote:

The air is going from the room to the duct. Yes the grill has slats. I thought about adjusting the slats. I'll need to get a ladder for this. Thanks for the suggestion! I'll try it out next week.
Eric
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Eric,
It seems that you are blocking off the ac in that room. First, that is usually bad for the equipment. Second, it is usually bad for comfort. Third, it is usually bad for the ventillation required by code in a classroom.
That said, the easiest way I know of to eliminate register noise is to use a register with larger spacing between the fins. residential grilles usually have 1/3 inch between fins and tend to be noisy. you can get grilles with 1/2 or 2/3 inch space between fins. That will be quieter. Imagine how much noise you could make as a kid blowing over the top of a soda bottle, it gave a deep resonent noise. Try the same thing with a mayonaise bottle, no noise!
Take the grille off altogether. Or bend the fins straight out like Stormin Morman said. Or get a grille with wider fin spacing. All should work without affecting the performance of the ac in a negative manner.
Stretch
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Stretch wrote:

Thanks Stretch for your tips! I certainly don't want to block the air flow in the room as all would suffer. I'll start with bending the fins and see how that goes.
Thanks again! Eric
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