Dual A/C Noise insulation options, driving me insane

releasedtruthAugust 4, 2011

This is a tough one. We have (2) 12-year old A/C units (3.5/3ton) side by side and are bound tightly on three sides by (2) cinder block walls (1) side of the house. That wall of the house is the wall of our guest room. With even one unit running, the wall vibrates and the volume is unbearable. I understand the age of the units doesn't help, but I'd like to know if this idea would be helpful. House is stucco over frame, 2x4 construction, drywall over batted insulation.

The plan:

- Remove baseboards on interior east wall (shared with A/C units)

- Green glue the wall

- Attach Suppress or Quietrock 5/8" drywall

- Refinish wall

I've been learning about STC values, but am not versed enough to know how much reduction we'd see. This site under section 5 appears to show little reduction with additional standard drywall. http://www.stcratings.com/assemblies.html

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Just a quick follow-up. I took some db measurements as a baseline. With both units operating (not including startup), here are the values:

Inside bedroom on shared wall: 68 avg, 72 peak
Inside bedroom on opp wall: 60 avg, 64 peak
Outside between units: 84 avg, 93 peak

Yes, that's 93 db, no joke. On startup it's even louder, but haven't taken that measurement. Talked to the guys at Green Glue and may do alternating layers of GG and 5/8" drywall, twice on that inside wall. Add a lot of mass, we'll see how much difference it makes.

Here is a link that might be useful:

    Bookmark   August 18, 2011 at 12:25PM
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seems to me that if the units were not surrounded by hard surfaces that they would be quiter.for me that would be a better option..
best of luck

    Bookmark   August 18, 2011 at 3:49PM
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I do wish I could fix the location or surroundings, but that's the property line. Not the best of designs, I'll grant. We just moved in last month so I'm attempting to help the situation as much as possible.

    Bookmark   August 18, 2011 at 4:30PM
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First off has anybody checked the units to see why they generate so much noise? Even the bottom of the line units back then didn't go much higher than 80-82 dbs. Compressor blanket might help some but don't think it's the solution.

I don't think you'll get the degree of satisfaction your looking for with green glue or quietrock solution. Additionally, if there's a window in the vicinity it will continue to transmit the same level of sound through it as before.

If indeed the units are checked out and found to be operating more or less as normally expected, based on the age of the units and your inability to move them somewhere else the only real solution is to purchase new quiet units.

    Bookmark   August 21, 2011 at 11:12AM
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Or use some vibration absorbers under the units and some noise blankets on the compressors. It also sounds like they maybe oversized. Or4 maybe another malfunction.

    Bookmark   August 22, 2011 at 8:54AM
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Thanks for the responses. I installed one sound blanket on the 3 ton unit and it has reduced the vibration and the higher pitched frequency on that unit. I will measure the difference. I've ordered another Brinmar blanket to do the same with the other.

I looked over the units when we added the blanket and there are several screws missing and some parts that don't look to be well secured so that'll be my next bit. Also, looks like some weatherstripping was added for vibration just under the top that's rotted away. I'll replace when I add the blanket. Still considering the GG + drywall option as the final piece.

    Bookmark   August 22, 2011 at 1:47PM
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One more update. I've now installed compressor blankets on both units, added rubber weatherstripping to reduce vibration and tightened up any loose fittings. The 3T unit is very quiet now, only hear the fan motor for the most part. The 3.5T unit is tempered a little, but the vibration and volume indoors has not abated much at all. I've done a frequency sweep before and after and it appears as though 100-250 Hz are the highest volume culprits, topping 80 db indoors. The wall appears unable to attenuate those frequencies when you compare the indoor and outdoor measurements.

    Bookmark   August 29, 2011 at 2:37PM
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Did you put vibration absorbers under the condensers?

    Bookmark   August 29, 2011 at 3:05PM
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I don't know much about vibration absorbers. The units are secured to a concrete pad. On there securely as far as I can tell. What should I look for?

    Bookmark   August 30, 2011 at 12:48PM
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