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Discussion Starter · #1 ·
Hi everyone,

I have been scouring the net trying to find an optimal solution to quelling the noise from my noisy outdoor A/C condenser unit, and my search has led me (surprisingly, at least initially) away from HVAC forums and towards acoustics, studio, and HT construction forums.

I have a relatively new (first summer of use, came with the house) 3.5 Ton entry-level KeepRite split central A/C system, and the condenser unit is situated along the side of my house that is adjacent to my neighbour. The compressor noise and vibration is noisy enough that it disturbs both my neighbour and i. Unfortunately, the condenser unit is on the side of my house where the study/library, family room, and HT/music room are. Even worse for my neighbour, her bedroom faces the condenser unit.

The mid-high frequency buzzing of the compressor and the hash of the fan is not too bothersome as long as the windows are shut tight against their seals, but the low frequency hum and vibration goes right through the ground / wall / foundation such that i can hear and feel it on both the main floor (family rm) and basement (HT/music rm), and my neighbour can hear and feel it in her main floor bedroom. Believe it or not, the condenser unit is sitting on foam-rubber blocks already, yet the vibration seems to go right through them.

I contacted 2 HVAC people in my area, one of whom came to physically inspect the A/C unit, and the consensus conclusions are as follows:

1. I have a cheap condenser unit, which is noisier than premium units as it does not have a dual-stage compressor, the compressor is not mounted to the base of the unit using rubber mounts, and it doesn't have a sound blanket.

2. It is technically possible to move the unit around to the backyard and put it under the deck, but both contractors recommended against it, saying that (a) it would be quite "invasive" and expensive re-routing the lines in the house; (b) there would be too much pressure loss along the line, resulting in low efficiency / cooling power, and (c) placement under the deck would result in restricted airflow, lower efficiency, and potentially just as much noise due to resonance under the deck.

3. The two best solutions from the HVAC contractor's perspective are: (option 1) wrap the compressor unit inside the condenser with a sound blanket and use softer rubber pads under the condenser unit (the foam-rubber blocks seem to be a bit too hard) (cost: $300-400), or (option 2) replace the condenser unit with a premium dual-stage model (cost: $3-4k).

Our neighbour is willing to help us defray some of the cost, depending on how effective the solution is. Naturally, she would only be able to help out so much if we were to spend a fortune on a new premium condenser unit. And my preference obviously is to avoid spending too much money. This has led me to the path I am down now: going with option 1 from an HVAC based solution and supplementing it with sound treatment solutions. It is my hope that the combined solution can give me superior results to getting a new condenser unit at a cost that's much closer to option 1.

Because the condenser is in the "alley" between our two houses, there seems to be a standing wave / resonance effect that is amplifying everything. Furthermore, because the majority of the noise seems to be vibrational from the compressor, i think the focus should be on stopping the noise and vibration at the source: the condenser unit and the virtual "room" created by the walls of our two houses. Ultimately, this means:

1. decoupling the condenser unit from the ground
2. damping resonance of the sheetmetal cabinet of the condenser unit, and
3. absorbing the standing waves between the two walls

So my questions are: 1. Do you think i have evaluated the issues and possible solutions properly, and 2. How might i best go about implementing the above three tasks?

I have posted a series of pics that illustrate the situation i have described above in the following gallery. Would appreciate if you could take a look and let me know what you think:

http://www.smugmug.com/gallery/13109852_9M9gJ#950588671_JBAxj
 

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The issues are airborne sound as well as conducted sound through the earth. I might suggest to try and determine if the conducted sound is really a component. Can she hear the system when in her basement?
 

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Discussion Starter · #4 ·
Yes, she claims that she can hear it in her basement and all the way in the living room at the front of her house. As well, you can readily feel the vibration in your feet walking anywhere along the alleyway and along the floorboards of the study/library, which is the room in my house right next to the condenser unit. Also, you can feel the vibration if you touch the wooden fence and doors that you see in the pics (but i don't know if that's through the ground or through the air).
 

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OK then a component may be ground conduction. I'd test a heavy, heavy blanket(s) draped over the entire exterior unit and see if that changes things much. This is just a test. The unit may also simply be outputting a frequency that finds resonance with some aspect of the house structure.

If the very heavy blankets remove most of the problem, then ground conduction can be scratched off the list. That's great, as it means no spring platform system required. That would point to some massive shielding structure / absorption to help
 

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Discussion Starter · #6 ·
Believe it or not, i don't have any heavy blankets... only light duvets! Any suggestions on alternatives/substitutes or where to get some cheaply?

Not to jump too far ahead here, but the "spring platform system" sounds expensive, and the "massive shielding structure" (which doesn't sound cheap either) seems like it would be immediately rejected by the board (aka, my wife) on the grounds of ugliness ;) Am i over-reacting, or is this a common reaction from newbies? :sweat:
 

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Maybe a lot of it is the echo off your bricks. Maybe a 4x4 or 4x8 by 2-4" thick panel made of mineral wool board, pressure treated wood (for the frame) and landscaping cloth (to cover it) will eliminate a lot of the echoing. That would cost under $25.
 

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Discussion Starter · #9 · (Edited)
Maybe a lot of it is the echo off your bricks. Maybe a 4x4 or 4x8 by 2-4" thick panel made of mineral wool board, pressure treated wood (for the frame) and landscaping cloth (to cover it) will eliminate a lot of the echoing. That would cost under $25.
I certainly like the price of that option! I'm just trying to visualize how it looks on the side of the house. Aesthetics of the solution needs to be wife-approved too, if you know what i mean. Do you think if i constrain the size of that absorber to the wall surface immediately behind the condenser unit, will that still be effective?

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What do you guys think about maximally treating the A/C condenser unit itself? As in, trying to heavily dampen, absorb, and block within the unit itself by:

- putting 2-3 layers of Dynamat on all metal panels
- either using several layers of Dynamat on the compressor or slathering viscoelastic damping compound on the compressor and essentially tiling it with mosaic tile
- wrapping the compressor with a sound blanket (as per all the mid/high end condenser units)
- adding mass to the base (bottom) panel of the unit by putting a couple 10 lb cast iron dumbell weight plates in the compressor chamber
- putting a 3.5" thick fiberglass batting inside the compressor chamber
- mounting the compressor motor with home-made rubber mounts (using a squash ball or vinyl eraser as the donor rubber)

Would these cheapo non-professional solutions focused on damping/absorbing for the condenser unit itself result in any significant gains? The reason i ask is not only to minimize cost, but to minimize any aesthetic impact to the house.
 

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The blanket concept was a *test* to determine airborne vs. Ground conduction. That's the first step... Identify the problem
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I am in the HVAC field and I do not think wrapping the compressor will solve any issues and will most likely overheat it as it was not designed to have a "blanket" around it. The vibration isolation pads under it will help but I would also suggest putting something under the entire condenser unit. My other concern is that the fan is another part of the noise as most older units did not take sound into the equation and the fan blades as they spin chop the air up causing vibration pulses in the air movement. This can be fixed as well by getting the fan blades replaced with more quiet ones that slice through the air much more smoothly.
 

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Again... the blanket was a test.
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I understand, there is a "sound blanket" that you can buy that is designed to wrap around the compressor but in my experience this only over heats it and causes pre-mature failure.
 

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Right-O. Which is why I don't think anyone is recommending its use long term. Its a 10 minute test in my mind.

If the physical blocking of the sound is accomplished with the blanket, we can assume this is primarily airborne noise and design the appropriate suppression system. I suspect your point will still come to haunt him as the space between buildings is tight and that starts getting him closer to closing off airflow around the unit.

If there's no change whatsoever with the heavy blankets (these have to be HEAVY) then we would presume more earth conduction.
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Planting some low 3'tall shrubs around the perimeter of the unit leaving about a foot of space all around can help dramatically as well and still allows for decent airflow.
 

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Discussion Starter · #16 ·
Planting some low 3'tall shrubs around the perimeter of the unit leaving about a foot of space all around can help dramatically as well and still allows for decent airflow.
Hmm... i like that idea! Aesthetically more pleasing than putting acoustic panels up, too. Don't know if we can put shrubs in front of the unit, but maybe ask our neighbour to plant shrubs in front of her wall directly opposite to the unit.
 

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Discussion Starter · #17 ·
Right-O. Which is why I don't think anyone is recommending its use long term. Its a 10 minute test in my mind.
I think you guys are talking about two different things. Tony was takling about a sound blanket designed to wrap around the 6" diameter x 16" tall cylindrical scroll compressor inside the compressor chamber of the condenser unit, whereas you were talking about throwing wool blankets on top of the entire condenser unit.

If there's no change whatsoever with the heavy blankets (these have to be HEAVY) then we would presume more earth conduction.
Would anyone know where to buy these large heavy blankets? I don't think this is something you can just pick up at Wal-Mart or Home Depot, is it?
 

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Discussion Starter · #19 ·
Ah, yes. Moving blankets. Good call. I was afraid i would have to buy all these blankets and then have no use for them (except for maybe 1 blanket) afterwards...
 
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