SD, since you seem to know quite a bit about building construction and acoustics, wondering if you had any ideas about my noisy A/C problem. I posted this on HT Shack.com, but wouldn't mind your take on things:
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/13109...50588671_JBAxj