Guess the frequency

[MDS]

My AC unit is behind a wall near the living room and is rather loud. It is basically enclosed behind the wall in a column that is 3' wide by 3' deep by 9' tall.

My first attempt at reducing the noise used sound dampening sheets applied wherever I could put them on the aluminum walls of the AC unit and the plywood that forms the enclosure. That helped to reduce the vibration and noise transmission through the drywall but really only changed the tone of the sound - not so much the loudness level.

Step 2 was to line the enclosure with acoustic foam (12" x 12" x 2" thick). That helped a bit more and the sound is definitely lower - but still not as quiet as I'd like. The next step will be to use more/different types of acoustic tiles.

So does anyone know the fundamental frequency of AC noise caused by high velocity airflow? I need to determine if I should be concentrating on acoustic tiles that absorb high or low frequencies or maybe even a broad range of frequencies.

GIK Acoustics guys care to comment and point me in the right direction?

 

[avaserfi]

I don't know how much this would work or if it is possible but have you tried decoupling the unit from the ground with some foam? That would help vibration and possibly make for an easier time of managing the other frequencies.

Also, to really absorb the noise I would look into getting something like a mineral wool as they tend to absorb pretty well across lower and higher frequencies. That would probably do a much better job over the foam you have right now.

This might help you pick the appropriate material: http://www.bobgolds.com/AbsorptionCoefficients.htm

 

[j_garcia]

I've used something from 3M before that is self adhesive and is intended for industrial machinery. It would be similar to something like Dynamat used in cars (and is probably where they got the idea). It is a very dense but thin foam that absorbs a LOT of vibration, the more you use, the better the insulation. That is for mechanical noise though, not so much sound damping per se.

So are you saying blower noise or is it mechanical noise from the unit itself is the issue?

 

[MDS]

The noise is blower noise but if you place your hand on the wall you can also feel the wall vibrate slightly. I figured the vibration would transmit sound through the drywall so dampening may help.

The sound dampening sheets I got from PartsExpress and are similar in concept to DynaMat but are lightweight vinyl. They did help but really only change the tone to be more agreeable (probably reduced harmonics from the vibrating wall).

I really should take some pictures of the enclosure so you can see what I am talking about.

 

[j_garcia]

How about some NoRez or Blackhole5? Expensive maybe, but if it works...

 

[MDS]

What is NoRez and Blackhole5?

 

[Davemcc]

I would lean toward the mineral wool. I've got a lot of it in my basement and I can barely hear a noise down there from the upstairs. Even when my teenage kids hung out down there, I only heard their music from the open stairwell. I don't hear my furnace running from the room next to the furnace room. Most commercial home construction suppliers (not box stores) will carry it and it's not really expensive unless you get the dense rigid, which is still about $10/2' x 4' x 2" sheet.

The best part is that whatever you have left over, you can use to make bass traps or custom acoustic panels.

 

[MDS]

Mineral Wool or the Roxul Safe'NSound product sounds like a great solution with excellent sound absorption and high resistance to flame.

However, it looks like that stuff is insulation that is to be installed between the studs of the wall. I would consider that a last resort as I don't really want to tear down two or three walls at this point (trust me I've thought about it. )

From aversfi's link, I see that porous foam type tiles like I bought are broadband absorbers. I have no idea what the noise reduction coefficient is though because it wasn't listed. Considering I got 16 tiles for $50 they probably aren't that good, although they do help somewhat.

When I first started researching this stuff I came to the conclusion that I need acoustic tiles with an NRC of 1 or greater for max effect. I'm sure Auralex or the like would have something that could work for my application but I'd bet it will cost 10 times as much as the acoustic foam I got from PartsExpress.

 

[avaserfi]

Instead of installing it between the walls you could possibly go to ready acoustics and buy some of these and fill them with the wool I think that would do the job. It would cost less then most places and should handle the noise .

 

[bpape]

If you could post some pics, that would definitely help. The frequency in question is really going to totally depend on the speed the fan is running, the speed the compressor motor is running (if different), and the air noise is going to be based on the airflow and the size of the openings.

If I'm understanding things correctly (and I may not be, only 1 cup of coffee at this point) you need to both isolate the vibration from the wall structure via some sort of decoupling) and also potentially build a sort of hush box like they use for loud projectors in home theaters. The trick is to absorb the noise but still allow the airflow.

Bryan

 

[mtrycrafts]

Do you by chance have a Real Time analyzer? That would at least show you the frequency spectrum produced.
I guess you cannot place anything under the unit to dampen the floor induced noise transmission?

 

[pzaur]

This would be a real P.I.T.A., but if you're going to end up putting vibration dampening material underneath the unit, you may as well look into moving the unit a little further from the house. The move would allow more space for you to work with.

In the last house I owned, I thought of bricking three sides of the A/C because it was next to our bedroom. That seems to redirect a lot of the noise and I know people who have done so.

As for the vibrations coming through the wall. If you're trying to figure out if it is low frequency or high frequency simply feel how fast it's vibrating. The slower the vibration, the lower the frequency. If you want to figure out the exact frequency being produced you need to record it to disc/computer and then find a program to analyze the pitch. I'm sure there are free ones out there.

Probably not the advice you're looking for, but the above people also had some very good ideas.

-pat

 

[avaserfi]

I am pretty sure you can use Room EQ Wizard to find the actual frequency of the sound that is being produced. That should be able help you decide exactly what kind of absorption would be needed.

If possible the best first step would probably be decoupling the unit if possible. If the unit is sitting on wooden beams replacing the wood with high density foam would probably do the trick (something like mopad type material). It is actually possible that if you decoupled the unit your current foam padding would actually be sufficient.

 

[Alex2507]

The noise is blower noise but if you place your hand on the wall you can also feel the wall vibrate slightly. I figured the vibration would transmit sound through the drywall so dampening may help.

I had a guy who owns an HVAC company tell me to insulate the inside of the air return duct using a 1" thick black fiberglass blanket. One side of it has a felt like material that keeps the fibers trapped. Exposed cut edges should be sprayed with 3M spray adhesive to trap fibers.

I wish I could tell you more about the product but I was just given a bunch of it. It is relatively rigid and I was told that a supplier would have it.

My return duct is only 6 or 7 inches long and the difference in volume was very noticable. I guess I should have asked you if you had done anything to the inside of the return first.

Alex

 

[MDS]

Tearing down the drywall and either filling the gaps between the studs with mineral wool/fiberglass insulation or using two layers of drywall with a constrained layer of insulation or sound dampening sheets would probably do the trick but is not an option at this time. Mainly because my list of things needed for the house adds up to tens of thousands of dollars and I want those things (new furniture, TV, electronics, etc) first.

I had a guy who owns an HVAC company tell me to insulate the inside of the air return duct using a 1" thick black fiberglass blanket.
That was kind of my thought but it is not simply one return air duct - it is the entire enclosure that holds the blower/air handler or whatever it is called.

I used sound dampening sheets to line the enclosure as well as on accessible parts of the aluminum cage that surrounds the fan motor. That helped a little bit. I then added the acoustic foam and that helped a bit more. I was just thinking that maybe I need more/better acoustic foam to reduce the noise further.

I appreciate all the ideas. I'm actually suprised that so many people would be interested in this problem. I will take pictures of it but they may not be that great with a 100 kb size limit.

 

[Alex2507]

I understand that Home Depot rents an insulation blower machine that you could use to the wall cavity in between the studs. A small hole would be required up high preferably in the room with the air handler. How is the air getting to the blower? A louvered door?

 

[Woodroc]

Acoustical liner

If the A.C. system is conventional duct work(sheet metal) have your A.C. contractor install acoustical liner in the main supply & return duct & lower the fan speed a little. You could also have him install canvas connections at any point where the duct connects to the air handler & put vibration isolators under the unit. That should all but eliminate any noise or vibration.

 

[MDS]

Pictures

Pictures of the AC Enclosure:

 

[MDS]

Pictures

A few more:

 

[avaserfi]

Looking at those pictures Dynamat actually seems like it would be a good idea to almost completely cut back on resonance caused by the unit.The material isn't expensive but it is rather heavy so you should make sure itwont cause any trouble sticking it to the unit. It will also help cut back on loudness of the frequency that is being output.

While I have never actually used dynamat in a house my experiences with it in cars has always been positive and I have a pretty good feeling that the dynamat mixed with your foam should do a good job. If anyone else that could chime in on this it would be great.