[By the way, this photo was taken with the camera, Panasonic G1, held out the window at arm's length, and the hinged screen turned back to front so I could frame the picture!]
I have two indoors air conditioning units. Each one has two vents as seen on the photo. I guess each vent is about 15cm x 15cm (six by six inches).
The noise, especially when the unit is actively cooling and not just the fan going, is bothering the downstairs neighbor. Last year I had the company come out and put some foam inside the pipe (I think only one of them for some reason). This helped about 50%, and since the neighbor didn't mention it again, I figured it was OK. Or I hoped so, at least. [Update: it was mentioned that it could be noise coming through the wall. I asked him, he says it is the vent, it's when his window is open.]
But now it turns out it still bothers him. Is there a noise engineer in the audience? What I figure is, I get a local craftsman to make a box which just fits around the two vents, and is open to the front so the air can move through. And then fit some waterproof foam inside the open box so the noise going directly up or down will be dampened.
It'll have to be a smallish box, because the housing association won't allow us to change the exterior of the building (which is why I had to get indoors units). But I think this should make quite a difference, since I have observed* that a huge percentage of noise travels in straight lines, like with light. What do you think?
------
*Observed this way: on an early morning, I was walking along a long and tall brick wall. The wall was shielding the sight of a noise source, a tall chimney. And I noticed the noise I heard, mainly reflected in the building opposite, was oscillating... And then I saw that it was oscillating in time with the reflection in the windows on the building, as I was walking along. When the chimney was reflected in a window, the noise was half of what it was when it was it was not!
Clearly the bricks of the building reflected the noise much more than the glass of the windows. And since the oscillations were so very clear, that must mean that the bulk of the noise traveled in straight lines.
19 comments:
I'm not an acoustic engineer.
I'd recommend first going downstairs and listening to the offending sound(s). If it is high pitched, some hard insulation might help. If it is low pitched, then isolating the air conditioner as much as possible will have a better effect.
My guess is that it is the low-pitched hum that is the offending noise. Mount the air conditioner from the floor (like on a table with padded feet) with foam rubber all around the sides, then in your vent, use more foam rubber to restrict air movement, rather than using a stiff wooden box, which will transmit the vibration downstairs.
Just an approach, cheap to try, from a systems engineer, retired.
My impression is that it's pretty low-pitched, but also that the sound from outside is the problem. But certainly it's worth a thought that some sound may go through the walls. In fact the way I found out that it still bothered him was because I wrote him a note to keep the bathroom fan running for a minimum of time, and I think the bulk of that noise goes through the walls (that one is a very small issue, and was just an attempt of mine at fine-tuning things). Thanks.
Good advice from Dave, but I would add the following:
If the offending noise is what comes out the vents, as it would likely be if your neighbor has no air conditioning and lives with his windows open, then perhaps a simple deflector (reflector?) would do the trick. The idea is that a judiciously located and oriented rigid surface, say a piece of wood for example, will act as a mirror and deflect some or even most of the sound away from his windows.
As a starting point, my guess would be an 8"x8" board located directly in front of the vent opening, 2~3" from the wall at its closest point, and angled 30~45 degrees upwards (as you would place a mirror if you were trying to deflect light towards the sky).
Simple enough too try, cheap to implement, and would not adversely affect cooling efficiency. Note that the optimal orientation may be very different, do look around and analyze the context. Be wary of all nearby walls, balconies, etc., as they can bounce directed waves around quite a bit. Aim for open space!
Interesting idea.
I guess hard wood best?
Why not going all the way to the wall? Seems to me that would be more effective.
BTW, I've yet to hear back from my *upstairs* neighbor if the noise every bothers him. My guess is not, based on various indications that he sleeps very soundly. But still, just in case I might consider not reflecting it all *up*...
Do you think reflection is more effective than absorption? (Foam.)
Reflection sure is easier, especially outdoors. Any rigid material will do, not necessarily hard (I would go with plastic, myself, but painted/stained pine is just fine too).
Porous and soft materials suitable for absorption are, well, absorbent and will soak up water, or be deemed suitable for nest padding by entrepreneurial birds, etc. Might very well result in a mess, and might not survive freezing very well either (because of absorbed water). Just not as simple to properly implement, in my view, because you have to protect the absorbing material from the elements.
Why not flush against the wall? Simply to try avoiding a situation where you end up directing the noise directly alongside the wall, thus potentially disturbing every neighbor above you. Separate the reflector from the wall, and try to keep the angle somewhat wide (30 degrees from horizontal, perhaps even less) rather that trying to go straight up. Keep in mind that you are deflecting a cone and not a straight beam, so it will flare towards the wall soon enough anyway...
I agree with Bert, certainly about leaving a gap from the wall at the bottom. Think dead leaves, bird poop, rainfall.
But I thought that you weren't allowed to change the exterior line of the building, and a reflector would definitely do that.
Great post. I lived in the United states and there is a company here called the American Green Group. They are a green foam insulation business with a unique product called "eco-safe premium foam."
The reason I am posting this here is that one of the major selling points is that it "seals the air." This spray foam insulation is monolithic and serves as a barrier of pure quiet. If you want to know more type in Paradigm Polymers on the internet and check it out. Hope this helps! BTW very nice site, I will be coming back for sure.
Feel free to read my blog any time, thanks!
So how about a tuning cavity. It works for RF and is much the same principle as the muffler on a car exhaust. The cavity is set to a length which is a function of the wave length, and the sound waves cancel each other out.
Not sure how to do it.
The airvents to our bathroom have tuned baffles on the top, added for air traffic noise abatement.
If it's a common problem then the HVAC supplier would/may have a solution. Go back to the manufacturer, see if they have any resources.
My two bits.
I learned about the basics of sound and acoustics in Biophysics.
Sound is a wave, but so is light, at a very different scale. So phenomenons of reflection and refraction are a very real thing, and it is correct that sound mostly travels in a straight line. After that, a lot of phenomenons (phenomena? manhamanha?) can get involved, but...
One thing that's very useful to know, is that similarly to light, the dominant properties of sound vary with the spectrum. Low pitch sound behaves much more like a vibration, a tremor shaking solids (and sometimes your insides or bones), so you need to fight it at contact points. "A table with padded feet" is a perfect example. High pitch sound is mostly transmitted through air, so for that you may need to smother it with complete foam enclosing. I think reflecting would work best in the case of medium pitched frequencies.
To make a comparison with light, red (low frequency) is more absorbed by matter, and even moreso infrared which heats at a distance. Blue (high frequency) is more diffused, as in sea and ocean depths and in the sky. Atmospheric diffusion of blue makes the sun's red hue more dominantly visible at sunset. Yellow (medium frequency) is known for optimally penetrating particle environments, like fog ; it gets less absorbed and less dispersed, great for fog headlights. Notably lower frequencies, like low infrared, radio waves, radar waves... can partially "leap" over macroscopic obstacles that would stop straight rays of visible light, so you don't need a direct line of sight to listen to your fave DJ on FM. The highest frequencies -X and gamma rays- will behave a lot more like a particle burst, capable of penetrating deep inside solid matter and atoms that are not too dense, hence the medical uses to penetrate the body tissues and highlight the more "opaque" calcium in bones. Very different behavior at very different scales.
But I sure hope you're not causing your neighbors annoyance with quantum electromagnetic wave experiments. I think this would be illegal without a military clearance. ;-)
"Not «mad» scientist! Angry! ANGRRRY scientist!"
Dave, there seems to be some small leeway, since I was asked to apply for permission to put up outdoors units, with photo mock-ups.
So I figure, if this little box or reflector is painted the color of the bricks, nobody will even see it. :-)
Bert, I hear you. My box idea, only without the foam and without touching the wall, wouldn't it do basically the same thing, despite the lack of angle? (I'd like it to work for my upstairs too, even though he never complained.) I have no neighbors right across.
By the way, the units are hung on simple brackets, and rest against the wall on the lower edge. I have just put some folded bubble wrap between the lower corners and the wall, that should remove some vibration transfer.
To your neighbour, Eolake:
http://tinyurl.com/pluggy-wuggy
www.rcmedia.com
I just found out now that he's had to use ear plugs already sometimes, which is why I feel so bad about it.
... Maybe I'll try Moldex ones myself, for the occasional night when the upstairs has guests. (He's not noisy these days, but sometimes I'm sensitive.)
"My box idea, only without the foam and without touching the wall, wouldn't it do basically the same thing, despite the lack of angle?"
The basic principle to keep in mind is that you cannot confine energy. Either you dissipate (convert) it by absorption, or you deflect it towards open space, where it will dissipate on its own. Period.
So, the best you can hope for from a box (muffler, etc.) is to convert a directional beam into diffused, scattered waves. The same amount of energy will come out of the box, only diffused and scattered in all directions. If you add some absorbent material in the path, hopefully some of the energy will be converted into heat and thus reduce the output noise level. But you will not trap sound in a box.
You certainly will eventually find a geometry suitable to your specific problem, but that may require a lot of tuning, because the direction in which the energy will propagate in the end is somewhat difficult to predict.
Also, and not to be neglected, any obstruction in the airflow path will reduce the cooling efficiency of the unit. Maybe you can afford the electricity bill and decide that you're willing to be less green in order to buy your peace, but if the A/C unit ends up working twice as long for a 25% noise reduction, I doubt your neighbors will be satisfied.
My advice is thus to try as hard as possible to keep things simple. Start by following Dave's advice: the first step is to identify what the problem really is. If it is conducted vibrations trough the walls, playing outside won't do much good.
But if the noise level inside your neighbor's place changes significantly when the windows are closed, then it definitely is worth having a look at how the noise is propagated from the vent.
I don't know much about your setup, so I can't propose any specific solution. I have however prepared a very crude sketch to illustrate what I had in mind in my previous comment. The top part of the sketch illustrates a common situation, where an opposing wall reflects most or all of the noise towards the source.
The bottom part shows what happens to the exact same setup when a small reflector in inserted in the acoustic path. You can see how such a simple change can sometimes produce very dramatic improvements, so it definitely is worth a try!
But keep it simple. As appealing this may be, it may not be the solution to your problem, I don't know. Yet it is easy enough to have someone in a ladder holding a board a various angles while another person is listening from inside your neighbor's place. Something like that.
This is all simple geometry, which I am positive you can handle. You're an artist, for crying out loud! If you can handle perspective in a drawing, you certainly can imagine a solution based on careful observation of your surroundings! ;-)
Good luck!
"But you will not trap sound in a box."
Yes, so much for that rather amusing, oft-seen, sci-fi idea.
The oft-seen problem always being, "where to get rid of that noise pollution afterwards?" (Well, d'uh!)
I've always wondered, how come a resonance box seems to AMPLIFY the intensity of sound in all directions? Just putting a music box directly against some wooden furniture makes it so much more powerful. Is it simply a matter of preventing some initial loss of energy and keeping it in the sound waves? Or concentration before release?
It's at times like this that you really need an acoustic engineer at hand!
"This is all simple geometry, which I am positive you can handle. You're an artist, for crying out loud!"
Um... bad idea. If you cry out loud, the downstairs neighbor is sure to be even more annoyed.
"You're an artist, whispered the soft voice of the muse."
There, much better. :-)
(By the way, I *have* contacted the installing company, and they've said they'll get back to me next week. But based on last time, it does not seem their expertise reaches much beyond just hanging up standard units, and I think I was their first indoors one ever, so I'm not that optimistic.
Nah, not the installation company, the manufacturer. They may have audio compliance specs, and may have approved baffle/muffler designs.
The installer has been trained on a procedure and that's all.
Oooohh....
I think they are Italian or something.
I'll have a look anyway, thanks.
Post a Comment