How do room treatments affect air-turbulence noise? - AVS Forum
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post #1 of 64 Old 04-21-2012, 07:19 AM - Thread Starter
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I have some air turbulence coming from my vents. I know now that I should have split it into two, to cut the CFMs down, but the drywall is already up and so doing that now is a bit involved. Right now, the room is all hard-surface, except for some bass traps, and the noise now is noticeable enough to annoy me. Will adding carpet/linaccoustic/fabric-chairs absorb this air-turbulance noise somewhat, or make it even more noticeable, as they absorb the other noises in the room, leaving this one to be more, um, appreciated . Ultimately, it comes down to this, should I rip open the ceiling before the carpet gets installed? - or roll the dice, hoping that it might be alright? Ripping open the ceiling after the carpet is installed is also an option, I guess, using lots of tarps and what not - what would you do?

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post #2 of 64 Old 04-21-2012, 09:22 AM
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Is this the offending component? I hope not - that would be troubling. If not, do you have any pictures of the problem area?
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post #3 of 64 Old 04-21-2012, 09:54 AM
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Just a technical note ... you don't want to reduce your CFM (cubic feet per minute ... amount of air), you want to reduce your FPM (feet per minute ... velocity at the vents).

One thing you can try (pictures of current vents would help), is to use "decorator" vents which have larger spaces for the air to flow through.

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post #4 of 64 Old 04-21-2012, 10:05 AM
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Right now there are no vents covers, just big holes and what I think is air turbulence noise is noticeable. The system is an air exchange design. One in-line panasonic pushing in another pulling out, equal size units but only using 6 inch round flex duct on the return side which draws from a large Linacoustic lined box up in the ceiling with a large opening into the theater. The supply is the picture Fred posted which is connected with an 8 inch flex. It is quieter than the return.

The question is now that the room is quiet with communicating doors with seals, is the HVAC noise going to me more or less noticeable with carpet, fabric chairs and some wall treatments. I'm thinking about the same. Which is somewhat annoying in a quiet room.
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post #5 of 64 Old 04-21-2012, 10:27 AM - Thread Starter
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Amazingly, that thing does give some noise, but it's the one in the back of the room that I'm more concerned about, given it's proximity to the back row.


The back vent is a 6" acoustical flex duct, with lots of bends, going into a ~22"x18" DD+GG box with it's interior lined with linacoustic. I've taped the lip of the 6" hole's opening into the box to provide as smooth a surface as possible. The bottom of this box opens to the room's ceiling near the back wall, almost immediately behind where the the PJ would be mounted .


Looking straight on:


Looking up:



I've already tried a few things to address the problem, including putting two 340 CFM fans (one on the supply, one on the exhaust) on a speed-controller (this helped) and trying to isolate the back fan from the joists (this didn't help). The one thing I haven't tried yet is to stuff lots of pink insulation around the fan, but it seems obvious that problem is too much CFM. The ultimate fix will be to open the ceiling and split the line but, before doing that, I'm wondering if anyone knows if carpet/linacoustic/chairs would help...do you know?

This is coming up now because I finally got the door with bottom-closer installed caulked everywhere. At this point, the room is sealed pretty tight, so I can no longer hope the sound coming from outside the theater.

BTW, the carpet is scheduled to be installed next Thursday. I can delay it, or let the installation proceed and find out the answer to this thread's question directly, but then, if it turns out no good, I would have to be very careful covering the carpet before tearing open tearing open the ceiling

Thanks for your post, hopefully the forum will point me in the right direction...

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post #6 of 64 Old 04-21-2012, 10:53 AM
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Do you have any sound measurements? I know I was concerned with this as well, but after the projector was installed I noticed its fan much more than the HVAC.

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post #7 of 64 Old 04-21-2012, 10:55 AM
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Again, it's not the CFM that's the problem, it's a combination of air velocity and turbulence. Is your fan near the exhaust side or near the room? I'm wondering if you are actually hearing the fan or if it's truly the air noise. If it is the air turbulence that you are hearing, you really need to create a more gradual transition from the vent opening down to your 6" pipe.

At any rate, I don't think the carpet and acoustic treatments will mitigate this noise, but you can always take several loads of dirty towels, blankets, clothes, etc. down there and spread them out to see if you get any noise reduction.

EDIT: IIRC, turbulent noise is high frequency, so you might be able to get away with adding a thin air filter up there to attenuate the high frequencies a bit (I'm just throwing out ideas, not sure how well this will work). It'll be a trade off, though. The thicker the filter, the better the noise attenuation, but it will be at the cost of flow.

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post #8 of 64 Old 04-21-2012, 01:24 PM
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I'm not confident in knowing where the noise would come from, exactly, but I'm with J_P_A: the noise, if not the fan itself, is related to high-speed air (as Dennis said, FPM, not CFM), which I think will tend to be turbulent and noisy at transitions, as J_P_A said. I don't think that sound absorbing material in the room is going to change your hearing it. In general, the noise will be so low intensity that direct sound is the only kind loud enough to hear - reflected sound (that you could absorb with furniture and carpet) will be already quieter and should not, IMO, add significantly to the overall SPL of the problem... Naturally, you are free to disagree; I make no claim to expertise.

Given that, I would suggest that the transition from the duct to the room (which is actually at least two transitions, in my book) is the problem (again, assuming it's not the fan itself). My understanding is that turbulence (and therefore noise) is generated when the aperture of a vessel widens abruptly. This will happen in the room if a small duct dumps directly into the room, or in the boot in your case.

I'll suggest two solutions, neither of which may actually work: first, replace a foot or two of duct just prior to the boot with larger ducting. I imagine this will be a fair hassle (HA!), as you'll need a new boot with larger opening as well as a small amount of duct, plus working in the closed ceiling will be no fun at all. Second, install some baffles in the boot that can taper the opening and/or some fins in the center of the aperture to encourage continued laminar flow through this transitional region in the boot (the mouth of the duct).

There is a third theoretical option to smooth this transition: encourage turbulence inside the duct, via a small set of fins or something. If the airflow is already turbulent, the transition will be less dramatic. Noise form the earlier transition could be better absorbed within the duct and ceiling. The natural outcome of this will be lower airflow overall, however - so it may be undesirable.

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post #9 of 64 Old 04-21-2012, 01:40 PM
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Why I think it is air turbulence noise is if you stick your fist up near where the duct takes off of the recessed box, decreasing the available duct size the noise gets louder as the air speed picks up.
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post #10 of 64 Old 04-21-2012, 01:54 PM
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Here's the simple analogy: subwoofer port noise. It's overcome by flaring the port - that's what you need (IMO). ...so there's another way of looking at it.

This is what I was trying to suggest above about tapering the opening of the duct into the boot. Maybe you can do this by building out the wall of the boot with successive layers of something (like wood or linacoustic) so that the opening is more gradual. I'm having a hard time with words this afternoon...

Here's a picture of the simplest way to test this, IMO.

I'll stop obsessing about how I would deal with it, and hopefully you can get more thoughts on whether or not filling the room with furniture and carpet will help, but from my perspective it doesn't matter; you'll hate it if you do nothing and it doesn't get better: do something now.
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post #11 of 64 Old 04-21-2012, 01:59 PM
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Interesting and helpful. Certainly something that could be tried.

BUT, back to the original question WILL THE ADDITION OF SOUND ABSORBING ROOM FINISHES DIMINISH THE IMPACT OF EXISTING ANNOYING SOUNDS? I suspect if the preponderance of opinion is no, there will be a ceiling ripped apart next week.
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post #12 of 64 Old 04-21-2012, 02:55 PM - Thread Starter
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Quote:
Originally Posted by BIGmouthinDC View Post

Why I think it is air turbulence noise is if you stick your fist up near where the duct takes off of the recessed box, decreasing the available duct size the noise gets louder as the air speed picks up.

To add further evidence, if I'm not mistaken, listening to the end of just the duct itself was very quiet - this was before hooking it up to that box...

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post #13 of 64 Old 04-21-2012, 02:58 PM - Thread Starter
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Quote:
Originally Posted by HopefulFred View Post

Here's a picture of the simplest way to test this, IMO.

Very interesting. I have a bunch of OC703 left over, I could sculpt a gradual curve out of it...

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post #14 of 64 Old 04-21-2012, 03:18 PM
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How did you calculate 340cfm x 2?

I think the issue is you are trying to move a tremendous amount of air. 6" flex is not even close to large enough to move that much air quietly. I would figure about 12"

If you have a db meter try taking some readings. I would wager the noise is where the 6" round meets the inside of the box.

If I have my math right, FPM *.19 = 340CFM, where .19 is the area of the 6" pipe in square feet. That's 1789FPM.

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post #15 of 64 Old 04-21-2012, 03:35 PM
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I don't think the other room acoustical treatments will help much. It looks to me that the duct size is too small for the volume of air, hence the velocity is too high.
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post #16 of 64 Old 04-21-2012, 07:09 PM
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I have the same problem with my projector ductwork. I have a panasonic whisper fan in another room pulling air from my projector box. I used 4" flex duct and the air speed creates noise. I wish I would have used a bigger duct for my fan. Maybe I can try a flared port also.
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post #17 of 64 Old 04-21-2012, 07:47 PM
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found a 6 inch flared port, they probably also have a 4. maybe some value in hooking the flex tube to a section of smooth tube and then the flair?

http://www.madisoundspeakerstore.com...lare-psp-6ofn/



Could be an interesting science experiment. May be able to shove it up the flex tube without cutting open the ceiling?
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post #18 of 64 Old 04-21-2012, 09:19 PM
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Big - that's exactly what I was thinking - I used some of those for some subs. I would not have to rip out the ceiling - the duct is in the soffit and I have removable fabric panels that I could remove to get to the duct but it would still be a bit of a pain. I may put a 5-4 inch reducer and increase the size of the last 3 feet or so to a 5 inch duct.
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post #19 of 64 Old 04-22-2012, 03:22 AM
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Adding acoustical treatments to the room will not help. It is more likely those treatments will exacerbate the problem indirectly. At the nearest to the supply and return, what is the current SPL (A weighted). If you can produce a full range plot (SPL by frequency), that would be more helpful.

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post #20 of 64 Old 04-22-2012, 08:39 AM - Thread Starter
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Quote:
Originally Posted by Dennis Erskine View Post

Adding acoustical treatments to the room will not help. It is more likely those treatments will exacerbate the problem indirectly.

Perfect. Thanks for answering that question, Dennis. The questions extends to include carpet and fabric chairs, but I assume that you took that into account.

Quote:


At the nearest to the supply and return, what is the current SPL (A weighted). If you can produce a full range plot (SPL by frequency), that would be more helpful.

I've tried to measure using an iPhone app, but I get 54 DBa if the fan is on or off, or if I'm in another room in the house or even outside at night. I'd think the thing's broken, if it weren't that it jumps to mid-60s when talking and my brother's iPhone has identical behavior. I wonder if the iPhone app has a limit on how low it can measure? I also have a Radio Shack SPL meter, I'll try that one in a just a little bit...

Dennis, do you think the flared port product Big posted and/or a sculpted "scoop" out of OC703 would help, or would that just be a waste of money and I need to commit to doing some ceiling surgery now?

FYI, I asked my brother about this last night. When I turned on the fan, his first comment was that it sounded like the Starship Enterprise, the constant low rumble of space, not the beep or laser sounds. When I put the fans on 50% speed, he said he could still hear it, but it was much less noticeable. He said that he wasn't sure if he would notice it during a movie, even during a quiet scene. Of course, he's not thinking about being able to hear sounds down to 22db, but then again, I imagine very few people have that ability, so it's a case of not missing something he's never had. To him, it's a toss if he would do anything about it, but says that I should do it before the carpet gets installed if I have an inclining to do it now

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post #21 of 64 Old 04-22-2012, 08:52 AM
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No amount of in room treatments are going to solve the problem.

I do believe you are looking at some drywall work, getting the final several feet of duct work into the 8 to 10" diameter size and a larger register. As to the rumble ... that sounds like fan noise. Remove the fan and see if it makes the same rumble while holding in your hand (does the fan feel like the motor/blades are not well balanced). If you don't hear the rumble in your hand, then you need to mount the fan with decoupling hangers (DC04's usually work).

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post #22 of 64 Old 04-22-2012, 03:49 PM
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Quote:
Originally Posted by Dennis Erskine View Post

If you don't hear the rumble in your hand, then you need to mount the fan with decoupling hangers (DC04's usually work).

I stood on a ladder and held the fan so that it was not hanging on any framing members while Morph listened in the theater and there was no change. It also has no noticeable vibration.
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post #23 of 64 Old 04-22-2012, 06:55 PM
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As mentioned earlier your 6" duct carrying over 300 CFM gives you a high FPM at the mouth causing a large change of pressures right over your seating location. The noise generated by that change of pressure at that spot is likely between the 1k-4k Hz range. Adding distance and absorption between your ears and the point where the pressure change occurs should help. As Dennis mentioned you could cut away some drywall and add some larger ducts. Perhaps a less drastic approach would be to make use of the large empty box you currently have at your disposal. You could look at that box as the outer frame of a baffle box that needs some baffles and absorption. Normally the size of the channel within a baffle box should be at least 50% larger than the area of the intake. In your case (6" round), you would have about 28"sq on the intake so you would want your channel to be at least 42"sq. If you were to add 2 baffles as in the sketch below you would still have 117"sq throughout the path to the outlet. If you wanted more baffles you might be able to reconfigure this but make sure to maintain at least 42"sq throughout. The outlet should also allow this much area otherwise it would once again cause an increase in pressure.
Taking this approach the pressure drop out of the 6" duct is significantly reduced (ie from 28"sq down to 114"sq rather than your current 396"sq of the open box). Adding the baffles also forces the generated noise to go through three 90 degree bends (padded with linacoustics) before being let loose into your room.

Just my 2 cents...


 

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post #24 of 64 Old 04-22-2012, 07:03 PM - Thread Starter
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The Radio Shack SPL only goes down to 50 db, and when it goes below that, it says "lo", meaning that the room's sound is below that level. This somewhat confirms my suspicion about the iPhone app being inaccurate - it's too bad it doesn't say "lo" rather then lie about it being 55 db.

SPL meters that can go lower cost quite a bit more. I'm now looking at Omnimic, it looks like version 2 can go down past 20 db over the listening spectrum...

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post #25 of 64 Old 04-22-2012, 08:41 PM - Thread Starter
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Quote:
Originally Posted by Mr.Tim View Post

How did you calculate 340cfm x 2?

I think the issue is you are trying to move a tremendous amount of air. 6" flex is not even close to large enough to move that much air quietly. I would figure about 12"

my bad, I had assumed since the fan came with 6" ports that it would be ok...

Quote:


If you have a db meter try taking some readings. I would wager the noise is where the 6" round meets the inside of the box.

My current meters aren't sensitive enough. Hopefully I can use an Omnimic soon to get a better reading.

Quote:


If I have my math right, FPM *.19 = 340CFM, where .19 is the area of the 6" pipe in square feet. That's 1789FPM.

I calc the current setup to be 1,732 FPM, using a four decimal digits (.1963), but still a lot.

Assuming the last few feet of the existing line are replaced with 8", it would be 974 FPM at the one register. Doing the same with 10" gives 623 FPM.

Alternatively, if the single 6" line is split into two 6", it should give 866 FPM to each branch. Doing the same with 8" gives 487 FPM per branch, and with 10" gives 312 FPM.

What FPM is the goal again?

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post #26 of 64 Old 04-23-2012, 02:29 AM
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Get as close to 250 fpm as you can.

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post #27 of 64 Old 04-23-2012, 03:49 AM
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Yes, it is deceiving when they put a 6" inlet on an inline fan. I suppose sound is not an issue for bathroom exhaust; or at least the manufacturer doesn't think it is.

I had a 4" inline bathroom exhaust fan with a 4" inlet on a different house and it sounded like a jet aircraft.

HVAC design for a typical residential room is no more than 700FPM on a flex inlet/outlet. As you can see above ^^ Dennis is recommending 250FPM for a theater room.

Doing the math: 250x = 340, x is 1.36

3.14*r^2 = 195.84, says you need an 8" duct. Or you can just refer to post #21
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post #28 of 64 Old 04-23-2012, 05:14 AM - Thread Starter
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Quote:
Originally Posted by Mr.Tim View Post

Doing the math: 250x = 340, x is 1.36

3.14*r^2 = 195.84, says you need an 8" duct. Or you can just refer to post #21

That's the radius for a single duct, the diameter would be 16"

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post #29 of 64 Old 04-23-2012, 05:42 AM - Thread Starter
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Quote:
Originally Posted by 235 View Post


Using the dimensions provided in this pic, the resulting FPM would be 418. That would be an nic low-hassle fix, unfortunately I fudged my numbers before...

Putting a tape on it, the box's actual dimensions, inside the linacoustic, are 16*11.5 and it goes 12.5 up. Tweaking your pic with the correct dimensions, my best case scenario would give a 8*11.5 opening, which should produce 532 FPM - much better, but still about twice DE's recommendation. But with the box being only 12.5 deep, the opening would feed directly into a 5*11.5, which would be 851 FPM - not even running the fan on 50% would get it close to the target

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post #30 of 64 Old 04-23-2012, 05:50 AM - Thread Starter
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Quote:
Originally Posted by HopefulFred View Post

Is this the offending component? I hope not - that would be troubling. If not, do you have any pictures of the problem area?

Measuring the box's opening, inside the linacoustic, to be 15*15, the FPM is 217 - finally some good news!

I don't understand yet why some noise is coming out of it, but clearly it's nothing to worry about until the rear is fixed...

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