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The Plains Theater - Page 39

post #1141 of 1535
Quote:
Originally Posted by J_P_A View Post

Agreed. My opening into the room should have a face velocity around 700 fpm. The idea was to dump it into my soffit where I have allowed roughly 7" x 16" for a lined duct. Then I was planning for a lined "chamber" at the end to slow things down the rest of the way. I didn't count on it being so loud that 1/2" DW wouldn't block the sound from entering the room.

I thought you were going with an 8x36 with fpm<250?

250 seems to be the recommended number for face velocity at the register. If you didn't use the 8x36, a 20x20 or 12x36 would work (return register, not diffuser)

Tim
post #1142 of 1535
Thread Starter 
That was the plan for the diffuser in the soffit face. I was planning to use my soffit as another 10' or so of duct to help slow things down, and then use a large bar diffuser to enter the room to get the face velocity down.

However, at the moment it seems like its going to be too loud where my dead vent enters the room. The noise is getting out of the soffit. Granted, the bottom face of the soffit/duct is just held up there for testing, and it's not sealed up yet, but it still seems pretty loud.
post #1143 of 1535
JPA you are (slowly smile.gif) becoming an expect at this theater building thing. Think you could take a month or two vacation in about 10 months and help me out? biggrin.gif
post #1144 of 1535
Thread Starter 
Quote:
Originally Posted by auburnu008 View Post

JPA you are (slowly smile.gif) becoming an expect at this theater building thing. Think you could take a month or two vacation in about 10 months and help me out? biggrin.gif


I don't know about expert, but I can certainly give you a list of things NOT to do smile.gif Let me know when you get started, I'm looking forward to helping out!
post #1145 of 1535
Thread Starter 
I thought I would try to clarify my issue a bit. I think it's probably tough to follow exactly what's going on for every one else.

My equipment room has a two chamber deadvent that's shown in this picture.



My fan is in the left chamber, and the right chamber is just for sound isolation. The box at the top was intended to slow down the flow in that 8" duct and transition into my soffit inside the room. Here's a rough layout of what the soffit on the inside of the room looks like.



On the right, you can see where the box enters the soffit. The opening is as large as I could make it and stiff fit inside my soffit, and between my studs with DD+GG. It's roughly 6"x12", which I figured would get the velocity down to around 800 fpm or so. I'm figuring less than rated flow on that 440 cfm fan. My soffit is roughly 16x6 after I install duct liner to one surface which gets the velocity down around 600 fpm.

I was hoping this duct velocity would be slow enough to not be audible through a large bar diffuser shown at the left of the diagram. However, what I've found is the noise at the entry to the room is loud enough that it can be heard through drywall I built the soffit/duct with.

Hopefully that clears things up a bit. Don't get too bogged down in the details of that sketch up model, I just put it together to help clarify things.
post #1146 of 1535
This is more or less what I was visualizing - thanks for the clarification and confirmation. I have a further question. In the model, we see the duct pass through the wall from the equipment room going back toward the main trunk at the height of the ceiling. Is that the same small stub wall we see the duct disappear into near the floor in the photo? Maybe I'm seeing the scale of the images wrong (I know you said that the photo doesn't show some of the parts of the system since they weren't installed at the time).

Unfortunately, I don't think we can make any real progress diagnosing the problem and determining possible solutions without spectral analysis. I was hopeful that there were smartphone apps that would function as RTA that might be helpful, instead of buying a mic and using REW. Those apps exist, but even the paid apps seem to be limited to mostly 1/3 octave bands (I've seen one android app that advertises 1/6). That's just not going to get the resolution required to be diagnostically significant - though you might enjoy them for other less precise uses. It might still be worth your time to search the app store of your choosing (I figure you for an iOS user?) to see if I've missed a worthwhile app. Maybe someone else will see this better from an outside-the-box perspective and come up with a simple way to know how to move forward.

The other thing to consider here, as you've mentioned, is that you don't have the soffit fully constructed, so it can't be counted on to contain sound as well as it will once complete. I hear you about it being louder than you want - but let's consider the outcomes. If the noise is broadband turbulence due to air velocity, we need to locate the location of the turbulence and either decrease the velocity or encourage laminar flow through changes to the architecture. If that source of turbulence is in the dead vent assembly (which seems likely to me, though I'm not sure why), then you still have access to make changes after the soffit is finalized. If the sound is resonant, it's again probably coming from within the dead vent assembly - so there's little risk, IMO, in continuing with construction as planned, as long as access to the ducts in the equipment room is maintained.
post #1147 of 1535
Ok, I am getting scared. I just finished doing 1/2 of my own return vent system and I probably did it with a lot less expertise than JPA has. I wish I had a solution for you but unfortunately I am just going to be on the sidelines here watching what happens and trying to learn something. Like Hopeful Fred said, at least you have access to the dead vent system to make changes if needed. After just doing some of that work yesterday I would not want to tear into mine either but I would if I had a real problem.

It is hard to come up with a solution without being sure about what the problem is. You could just do trial and error and maybe get lucky. It looks like I am going to need some luck with mine as well.
post #1148 of 1535
Thread Starter 
Jedi, I wouldn't worry. My main issue is I didn't listen to my own advice smile.gif

A bit of an update. I put in a cheap fan speed controller last night, and first impressions are not great. This fan doesn't like to be speed controlled, and starts to hum as soon as you start adjusting the speed. I don't think it will hurt anything immediately, but I would be willing to bet that it will lead to premature cap failure. I don't know for sure that the caps are what's singing, just and educated guess.

However, I did fiddle with it a bit, and while I would like for it to be quieter than it is, If I slow the fan down enough, it does get quiet enough to be acceptable.

That said, I think I'm going to follow HelpfulFred's advice. I'm going to finish up my soffit and see just how much noise gets into the room once the soffit is sealed up completely before I do any more surgery. I spent some more time listening to everything, and I'm beginning to think the noise is originating in the box that connects my dead vent to the room. Worst case scenario is I need to cut a hole in my dead vent, tear out that box, and put in a better transition into the room.......... Hopefully it won't come to that! I've also considered putting a short piece of duct into the opening and sealing around it in hopes of "detuning" the resonance. It's a bit of a long shot, but it might be worth trying.

Fred, to answer your question, that stub wall just divides the two sections of the dead vent. I don't have an image with both sides open, but here's one where the first section has been closed up, and the fan and duct added to the second section. You can see just a little of the stub wall in the right of the picture, and the duct goes up and through the back wall of the equipment room as shown in the sketchup model.



Hope that clarifies more than it confuses smile.gif
post #1149 of 1535
To slow down the air flow you also might try restricting the "dump" end of the vent with a throttle. This would be kinda noisy on that end but hopefully its away from where that matters.....

Hmm weird mine didnt make any noises at all, what speed controller did u get? The multi position switch type i hope.....
post #1150 of 1535
Hey JPA....been following your saga here....

I'm with Fred (and you) in that you should finish things up with the soffit before really going back to re-engineer the system and mess around with swapping fans, speed controllers, etc.

However, if I were to suggest a change, it would be getting rid of that box and having a local HVAC sheet metal place custom-fab the transition, including a right angle stub-out "bend" that will fit perfectly with the internal dimensions of your soffit so you have as laminar flow as possible. I'm willing to bet they can make a really nice transition from a round piece of flex to a rectangular opening inside your rough construction for $100 or less. As always, just my two cents!! smile.gif
post #1151 of 1535
post #1152 of 1535
Hey i just realized something, are the panny fans rated for vertical install? All the mounting options on mine were for horizontal installation?
post #1153 of 1535
Thread Starter 
Quote:
Originally Posted by TMcG View Post

Hey JPA....been following your saga here....

I'm with Fred (and you) in that you should finish things up with the soffit before really going back to re-engineer the system and mess around with swapping fans, speed controllers, etc.

However, if I were to suggest a change, it would be getting rid of that box and having a local HVAC sheet metal place custom-fab the transition, including a right angle stub-out "bend" that will fit perfectly with the internal dimensions of your soffit so you have as laminar flow as possible. I'm willing to bet they can make a really nice transition from a round piece of flex to a rectangular opening inside your rough construction for $100 or less. As always, just my two cents!! smile.gif

I could just about use a normal register boot. They are 6" x 14". That's only 2" too wide. That's a good idea, though. I have a feeling this is going to bother me until I get it fixed. Oddly enough, I think my HVAC supply would push enough air out to keep the room cool even without the fan. I'd just like to have as much air moving as possible.
Quote:
Originally Posted by HopefulFred View Post


I'm speechless eek.gif Do you have any first hand dealings with that particular piece of equipment? I'd like to know the reasoning behind that!
Quote:
Originally Posted by NicksHitachi View Post

Hey i just realized something, are the panny fans rated for vertical install? All the mounting options on mine were for horizontal installation?

That's odd. The manual looks generic for all four fans, and there is an
Quote:
Installation III (Vertically on Joists or Pillars)

section. The fan didn't make that noise before the controller, but I actually used the variable one rather than the three position one. Is there a a reason to use the three position one instead? The three position switch is rated for 1.5 A which is getting pretty close to the 1.1 A rating of my fan.
post #1154 of 1535
Thread Starter 
Quote:
Originally Posted by NicksHitachi View Post

To slow down the air flow you also might try restricting the "dump" end of the vent with a throttle. This would be kinda noisy on that end but hopefully its away from where that matters.........

Yeah, I tried that, too. You have to choke it WAY down to get much change in flow.
post #1155 of 1535
There's two different ways (broadly) to use mufflers/resonators to attenuate resonant noise inside a pipe or duct.

The first is the most familiar, and it's a muffler, as in a vehicle exhaust. The basic design involves dimensioning the cavity to reflect sound off the end of a cavity, then again off the originating end. The twice-reflected wave is now a half-wavelength delayed and effectively out of phase with the drone sound that produced it = cancellation. Real mufflers, especially for dynamic engine applications (not stationary, constant-speed applications), are far more complex, but this basic muffler design can be applied in-line with an air duct to good effect. (See Alton Everest - Master Handbook of Acoustics) The absorption characteristics are periodic, and have notches with very low absorption - but in general absorb fairly widely. THe effectiveness of the absorption will be related to the relative cross-sectional areas of the normal pipe and the larger muffler. A ratio in excess of 10:1 seems like it gets you into the neighborhood of significant dB loss, but that's really big, and we don't yet know how much absorption you might need, and at what frequency (plural?).

The second design would probably be easier to build as is shown in that image. It's just a blind end branch. There's (best I can tell) two ways to back into the dimensions for the branch - you can base it on similar 1/4 wave cancellations, or you can calculate the volume as a classic Helmholtz cavity. This is simpler to build, but has pretty significant disadvantages: predicting the absorption frequency is going to be tough - I would not recommend building a device that cannot be adjusted; and the Q of the absorber is pretty high - it resonates at one frequency. There are things that we can do to broaden the Q (at the expense of dB loss), but it's still going to take some trial and error.

I post this link with the caveat that I have not read all of it. It is what lead me to the image I posted. This guy was trying to engineer an exhaust add-on for his modified Porsche. He seems to have a pretty good grasp on the mechanics of it, and does a nice job explaining, but has mixed results at least as far as I read (not to the end). http://www.planet-9.com/987-cayman-boxster-modifications/50861-exhaust-drone-resonator-fix-design-construction.html
post #1156 of 1535
Quote:
Originally Posted by HopefulFred View Post

The basic design involves dimensioning the cavity to reflect sound off the end of a cavity, then again off the originating end. The twice-reflected wave is now a half-wavelength delayed and effectively out of phase with the drone sound that produced it = cancellation.
Something's not right in there - it'd need to be 1/2 wavelength, right?

No, wait. It's gotta be your bull.
post #1157 of 1535
Variable controllers use resistor based control. The toggle ones use capacitor control which is what you want to use...... Resistor control wont work from what i remember.

Mine is the toggle 1.5A model. It uses capacitors to control speed. IDK how its important but during my reading i found somewhere I think Dennis E said those were the ones to use.

Ok, cool i wasnt aware of the vertical install option.

If its resonance i agree with you guys though, gotta kill it.
post #1158 of 1535
Thread Starter 
Wow, Fred! I don't know why that's so surprising to me, it's the same principles used in designing microwave transmission lines. You can add stubs at specific locations along the transmission line to match the impedance in order to eliminate standing waves, etc. I suppose it's the same wave equation.

Good to know Nick. That certainly makes sense. TBH, I was expecting to have issues with it, but thought it was worth a try. I didn't realize the three position switch was fundamentally different in how it worked, but I think adding some reactance should help with the humming issue.

I think once I get my soffit built and sealed up, I'll see where I'm at from a noise standpoint when the fan wide open, and just adjusting my equipment room return to get the flow balanced in the room. If it's still too noisy, I think the next step will be to tear out the box going into the room, and replace it with a smoother transition into the soffit.

If that doesn't work, we'll have to look at some resonant absorbers. As much fun as I think it would be to design one (I'm being serious, too), I've got so many other things that I want to finish in the theater before I get tied up in stuff like that. My 3 y.o. asked me tonight if the "Big TV" is finished yet. Talk about a motivator!
post #1159 of 1535
Quote:
Originally Posted by J_P_A View Post

I think once I get my soffit built and sealed up, I'll see where I'm at from a noise standpoint when the fan wide open, and just adjusting my equipment room return to get the flow balanced in the room. If it's still too noisy, I think the next step will be to tear out the box going into the room, and replace it with a smoother transition into the soffit.

If that doesn't work, we'll have to look at some resonant absorbers. As much fun as I think it would be to design one (I'm being serious, too), I've got so many other things that I want to finish in the theater before I get tied up in stuff like that. My 3 y.o. asked me tonight if the "Big TV" is finished yet. Talk about a motivator!
Two things - 1 - isn't your flow and ACH goal based on running maxed out? 2 - The adapter box you built may in fact be the source of resonant amplification - couldn't it?
post #1160 of 1535
Thread Starter 
Quote:
Originally Posted by HopefulFred View Post

Two things - 1 - isn't your flow and ACH goal based on running maxed out? 2 - The adapter box you built may in fact be the source of resonant amplification - couldn't it?

Yes, and yes.

I'm willing to let the ACH drop in order to get the duct quiet. As long as the room is cool, I can live with lingering popcorn smell.

I highly suspect the adapter box. I listened to the T last night, and I couldn't make out any noise there. That doesn't necessarily mean there isn't, but it's certainly a lot louder at the box.
post #1161 of 1535
I love watching you guys work through all the same issues I am going to have.
post #1162 of 1535
Thread Starter 
Quote:
Originally Posted by Mfusick View Post

I love watching you guys work through all the same issues I am going to have.

Gee, thanks. Glad my misery is helpful to someone biggrin.gif
post #1163 of 1535
Thread Starter 
I seem to be on a roll overlooking things that come back to haunt me later. I usually plan every detail out way before I start driving the first nail, so this is driving my NUTS! After the first mistake, my OCD has me second guessing everything.

To the question, I was planning to use a non-IC can light, so I framed everything up with just enough clearance to get the clips that hold the fixture in between my framing. Well, I happened to look at the instructions today, and they say to leave 3" between the can and insulation. No mention about framing. For some reason, I thought non-IC lights had an insulation rating and a framing rating (something like 1/2" to framing). Anyone know if the 3" spacing applies to the framing as well?
post #1164 of 1535
It depends on the fixture. If it's not listed, I would call the manufacturer's technical support department on Monday. It goes without saying that you want to avoid any potential fire hazards at all costs.
post #1165 of 1535
Thread Starter 
That's probably the only way to be sure. I think the code requirement is 1/2", but there's no way to know if the manufacturer built the can to meet that requirement or not without verification.
post #1166 of 1535
You raise a good point. If you contact the company on Monday and they say 1/2" airgap with that fixture, your local code guys always have the "trump" card available to demand more, assuming this is a permitted job. Probably wouldn't hurt to call your local code guy after you have the info from the lighting manufacturer.
post #1167 of 1535
Quote:
Originally Posted by HopefulFred View Post


I swear if I didn't know better, I'd say that came off a Harley!!! biggrin.gif
post #1168 of 1535
Quote:
Originally Posted by J_P_A View Post

I seem to be on a roll overlooking things that come back to haunt me later. I usually plan every detail out way before I start driving the first nail, so this is driving my NUTS! After the first mistake, my OCD has me second guessing everything.

To the question, I was planning to use a non-IC can light, so I framed everything up with just enough clearance to get the clips that hold the fixture in between my framing. Well, I happened to look at the instructions today, and they say to leave 3" between the can and insulation. No mention about framing. For some reason, I thought non-IC lights had an insulation rating and a framing rating (something like 1/2" to framing). Anyone know if the 3" spacing applies to the framing as well?

Ran into same issue with electrician.............and we had a quick fix.......... solution was to use IC rated cans which may have direct contact to framing and insulation. Of course building inspector had to have a look at can specs after failing our first inspection............after looking at manufacture specs/ratings and a quick look at his resources we were good to go.
post #1169 of 1535
Quote:
Originally Posted by doublewing11 View Post

I swear if I didn't know better, I'd say that came off a Harley!!! biggrin.gif
it may have. I think I found it in relation to an s2000, but google image search is a little unwieldy sometimes.
Edited by HopefulFred - 8/24/13 at 11:40am
post #1170 of 1535
Thread Starter 
Quote:
Originally Posted by doublewing11 View Post

Ran into same issue with electrician.............and we had a quick fix.......... solution was to use IC rated cans which may have direct contact to framing and insulation. Of course building inspector had to have a look at can specs after failing our first inspection............after looking at manufacture specs/ratings and a quick look at his resources we were good to go.

Sadly, there are no 4" IC rated remodel cans that I've been able to find. Plenty IC rated new construction, and I've even found a low profile IC rated 5" remodel can, but I'm afraid the mounting tabs won't fit in the area that I have. It being $30/can also doesn't help. I only have about 7-1/2" of mounting surface to work with, which is too narrow for many of the new construction lights.

I think I have a solution, though. I went to Lowes this morning and looked at some of the lights. The 3" accent can specifically states that it requires 3" to insulation and 1/2" to combustibles/framing, and it's less than $10. It's a little taller, but I think it will work. The down side is the connection box is tiny, and they only come in a gimbal type.
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