How do room treatments affect air-turbulence noise? - Page 2

Good thread lots of concerns with in-line ventilation i wasnt aware.

Ill follow this and see how it turns out.......

If you can reach the fan, why not split the line there and run the other resulting vent(s) to another area? Is there another area in the theater you could add a supply line without too much drywall work?

Another thing to keep in mind is the fan is not going to operate at 100% of it's rated CFM with an inlet and/or exhaust line that is very long as well as having several 90 degree turns in it. I'm no mechanical engineer, but the fan has to work against the losses in the line, and since air is compressible you will end up with a lower operating flow.

However, you're probably better off redesigning based on the assumption that you will get 100% flow. If you reach that 250 fpm at rated flow, you'll be well under with the actual installation.

EDIT: With regard to the other inlet, sharp transitions cause turbulence in the air, and turbulent flow does funny stuff. Seems like I've heard my aerospace buddies talk about local velocities and the like for turbulent flow, so even if the bulk velocity is below 250 fpm you may have local velocities higher than that. You could try adding a diffuser of some sort in the outlet to change the direction of the flow and see if that helps. I'm really speaking outside my area of expertise here, but maybe someone with a better background in this can correct me where I'm wrong.

Don't forget even with your fan rated at 340CFM, with all those bends you won't be getting that much at the opening in your room. How much distance do you have between the untreated end of those 2 baffles and the wall above/below? May be just an optical illusion but I'm guessing 7"? If that's the case then 7x15 would give you an FPM around 465 when your air is making the bend around the end of the baffles. I would shave those 2 baffles back until they only overlap by about 1" vertically. If that doesn't give you 15*15 and you still find it too noisy then you should make your box big enough vertically to allow for a constant path size throughout the box.

+1 for this approach.

Some food for thought if you decide to add a second return......

I'm hedging my bets in my build with 2 returns on separate dampers. One return low and the other high. I'm hoping they will be quiet enough to only use one at a time. This would give me the flexibility of using low return in winter and high return in summer (or high return in winter when lots of people in the room). If I find a single return too noisy then I could just leave both dampers open cutting the FPM to any single register in half. I'll be posting detailed sketchup drawings to my build thread shortly so people can poke holes in my plans

Thanks for reminding me of every math test I did poorly on!

I was under theimpression that 600fpm in flex was the accepted norm, and 250fpm was the desired fpm at the face of the grille.

If that was the case, you may have a little more leeway to work with. I would of course defer to Dennis on the matter.

Tim

What was your logic for that approach?

My thoughts are to have lower return damper open in heating season (~9 months/year) and upper return damper open in cooling
season. If the room occasionally gets too hot in winter (eg lots of people in the room) then I could reverse the dampers.
I haven't seen any lab tests comparing these two options but there are recommendations from hvac pros on these forums to do this.
Drawing out cooler floor air in the winter and the warmer ceiling air in the summer seems logical to me.

A well insulated basement theater in Virginia has relatively few days requiring heating. It may start out on the cool side but that changes fast.

Even in Adak, a "proper" theater will have very, very little need for heating.

Its true mines hot even in January.....

For consideration:

1. Supplies and returns are placed high or low based on the *primary* use of the HVAC system. Dual mode placement doesn't work and can totally screw with static pressures in your system.
2. Residential HVAC contractors (in J-School that is called a glittering generality ... I call it fact of life) don't have a clue about the special use requirements of a reasonably air tight, well insulated dedicated home theater...*including* consistently ignoring the requirements (ASHRAE) for fresh air exchange. It is your job to educate them (and while doing so, remember it is your room and your money, not theirs.)
3. If you tell a residential HVAC contractor the system must be designed to an NC20 requirement, in all likelihood they won't know what you're talking about (granted, I've been surprised a few times).
4. If you engage an outside mechanical engineer to design the HVAC system, the vast majority of residential HVAC contractors will use phrases like "you really don't need to do that" (the hidden meaning of that is "I don't want to be responsible if it doesn't meet spec", or "I'm going to charge you five times the normal rate to install this mess", or "I don't want to do this job because I have to maintain something I don't understand". Also, "you don't need a dehumidifier, the Air Conditioner will do that" (air conditioners are very inefficient at dehumidification and with high humidity, very inefficient at cooling to boot).
5. Residential HVAC contractors know more about HVAC systems, cooling/heating needs than anyone else on planet earth so whatever whomever told you x, y, or z is BS.
6. The location of supplies returns is very important and very often overlooked. For example, I've seen an HVAC contractor determine the "designer" was an idiot and since returns in the back of the room/supplies in the front would be "inconvenient", they installed the returns behind an acoustically transparent screen (now called an air filter). That cost the HVAC contractor $15K to "do over". You don't want cool air blowing down the necks of the people in the seats for two hours.
7. "This is White Horse (Adak, Barrows, Unalakeet, Northern Vermont) and you don't need A/C." Yes you do. End of Discussion. Full Stop. The method of cooling may not be by means of a compressor, but cooling you need. (If you use an air exchanger, use a right proper one that will provide filtering and humidity control.)
8. If you are going to stuff your room with eight seats, your cooling needs will be way beyond what you (or your residential HVAC contractor) would initially expect. ("...but, it will only be the wife and I except for about four times a year when we invite friends over." Ok. Fine. Why are you then spending $6K to $12K for excess seats that will be used four times in a year rather than getting a, say, better projector?

So, how do you deal with these guys? Very simply. You tell them the specifications they must meet (which includes where supplies/returns should be installed and what they can, and cannot do with respect to your overall sound isolation.) "Look, bud, I don't care how you do it, I don't care what you use, you won't affect my ceiling height, you won't degrade my isolation envelope, and when you're done, I'm going to stick a $2000 microphone 1 meter from that supply and if it doesn't meet NC20, you're paying to rip it out and start over again. Questions?"

Some A weighted measurements , overall DbSPL with fans on was around 39

Baseline, nothing on, Prime seat location



Room Air Exchange fans on, Prime seat location



Under the supply, but not in the breeze



Under the return

After pulling the box and letting the flex duct dangle in the room the air turbulence noise disappeared and now we have just a low rumble. Also a big hole in the ceiling. We put a loud radio near the fan and could barely hear it in the theater through the new hole.

Larger duct, a mega dead vent with two exits and two 12x12 grills are in our future, Also more demolition, clips and channel to hold the dead vent, MDF, Green glue and drywall. We intend to build a 14 ft long coffin with the duct take off in the middle. Come back into the room at the two ends. All lined with Linacoustic.

With a decent quality damper on both takeoffs (high/low) and similar distances/bends/duct sizes in both, then why would static pressures be screwed up? The HVAC professional I was referring to is a commercial HVAC engineer so I'm still hopeful that the high/low approach is of benefit.



I agree most residential HVAC wouldn't have experience in meeting the specialized needs of this type of room. Any tips how to go about finding one that will be willing to take on requirements they may not even understand let alone know how to achieve? I have little faith in my ability in finding someone willing to do this so I have cobbled together a design based on various sources of information. Hopefully this will be 'close enough' to keep me happy in the end. I just posted my design in my build thread so anyone willing to provide feedback is welcome to have a look.

Doesn't your seating position curve satisfy NC20? If I'm reading this right it almost satisfies NC15. How far from the openings were these measurements taken?

I'd respect a guy that used that excuse!

I was looking at the "Room Air Exchange fans on, Prime seat location".
That curve appears to be NC20. How do you find the noise levels when seated?

It is noticeable at the prime location, more so as you move towards the S/R.

Found this reference at the Engineers tool box for this discussion. From my reading you have to fall below the curve to meet the standard. So what does it mean when you meet the NC standard and can still hear the system? Did we not measure correctly? Is our interpretation of the measurement graphs incorrect? are we looking at the wrong graphs?

Maybe you have bionic ears? I believe the measurements should be taken 1 meter from the register. Maybe your measurements were taken too far away?

Examination of the plots (noting we don't know the accuracy of the microphone/meter) indicates a few things for consideration:

1. The contribution to the noise floor by the HVAC system is 500Hz and below. The rumble being in the 20Hz to 40Hz area which is a low frequency noise likely from the fan blades or the motor/mount.
2. This is a common issue in quiet rooms. Sounds you'd never normally hear, suddenly become very obvious but disappear during normal listening.
3. The higher frequency contribution from the box itself can be attenuated with absorption. The lower frequency contribution can only be addressed by dealing with the source (fan/motor). A quick and easy approach might be to line the box with two layers of 1.5 PCF InsulShield with a layer of 3mil plastic or vinyl between the layers.

Thanks Dennis, that box originally had two layers of Linacoustic and it was better than none, no plastic though. But, the noise generated by the high velocity air being sucked out created a noise and given the proximity of the outlet to the face we could hear it. After disconnecting the box we hung the acoustic flex duct straight down into the theater and the noise went away. That gave us more evidence that the pipe to box interface was one issue that we intend to attack by reducing the velocity and putting the outlet much further from the face via a much longer box with two openings, one on each side of the room with a larger outlet takeoff in the middle. It will be lined.

IB-1? I can Google a picture, but it isn't on Ted's site. Do they offer any isolation? looks like attaching hat channel directly to the joist is about the same? Or is it the metal clip part of the whisper clip which has some spring effect. Used them on another job.



Also add to your shopping list a take off collar for the coffin if there isn't one laying around. Grow the width? We may need center supports to keep the coffin from acting like a drum?

Ask Ted if you are passing the point of optimal isolation with so many contact points. How about the IB-3 clips instead? no channel But reaching up to secure might be nearly impossible.