Home theater HVAC soundproofing suggestions

[Matthew J Poes]

ok so I spent a great deal of time educating myself on soundproofing including hvac. One problem I ran into was I needed the hvac lines to have minimal sound radiation (also known as breakout noise) as the ducts were exposed in adjacent rooms and hurried in the ceiling very close to the floor above. It is illegal here to creat a wood baffles plenum or to use dead vents so that wasn’t an option.

That all meant that I needed to use round steel ducts, and then make them as non resonant as possible and add sound absorption to minimize sound transmission down its path to minimize noise escaping. I had special steel double wall spiracoustic mufflers built with Two 90 degree bends. I also bought round duct liner to line the rest of the duct length. Further I added an expansion box on the return line with sound absorbing material.

It didn’t work. I mean I’m sure it worked better than had I not done anything, but sound travels through the ducts. Strangle voices can’t be heard in the dictwork directly connected to the theater lines, which have vents in the room right next to it. However you can hear sounds in rooms 2 floors above. A good amount of bass also escapes the theater, but that’s a known issue with soundproofing, you can’t achieve anywhere near the same TL values.

Any suggestions on how to reduce the amount of sound escaping through the hvac supply and return vents? I feel like the only option will be hiring a company to custom fabricate larger proper mufflers engineered for my purpose. Way more than I can afford.

 

[Sonnie Parker]

Use a separate system for your theater/listening room. lol.. Okay, so maybe that's not feasible for about 99% of folks. It worked great in my case. My problem is the outside unit is loud, despite super thick and well insulated walls. I have a compressor that purrrrrrrrrrrs like a tiger on steroids.

 

[Wayne A. Pflughaupt]

However you can hear sounds in rooms 2 floors above.

Are those rooms all on the same duct line? I can’t imagine that if each was on a separate duct line that the sound would be able to go all the way down the duct, through the air handler, and out to the room in question.

It also seems to me like steel ducts would not be the way to go. Modern flex duct has a fiberglass interior, which has a certain amount of absorption characteristics, and the irregular flexing and bends in the tube would only aid sound absorption.

Regards,
Wayne

 

[Matthew J Poes]

Are those rooms all on the same duct line? I can’t imagine that if each was on a separate duct line that the sound would be able to go all the way down the duct, through the air handler, and out to the room in question.

It also seems to me like steel ducts would not be the way to go. Modern flex duct has a fiberglass interior, which has a certain amount of absorption characteristics, and the irregular flexing and bends in the tube would only aid sound absorption.

Regards,
Wayne

Hi Wayne, they must be on the same trunk but there are many branches between them if so.

A lot of people think flex is the way to go. THat is actually not what I found in the literature from architectural acoustics, they specifically warned engineers from speccing it in soundproof rooms. I also found warnings about its use even from manufacturers say it radiates considerable sound and should not be used where that’s an issue. Remember I lined my ducts with fiberglass and it’s far denser than that of flex duct, roughly 4lb density compared to the 1-1.5lb of flex duct. It’s also 1.5” thick which is thicker than most flex duct.

Flex duct is commonly used but fiberglass only blocks about 3-4 db of sound passing through at mid and low frequencies. Damped steel duct with fiberglass has an average TL of around 30-35db. The plastic or metalyzed body material is far too light to block any sound. There are even tests from both manufacturers and academics showing how much sound leaks out of them. That scared me off of using them. It does a good job absorbing sound going down the length of the duct and it’s likely true that less sound would make it to the upstairs, but I really believe that would only be true because it leaks out the sides of the material.

 

[Matthew J Poes]

Use a separate system for your theater/listening room. lol.. Okay, so maybe that's not feasible for about 99% of folks. It worked great in my case. My problem is the outside unit is loud, despite super thick and well insulated walls. I have a compressor that purrrrrrrrrrrs like a tiger on steroids.

This IS what I should have done. My HvAC guy says I have the space to do this if I want in the future. It is expensive, roughly $4500. I have the access to add the ducts but would need to run a new supply. I couldn’t tap into the existing supply. It just seems like for less than $4500 I might be able to make it better. I cheaper out on duct mufflers so maybe that’s an answer. I’m actually a design partner with ASC so maybe I should ask them the cost.

 

[bkeeler10]

ok so I spent a great deal of time educating myself on soundproofing including hvac. One problem I ran into was I needed the hvac lines to have minimal sound radiation (also known as breakout noise) as the ducts were exposed in adjacent rooms and hurried in the ceiling very close to the floor above. It is illegal here to creat a wood baffles plenum or to use dead vents so that wasn’t an option.

That all meant that I needed to use round steel ducts, and then make them as non resonant as possible and add sound absorption to minimize sound transmission down its path to minimize noise escaping. I had special steel double wall spiracoustic mufflers built with Two 90 degree bends. I also bought round duct liner to line the rest of the duct length. Further I added an expansion box on the return line with sound absorbing material.

It didn’t work. I mean I’m sure it worked better than had I not done anything, but sound travels through the ducts. Strangle voices can’t be heard in the dictwork directly connected to the theater lines, which have vents in the room right next to it. However you can hear sounds in rooms 2 floors above. A good amount of bass also escapes the theater, but that’s a known issue with soundproofing, you can’t achieve anywhere near the same TL values.

Any suggestions on how to reduce the amount of sound escaping through the hvac supply and return vents? I feel like the only option will be hiring a company to custom fabricate larger proper mufflers engineered for my purpose. Way more than I can afford.

Ha, I read the first two paragraphs and said to myself, "Hey, he seems to have done it right." Then the deflating comment: "It didn't work." Sorry to hear about it. I'm grappling with what to do for ventilation in my theater under construction now. Of course I want good sound isolation from the rest of the house, but I also want as low a noise floor in the theater as possible. Which makes it hard to consider a minisplit system for just the theater. I may do a dead vent and exchange air with the rest of the basement that way.

But I'm afraid I don't have any great ideas for your situation.

 

[Matthew J Poes]

Ha, I read the first two paragraphs and said to myself, "Hey, he seems to have done it right." Then the deflating comment: "It didn't work." Sorry to hear about it. I'm grappling with what to do for ventilation in my theater under construction now. Of course I want good sound isolation from the rest of the house, but I also want as low a noise floor in the theater as possible. Which makes it hard to consider a minisplit system for just the theater. I may do a dead vent and exchange air with the rest of the basement that way.

But I'm afraid I don't have any great ideas for your situation.

Thanks. I thought I did it right too!

Being honest I am sure what I did made a big difference. If you talk at the vent outside my theater you can hear it clear as day in the upstairs guest bathroom. If you do the same thing in my theater you can hear it at a very reduced level and mostly only hear bass. The vent outside the theater is tapped directly into the trunk, my theater vent is connected to the trunk 1 foot farther down the line and is just 6 feet long.

I expected total silence and I didn’t get that. I listen to music at levels between about 75db and 100db average volume depending on my mood. Anything above 85db can be faintly heard in these other places like the bathroom. My guess is there is still about 40-50db or transmission loss.

A dead vent should work better than what I did, I had code issues preventing that.

 

[bkeeler10]

Bass is a bear . . .

 

[Matthew J Poes]

Bass is a bear . . .

Yeah it is. My rooms soundproofing is pretty extreme but bass can be heard outside the room. Especially on the floor directly above the theater. Movies are not noticeable other than during a loud action scene. Music however is killer. My wife tells me you can tell what I’m listening too sometimes. I’ve done tests and I am getting about 50db of isolation down to about 100hz and quite a bit more above 2khz (which is correct for an STC65 structure in situ). The problem is that if the bass is 100db, then that is still 50db, and that’s still pretty loud.

As you probably saw in my response to your theater post, the bass region isn’t controlled by mass, rather by stiffness. The resonant region is also very wide, it actually starts around 100hz and extends down to about 70hz before the structure transitions into a pressure vessel. That means you need a lot of damping to control bass in that critical kick drum range.

 

[bkeeler10]

If you're getting transmission loss of 50 dB at 100 Hz, you're doing extremely well. What's the construction on your ceiling?

In my theater, I'm planning for the floor above to be laminate on top of SoundMatt (or similar) underlayment and a gypcrete subfloor. The joist cavity (24" on-center) will be filled with fiberglass, and a layer of 5/8" gyp on clips with a finishing layer of OSB for the ceiling. May add green glue, per your thought. My modeling (without the green glue) suggests STC 65 and transmission loss of 43 dB at 125 Hz for this construction.

 

[Matthew J Poes]

If you're getting transmission loss of 50 dB at 100 Hz, you're doing extremely well. What's the construction on your ceiling?

In my theater, I'm planning for the floor above to be laminate on top of SoundMatt (or similar) underlayment and a gypcrete subfloor. The joist cavity (24" on-center) will be filled with fiberglass, and a layer of 5/8" gyp on clips with a finishing layer of OSB for the ceiling. May add green glue, per your thought. My modeling (without the green glue) suggests STC 65 and transmission loss of 43 dB at 125 Hz for this construction.

I haven’t delved into the precise reason for this but apparently there is a official correction factor for converting an Stc value to an in situ value. It’s typically about ten points. My assumption is this is the difference due to natural flanking problems that the lab is better able to mitigate.

My ceiling is 5/8” t&g plywood, 16” spaces joists, R15 mineral wool insulation, rsic clips, hat channel, two layers of 3/4” drywall with green glue. A sheet of plywood replaces the inner drywall where the projector is. Above the theater is a formal living room that we don’t use and a formal dining room. The floors are carpeted.

I haven’t measured the transmission loss yet. I tried and got unreliable values.

I plan to add a second layer of T&G with green glue and a layer of sound isolation Mat when we put hardwood floor in the house.

 

[bkeeler10]

Ah, when you said you were getting 50 dB of isolation down to 100 Hz, I thought you meant that you had measured transmission loss.

The rule of thumb I'm familiar with is that you can expect real-world, in-field construction to yield isolation (NIC) values about 5 points lower than the idealized STC number would indicate. Construction is never perfect, and unless you are extremely careful (and probably rather lucky too), you will end up with flanking paths that compromise your isolation to some degree.

 

[Matthew J Poes]

We are talking about two different things. Sorry for the confusion. You asked how much transmission loss I had and mentioned the ceiling construction. What I mentioned earlier referred to transmission loss through the hvac duct to an upstairs bathroom (not the room above the theater) that was readily transmitting voices through other ducts in the same trunk. I did measure that at an earlier stage because I was able to pass a microphone cable down the duct into the basement and measure with Two mics. I can’t do that right now because it’s all closed up. I also was just using an 2 channel RTA and the mics I used we’re spl calibrated but not matched. Not so professional.

I plan to measure the spl transmission loss between the theater and room above using two instances of REW. The problem I have is that it appears to be at least 50dbs transmission loss so I’m mostly measuring noise even with pink noise at 100+ db. That’s pretty loud.

 

[bkeeler10]

Oops, yep, I conflated two different things. My bad. Carry on

 

[Matthew J Poes]

Oops, yep, I conflated two different things. My bad. Carry on

No problem. The method I used for HVAC worked because I could turn the HVAC off and put the mics into the hvac duct which had lower ambient noise. I also had a 5 hour period with my wife and daughter gone to set it up. You can imagine the eye rolls I get when she comes home and finds I’ve taken apart some of the ductwork and run cables through them.

As soon as I get a quiet time with nobody home I’ll run the Two mics again and see what I come up with.

 

[NBPK402]

I have heard of people getting the Panasonic multiroom units (around $1k per room module, and available at Home Depot) for their theater, and they were very happy. These are a separate ac/ heater unit that mounts on the wall, and connects to an outside unit. Supposedly you can easily do it yourself. I am going to look at one of these if it turns out we need heat or AC in our new home.

 

[Matthew J Poes]

I have heard of people getting the Panasonic multiroom units (around $1k per room module, and available at Home Depot) for their theater, and they were very happy. These are a separate ac/ heater unit that mounts on the wall, and connects to an outside unit. Supposedly you can easily do it yourself. I am going to look at one of these if it turns out we need heat or AC in our new home.

The main problem with this approach is that in a soundproof and airtight room like my theater there is no fresh air makeup. In a basement that isn’t really ideal. They are also noisy compared to what I did. Even 25dB is well above the ambient noise of the room. My projector is far and away the loudest thing in my room. I wouldn’t have wanted another such source. My situation is highly unusual though. For most people it’s a good option and in fact offers better isolation between rooms.

 

[Sonnie Parker]

I ended up with a separate complete unit for my room, but the Panasonic sounds pretty interesting if were not all that loud (sounds like it might be).

 

[Matthew J Poes]

I ended up with a separate complete unit for my room, but the Panasonic sounds pretty interesting if were not all that loud (sounds like it might be).

By split system standards it is not loud. I think between 25 and 45dB. That is just louder than what I could tolerate.

I should have done a separate system myself. Would have cost more but would have avoided the sound escaping to other spaces.

 

[NBPK402]

I wonder if you could do something like a hush box on one, and still get the benefit of AC, and also make it quieter?

 

[NBPK402]

Our last room was soundproof...per say double wall, green glue clips, seals, etc., and the return line is what released all the noise throughout the house, but it was under 70db outside while listening at reference levels in the theater.

 

[Matthew J Poes]

I wonder if you could do something like a hush box on one, and still get the benefit of AC, and also make it quieter?

I’m sure you could. I still would have needed a fresh air supply myself.

I looked into the split system option but it was something like $3500 installed. A second system was about $4500 installed so at least for me it wasn’t a huge difference.

In my case I knew that a soundproof duct system that heavily attenuates sound traveling through the duct would require pretty specialized design and to be very large. Probably around the same total cost as a second system or split system. I decided design my own small versions of these mufflers hoping they would be good enough. While a lot of sound is attenuated, it wasn’t as good as I hoped. I also used modeling systems designed to focus on sound in the mid range and treble. Turns out that bass travels through the duct pretty readily. I also have found one upstairs duct in my bedroom (of all places) that picks up sound from the other ducts more readily. I don’t know how or why. It is still heavily attenuated but 115dB seems to be about 45-55dB in the bedroom through the duct. Enough to be noticeable if you are trying to sleep. If i close the duct then you hear nothing at all, so I at least have a solution.

I’ll also say that adding fresh air with a split isn’t that hard. You could make an air exchanger with a dead vent style and likely keep good sound isolation. If I had this to do over, I might have used only this method. Two dead vents. The room is actually fine without heating and cooling (if I close off the vent to the room it’s still comfortable).

 

[Matthew J Poes]

Our last room was soundproof...per say double wall, green glue clips, seals, etc., and the return line is what released all the noise throughout the house, but it was under 70db outside while listening at reference levels in the theater.

Hah our ships crossed in the night. My return doesn’t leak all the sound, the supply does .

It was just from me being cheap and trying to simplify. The return has a large expansion box with an indirect soundpath and sound damping inside. Both side of the duct have over 10-15 feet of muffler. There is even a LF resonator on that duct to address fan noise that was coming through. I didn’t have that much room on the supply side and rather than running a really long muffler parallel with the trunk, I took a short cut and tapped in close to the room. The total length of the lined duct area is 15 feet, but the actual muffler is only 24” long. There is also an expansion box that is tiny and has a line of sight air path that leaks more sound than I hoped. Oh well!

 

[NBPK402]

Well you did the best that you could. We had 25' of lined flex that was zig zagged for its entire length, and it still let the bass through to the main part of the house. I am sure if we would have paid someone big buck it could have been done, but I was not rich, and I am now retired so my new setup will be even more budget. Just do not have the money to play with the Big Boys...gotta do the best we can with what wr have to spend.

I am sure you are happy with the results you are able to attain with your budget, I know I was with mine.

 

[Matthew J Poes]

Well you did the best that you could. We had 25' of lined flex that was zig zagged for its entire length, and it still let the bass through to the main part of the house. I am sure if we would have paid someone big buck it could have been done, but I was not rich, and I am now retired so my new setup will be even more budget. Just do not have the money to play with the Big Boys...gotta do the best we can with what wr have to spend.

I am sure you are happy with the results you are able to attain with your budget, I know I was with mine.

I think when you set certain expectations it’s easy to complain and see problems and fault. It creates a lot of “I wish I had...” thinking. However in the big picture I recognize that what I’ve achieved is quite extraordinary. I’ve taken measurements in dozens of rooms, including dedicated and soundproof rooms. One thing I achieved that I don’t find true of other similar spaces is the noise floor. It is so low that we are so far unable to measure it when i turn off the projector and fanned amplifiers. In fact it is so low that every attempt I’ve made has turned out to be a bust. Every time I think i have a noise floor measurement I find a problem with the method. One problem I have is that even my breathing is enough to raise the measurement at certain frequencies. It frequently just fails a certain noise threshold due to laptop noise, breathing, etc. Normal measurement devices, even the basic NTi suite cannot measure below 20dB very well, so I’m on the edge of accuracy here.

Even duct attenuation and wall transmission loss has been a real problem. Unless the source is extremely loud I am picking up as much ambient noise as I am source noise. Many of my measurements have the same absolute spl levels, by the spectrum is different and I can clearly hear sound from the source. For example I can play a 200hz tone and hear it through the soundproof wall, but it might not actually be louder than the 40-50dB ambient sound level. As such a measurement of TL won’t reflect what I hear and see.

A few weeks back I attempted to “scan” the wall while a test tone played. I scanned both the source and receiver side. In theory the wall would have about 80dB TL if it had no door, but with a door Like I have, it’s more like 60dB. I should get over 30dB below 100hz based on lab tests of a similar wall. I kept showing as little as 10-15dB at certain low frequencies. I couldn’t figure it out. Was my communicating door assembly that bad? Was the ducts between rooms the problem? Turns out it was modes. Move a few feet from the wall on either side and the measurements change quite a bit. Certainly the doors and ducts are the major weak points, but modal peaks and dips form both inside and outside the room and if you just happen to stand in a dip on the source side and a peak on the receiver side, you get a really low TL value.