Hello all,
First post in this forum. I've done searches and not found much that appears to help my situation. So here goes.
I have an existing completed DIY studio build with a 2 ton air condition system feeding 2 rooms. I have a low frequency rumble in the live room and I don't know what causes it. The rumble is audible in the live room but almost negligible in the control room. The air handler/fan unit is 20 feet away from the live room and at least 15 feet away from the control room. The main feed (and return) to the live room is 16" flex about 20' long (and very straight). When the feed enters the live room the duct size is reduced to 12" with 3-12" plenums which are all located in a fabric covered drop structure across the 12 foot high ceiling. The live room returns are the same number and size but on the opposite side of the drop ceiling structure. The feed to the control room is a single 12" flex with an 8"x12" plenum.
I haven't measure it yet but would guess its around 100-120Hz. Console rumble filters remove it.
How do you diagnose the source of the low frequency rumble?
Regards to all,
Jeff
Air Conditioning Rumble
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Soundman2020
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Re: Air Conditioning Rumble
Hi Jeff, and welcome! 
There are basically only two possible sources for that rumble: it could be mechanical noise produced by the AHU and/or heat pump, or it could be air noise, created by some form of turbulence and/or resonance in the duct system.
First, are you sure the air flow rate and velocity are correct? In other words, is there enough volume of air moving through the system (cubic feet per minute), and is it moving at a low enough speed (feet per second), to be suitable for studios? If the air is moving too fast, it would well be creating its own noise just from moving, or from generating turbulent flow at some point. If the system was designed correctly, then the air flow velocity should be less than 300 fps at the registers (considerably less if you need a very low noise floor), while at the same time the flow rate remains sufficient to produce the correct number of "room changes per hour". Air flowing too fast is a major cause of unwanted noise in HVAC systems. Was your system specifically designed by someone with experience in designing HVAC systems for studios? Or was it just done by the cheapest HVAC contractor in town?
Assuming the basic parameters are correct (flow rate and flow velocity), then there are several other possibilities:
I would start by checking the AHU and the heat pump themselves, to make sure they are properly supported on vibration isolators that are correctly sized and tuned for the system, and also that the ducts are decoupled from the AHU by canvas sleeves. Some installers just bolt the units directly to the building structure, with no isolation pads, or use anu old piece of rubber without doing the math to make sure it has the right characteristics, and also "forget" to isolate the ducts from the units, thus allowing direct flanking of machinery noise into the ducts.
After checking (fixing) that, I'd make sure that all of the ducts are of the right size, and are correctly lined on the inside with proper duct liner. That's another place where installers skimp. It is cheaper to just install small ducts without liner, but that forces the air to move faster, increases the static pressure, puts extra load on the AHU, and increases noise. Larger size ducts (needed to keep large volumes of air air moving slowly) are more expensive, and duct liner is not cheap at all, so skimping on those can save the contractor a lot of money. But hopefully if your system was designed correctly by a guy who understands studios, then that would all be correct.
If the problem still persists after checking/fixing the above, my next move would be to fix the problem of those very long and straight ducts, and also to replace the flex duct with proper round duct of the right size. That's a red flag right there: long straight runs and flex duct? Hmmm....
There's also the issue of "When the feed enters the live room the duct size is reduced ". Duct size reductions should NOT occur right where the feed enters the room: that should occur way up-stream. Size reductions cause speed increases and turbulence: when that flows across the register vanes, you get even more noise. Size reductions should occur upstream, and the final duct run into the register should be as straight and direct as possible, so that the airflow is smooth. If the final duct size is 12", then the last change in direction or cross section should be at least 36" upstream of that point, and preferably more.
If there's still an issue after fixing all the above, then I'd take a look at the design of the silencers ("baffle boxes"). You didn't mention those in your description, but they must be in there some place (assuming that your HVAC system was properly designed for studios). It is possible that the silencers are not big enough, or that they don't have the right cross sectional area to do what they are supposed to do, or that they are not correctly mounted, or that they are not correctly decoupled where they pass through the isolation walls of the rooms.
So that would be my plan.
Hope that helps!
- Stuart -
There are basically only two possible sources for that rumble: it could be mechanical noise produced by the AHU and/or heat pump, or it could be air noise, created by some form of turbulence and/or resonance in the duct system.
First, are you sure the air flow rate and velocity are correct? In other words, is there enough volume of air moving through the system (cubic feet per minute), and is it moving at a low enough speed (feet per second), to be suitable for studios? If the air is moving too fast, it would well be creating its own noise just from moving, or from generating turbulent flow at some point. If the system was designed correctly, then the air flow velocity should be less than 300 fps at the registers (considerably less if you need a very low noise floor), while at the same time the flow rate remains sufficient to produce the correct number of "room changes per hour". Air flowing too fast is a major cause of unwanted noise in HVAC systems. Was your system specifically designed by someone with experience in designing HVAC systems for studios? Or was it just done by the cheapest HVAC contractor in town?
Assuming the basic parameters are correct (flow rate and flow velocity), then there are several other possibilities:
I would start by checking the AHU and the heat pump themselves, to make sure they are properly supported on vibration isolators that are correctly sized and tuned for the system, and also that the ducts are decoupled from the AHU by canvas sleeves. Some installers just bolt the units directly to the building structure, with no isolation pads, or use anu old piece of rubber without doing the math to make sure it has the right characteristics, and also "forget" to isolate the ducts from the units, thus allowing direct flanking of machinery noise into the ducts.
After checking (fixing) that, I'd make sure that all of the ducts are of the right size, and are correctly lined on the inside with proper duct liner. That's another place where installers skimp. It is cheaper to just install small ducts without liner, but that forces the air to move faster, increases the static pressure, puts extra load on the AHU, and increases noise. Larger size ducts (needed to keep large volumes of air air moving slowly) are more expensive, and duct liner is not cheap at all, so skimping on those can save the contractor a lot of money. But hopefully if your system was designed correctly by a guy who understands studios, then that would all be correct.
If the problem still persists after checking/fixing the above, my next move would be to fix the problem of those very long and straight ducts, and also to replace the flex duct with proper round duct of the right size. That's a red flag right there: long straight runs and flex duct? Hmmm....
There's also the issue of "When the feed enters the live room the duct size is reduced ". Duct size reductions should NOT occur right where the feed enters the room: that should occur way up-stream. Size reductions cause speed increases and turbulence: when that flows across the register vanes, you get even more noise. Size reductions should occur upstream, and the final duct run into the register should be as straight and direct as possible, so that the airflow is smooth. If the final duct size is 12", then the last change in direction or cross section should be at least 36" upstream of that point, and preferably more.
If there's still an issue after fixing all the above, then I'd take a look at the design of the silencers ("baffle boxes"). You didn't mention those in your description, but they must be in there some place (assuming that your HVAC system was properly designed for studios). It is possible that the silencers are not big enough, or that they don't have the right cross sectional area to do what they are supposed to do, or that they are not correctly mounted, or that they are not correctly decoupled where they pass through the isolation walls of the rooms.
So that would be my plan.
Hope that helps!
- Stuart -
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deveng
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- Joined: Fri Oct 09, 2015 7:05 am
- Location: Los Angeles
Re: Air Conditioning Rumble
Thanks for the quick and thorough response!
Just to clarify, its a DIY studio, and the cheapest man in town was me! So I did everything myself including the air conditioning. When I built it (30 years ago during the start of the home studio invasion) I didn't consider all the design issues with the air system. I'll take your advice and start on the easiest item first. I had already purchased 2" metal straps to hang the air handler and decouple from the structure. Its not difficult to do and would be quick to do and/or remove if necessary. Along with that I'll try my hand at the calculations for air velocity and volume. On your list of items to check here's the quick info:
1. no canvas sleeves at plenums or registers
2. the flex duct is as straight as you can get flex duct where the only turn is at the supply and return plenums.
3. the 16" to 12" reduction happens at the wall partition to the live room in the attic and the first register is 6' from that point.
Would it be better to make this reduction some distance before the wall partition?
4. there are no baffle boxes installed.
The interesting thing is the rumble is only in the live room. So I would presume the noise would be related to ducts size, pressure,
distance, etc as related to the live room. The control room has the same "straight" flex, but only 12" and there's no rumble there.
Regards,
Jeff
Just to clarify, its a DIY studio, and the cheapest man in town was me! So I did everything myself including the air conditioning. When I built it (30 years ago during the start of the home studio invasion) I didn't consider all the design issues with the air system. I'll take your advice and start on the easiest item first. I had already purchased 2" metal straps to hang the air handler and decouple from the structure. Its not difficult to do and would be quick to do and/or remove if necessary. Along with that I'll try my hand at the calculations for air velocity and volume. On your list of items to check here's the quick info:
1. no canvas sleeves at plenums or registers
2. the flex duct is as straight as you can get flex duct where the only turn is at the supply and return plenums.
3. the 16" to 12" reduction happens at the wall partition to the live room in the attic and the first register is 6' from that point.
Would it be better to make this reduction some distance before the wall partition?
4. there are no baffle boxes installed.
The interesting thing is the rumble is only in the live room. So I would presume the noise would be related to ducts size, pressure,
distance, etc as related to the live room. The control room has the same "straight" flex, but only 12" and there's no rumble there.
Regards,
Jeff
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Soundman2020
- Site Admin
- Posts: 11938
- Joined: Thu Aug 21, 2008 10:17 am
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Re: Air Conditioning Rumble
It's not quite as easy as that, unfortunately!I had already purchased 2" metal straps to hang the air handler and decouple from the structure.
Nothing is, in studios... Hanging the AHU from metal straps will only provide limited decoupling. What you need are proper isolation hangers. Such as these:
And here too, there's some math involved; you can't just grab any old isolation hanger! It has to be correct for the weight of the unit that it is supporting, and also tuned to the right frequency. For example, you say that you suspect the rumble is around 100 Hz, so if you accidentally got an isolation hanger that was tuned to 100 Hz, then not only would it NOT isolate the problem, it would probably make it much worse, since it would be resonating at that exact frequency, and therefore amplifying the vibration, instead of isolating it. In reality, yours would need to be tuned way down low, no higher than about 15 Hz, and preferably much lower. But that has to be calculated, not guessed....
The actual physical installation isn't hard, agreed, but on the other hand, the calculations can be ... ummm ... "fun"!Its not difficult to do and would be quick to do and/or remove if necessary.
A couple of weeks back, I spent four entire days calculating the correct isolation for a pair of speakers....Ooops! You only really need it at the point where the supply duct leaves the AHU, and the point where the return duct arrives at the AHU. Those are the critical ones.1. no canvas sleeves at plenums or registers
Is it stretched out taught, to its full length? Is it correctly supported at regular intervals with proper broad saddles? Flex duct kinks very easily when not supported correctly, and even a slight kink can reduce the interior corss section significantly, thus causing turbulence, noise, etc.2. the flex duct is as straight as you can get flex duct where the only turn is at the supply and return plenums.
Not sure: it would help if you could provide photos and diagrams, to show what you have.3. the 16" to 12" reduction happens at the wall partition to the live room in the attic and the first register is 6' from that point. Would it be better to make this reduction some distance before the wall partition?
Also, when you say " 16" to 12" reduction", are you talking about duct diameters? So you have a 16 inch diameter duct transitioning to a 12" diameter duct? Are those sheet metal ducts? Are they lined on the INSIDE with duct liner?
4. there are no baffle boxes installed.
Well, there's your problem, right there. That's the purpose of silencer boxers: to allow the air to pass while also preventing sound from passing. That said, silencers for 12" diameter duct and for killing 100 Hz, are going to be pretty darn big. As in BIG! No, I mean REALLY BIG! (Did I mention they would be big?)Not necessarily: it could be a resonance thing too. It could be that the live room also has a natural resonance at 100 Hz, which exacerbates the rumble... What are the dimensions of the LR?The interesting thing is the rumble is only in the live room. So I would presume the noise would be related to ducts size, pressure, distance, etc as related to the live room.
Also, you might want to run a simple check with the free REW software, to try to determine better what the frequency and spectrum of the noise look like, as well as the acoustic condition of the room. Run the tests, upload the data files to a file sharing service, then post the link here so I can download the file and analyze them.
- Stuart -