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HVAC Issues/Low End Rumble - Help!
Old 25th May 2016
  #1
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HVAC Issues/Low End Rumble - Help!

Hello everyone. I built my studio nearly 5 years ago now. It's nice and generally works very well. It was a dream come true for me at a very young age that has afforded me many opportunities. However, I have always had a bit of a problem with my control room holding too much low end. The main culprit certainly HVAC, so I could really use some pointers there or even referral to individuals that I could hire to help me out. Basically the AC units are very close the control room (just outside our building, back right corner from mix position). That right there was my biggest mistake in the build, and is I'm sure setting off some alarm bells. Effects of this are that I can hear a slight low end boom and ensuing rumble coming from it which my room holds until AC shuts off. I assume this is due to location. But before I get any further, let me tell you a little about the build so you are aware.

The overall layout is somewhat typical of those found on the John Sayers forum. I have attached a diagram that roughly corresponds to my room and overall studio layout. My rooms are a little more than "slightly" larger than these, and my live room is not square. This is just so you have a general idea of what I'm working with, as this is not a general build question.



The studio was built painstakingly for isolation between all rooms which was achieved with flying colors. Generally speaking it was done with great care and high quality materials. Green glue, mass-air-mass construction, floating floor, lots cedar and stone, dual "soundproof" sliding doors between rooms, a lot of bass trapping and a fair amount of diffusion. The works.

Now...on to some problems. Point out the issues where you spot them!

The live room is great and I have no issues there other than a pesky thermostat that makes a little clicking sound when AC is turning on and off. Any ideas or recommendations there? This mostly is a non issue as the majority of our work is loud rock, though it is somewhat troublesome to me just because I know. Really though it is a control room issue that I am dealing with right now, likely due to proximity to AC/Heat units nearby as I stated.

As far as design, the control room features floor to ceiling bass trapping in the corners - and I mean all corners - and also in the middle of the back wall. Now I know that bass trapping in the back of the room is one thing, but I am also aware that this may be unusual in the front corners. Picture the place where studios would normally mount large mains - instead with large floor to ceiling bass trapping inside a full wall soffit. A cloud above the mix position that is a little further forward than I would like it sometimes, but generally works well when I'm up on the desk getting busy. Also skyline diffusion across back wall. Check it out:




So on to the HVAC issues. Now bear with me as I describe this. While I am still proud to this day of my studio build, in all honesty we probably winged the HVAC part of the build in some ways. We have 3 zones: control room, live room, and kitchen/lounge, the 2 important ones are supposedly of the fairly "quiet" variety). Placement of units just outside the control room was a large oversight. It is worth mentioning when I turn the AC off, the low end problems largely disappear. The main issue is clearly caused by the "climate control" being on. Keeping it off is not a feasible option in any sense as we are based outside of Houston - from now until late October, we will have very few days if any under 85 degrees, and many at or above 100 degrees. Also, the room maybe holds the low end a little to much from passing street noise during rush hour (big dumb trucks) but this is largely not an issue. We are about 75 yards or so from the road.

I guess my question is, what are my options? Moving HVAC units? Housing them? How involved is this or could it be a fairly painless thing in some ways? Could my front corner trapping be exacerbating the issue? One last thing to mention - when the AC kicks on, the power will dim. Obviously this is far from ideal. We had some issues with our power in the past related to lighting and that would cause noise, but this has been fixed for a good while now. All power issues on the "audio" side of things have been gone for some time - signal over here sounds great.

So...Educate me, send me links, scold me...but please help me! If any pros on here are available I would very much like to consult with you about correcting my issues. Thanks guys!
Old 25th May 2016
  #2
JWL
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I'm not clear on what the problem is exactly. Is the HVAC itself making too much noise? If so, often the cause is too much air moving through ducts that are too small. Is outside sound getting into the control room through the HVAC? Then it might be possible to build silencers.

From what I am reading, it seems like you are saying the HVAC is changing the way bass sounds in the room. I'd need to hear more about it, as well as possibly see a measurement when the HVAC is on and off (to compare).

The room looks fantastic by the way!
Old 25th May 2016
  #3
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Low frequency rumble in HVAC systems is a well known problem - have a read in the ASHRAE handbook on the subject. In essence - it is most likely that the noise is due to resonances inside the duct system, which in turn is generated by flow turbulences. There is no easy fix. Adding a silencer near the fan will (most likely) not fix this since the duct resonance and the overall mass flow rate doesn't reduce a lot (unless your silencer has a massive pressure drop). You could add an acoustic high-pass filter near the outlet of the HVAC into the room - but this thing is going to be huge and you'd have to tear part of the room walls down to install it.

I'd investigate splitting the HVAC system in two parts - totally disconnect the control room from the other system. Then design an independent HVAC system just for the control room (or better - get someone who knows about this). Important aspects are air exchange rate calculations, cooling requirements, max flow velocity in ducts, staged reduction in flow velocity towards the control room, acoustic calculations of the duct design to check for low frequency resonances, proper in-line silencer, careful design of bends, proper design of air outlets, check calculation that the air pressure inside the control room is actually above ambient pressure to keep the door seals tight, etc. pp.
Old 25th May 2016
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Quote:
Originally Posted by JWL View Post
I'm not clear on what the problem is exactly. Is the HVAC itself making too much noise? If so, often the cause is too much air moving through ducts that are too small. Is outside sound getting into the control room through the HVAC? Then it might be possible to build silencers.

From what I am reading, it seems like you are saying the HVAC is changing the way bass sounds in the room. I'd need to hear more about it, as well as possibly see a measurement when the HVAC is on and off (to compare).

The room looks fantastic by the way!
Thank you sir! We're called Longhorn Sound Studio and are down here in Texas. Yeah it definitely sounds like good idea to take a measurement. I'll see if I can knock that out later today.

So the problem is that when the AC kicks on and while it runs, there is a substantial amount of droning low end happening in the sub region. I'd guess somewhere around 5O Hz roughly. It is noticeable to trained ears like mine and the engineers that work here after awhile, though most do not hear it until I point it out. It extends really low, like low enough that when you are mixing it just doesn't "feel" right sometimes. It is not a "pressure" issue - it isn't coming through the floor or anything like that and doesn't cause any micrphones to pick up rumble (good construction!). I don't think it is being brought in as sound through the actual ventilation/ducts either. The issue is almost certainly just the proximity of the units just being too damn close. They are literally like 1-2 feet from the outside of my mix room.

I am assuming there is also a resonance issue contributing to it. We did a mass-air-mass, multi-leaf build. Even with all its strengths for isolating, I have read that there is often a resonant frequency that occurs between the air gap in these sort of constructions that is not dealt with nearly as well as the rest of the frequency range. So it's hyping in that frequency and it's getting through accordingly best I can surmise. I had the most experienced engineer I know in the studio today and he concurred with me pretty much 100%. He hadn't even noticed it...maybe I'm working too much!
Old 25th May 2016
  #5
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Quote:
Originally Posted by svenr View Post
Low frequency rumble in HVAC systems is a well known problem - have a read in the ASHRAE handbook on the subject. In essence - it is most likely that the noise is due to resonances inside the duct system, which in turn is generated by flow turbulences. There is no easy fix. Adding a silencer near the fan will (most likely) not fix this since the duct resonance and the overall mass flow rate doesn't reduce a lot (unless your silencer has a massive pressure drop). You could add an acoustic high-pass filter near the outlet of the HVAC into the room - but this thing is going to be huge and you'd have to tear part of the room walls down to install it.

I'd investigate splitting the HVAC system in two parts - totally disconnect the control room from the other system. Then design an independent HVAC system just for the control room (or better - get someone who knows about this). Important aspects are air exchange rate calculations, cooling requirements, max flow velocity in ducts, staged reduction in flow velocity towards the control room, acoustic calculations of the duct design to check for low frequency resonances, proper in-line silencer, careful design of bends, proper design of air outlets, check calculation that the air pressure inside the control room is actually above ambient pressure to keep the door seals tight, etc. pp.
Thanks for that. Problem is that I don't even know if that will do the trick since the units for the other zones (live room and lounges) would still be right behind me and just outside kicking on and off. & I can tell you right now that there is no way I'm tearing down walls! Sounds cool though. I find myself wishing I could load up an instance of Waves Q1 as a high pass and use it on my room. Makes no sense but glad to know there is indeed acoustic high pass HVAC technology.
Old 25th May 2016
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So...at this point I think I might have a plan. If I move the AC units about 8-10 feet or so back, and build a nice high STC rated wall between the units and the studio, it just might be on the money and solve my issues. Been reading and researching this idea since last night. 2 concrete block walls with sand cores, 3 inch Roxul Safe and Sound housed between them, all about 8-9 feet high. Going to draw something up and post soon. My mad scientist plan. Could definitely use some advice
Old 26th May 2016
  #7
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Ah - so you are seeing that what you hear could be the actual airborne and / or structural borne noise from the HVAC units, rather than noise that enters the control room via the ducts? In this case, I'd do a simple test. Take a microphone with good low frequency response and set it up near the monitoring desk. Record the rumble noise and look at the spectrum. Then disconnect the ducts from the HVAC unit completely, such that there is no air flow inside the ducts. Switch HVAC units back on and repeat the measurements. If there is very little difference, then the problem is indeed due to airborne and structural borne noise.

Then there are two things to do - reducing airborne noise emission from the HVAC units and preventing structural borne noise transmission into the building. The second one requires to install properly designed spring supports between the foundation and the HVAC units. For the design, you need to know the total weight of the HVAC units. Suitable supports are readily available on the market. Airborne noise is best dealt with by building an enclosure around the HVAC unit rather than increasing the transmission loss of an entire building wall. Obviously, a HVAC noise enclosure has to provide very good ventilation - hence you need someone to design this for you.

But in any case - you need to establish first if the noise source is the HVAC unit or the air flow inside the ducts. That determines which actions you need to take. Post a couple of photos of how you HVAC unit is currently mounted - that tells a lot about potential problems.
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