John Sayers' Recording Studio Design Forum Archive

Hello there,

my studio has a plenum providing fresh air that goes through the leaves with a wooden sleeve cut in the middle to avoid flanking as suggested by Stuart, and the same goes for the plenum that takes stale air out.
The fresh air plenum has 4 baffles, whereas the stale air plenum has 3 for space concerns.

However, the air rumbling on the inlet plenum is louder than I would have expected, whereas there is none on the outlet.
This is with a 27x24 cm or 648 cm2 cross section for 350 m3 of fresh air supplied per hour, which should have the air flowing at 1,5 m/s or 295.34 ft/min.
What I find really odd is that even by setting the air supply machine at low speed, which is 210 m3/h, I still get a (less noticeable) low frequency rumble. That should give me 0,9 m/s speed for the air! Isn't that "Foley studio specs"?

Is there anything I can do to fix this now that the studio is already built? I do have access to the plenums. What if I built an "extension" baffle to put on top of the plenum, so that the air has, say, 2 extra baffles to go through before coming in?
Or is this simply a matter of the size of the register? (which at this can't be touched)
Or maybe it is a matter of turbulence: I have screwed the lid on the boxes, but not all the way (especially on the inlet) and still haven't caulked them cause I wanted to test before committing. Maybe the small air gaps are creating turbulence? However I don't know if that could be such a low end rumble.

Eager to hear your thoughts.
I'm attaching a sketch of my baffle. The duct liner is not pictured in the sketch, but the dimensions I provided (27x24) are the actual opening, already accounting for the 4cm thick duct liner (actual opening is 35x32).

However, the air rumbling on the inlet plenum is louder than I would have expected, whereas there is none on the outlet.

Did you build them identically? Same dimensions?

which should have the air flowing at 1,5 m/s or 295.34 ft/min.

Did you measure that? Or are you just calculating it? You should actually measure the flow velocity, to make sure it is within spec.

Try removing the register from the inlet duct, and see if that makes a difference. Some registers create turbulence in the the airflow, which creates noise.

How long is the straight section of the duct or sleeve, right before it reaches the register?

Please post some photos of the silencer box, as it is right now, and also the actual air speeds.


- Stuart -

Hi Stuart, thanks for your help.

The inlet and exhaust plenums are not identical: the inlet plenum (pictured in the first post) has 4 baffles, whereas the outlet plenum (pictured here) has 3.
Their openings leading into the room (which I was referring to as registers, but I'm not sure that's the correct term) are close to identical: 648 cm2 for the inlet, 653,25 cm2 for the outlet. Also, the flex duct providing air to the inlet plenum and the duct taking air from the exhaust plenum to the outside of the room are identical: 14ø cm.

The figures I provided for air flow are just calculated, as I have no means to measuring them in the real world. If that's necessary, I'll do it. What kind of instrument should I get ahold of?

So, to clarify, the "sleeve" that brings air from the the inlet plenum into the room ends up open, there is nothing covering it: it is literally a 648 cm2 opening into the wall with no grill or anything else to remove.
Same for the sleeve that takes air from the room into the exhaust plenum: an open rectangle of 653,25 cm2.
You can see the inlet (on the bottom) and the outlet (on the top) registers (or at least what I was referring to as registers). This is a picture of the inlet plenum right before being installed (the outlet plenum is on the floor to the left), minus a couple of missing pieces of duct liner. I can open it up as it is not sealed, I just drove the screws through to hold the lid in place. I'll do it as soon as I get my screw driver back. This is the same plenum, installed. This is the outlet plenum in 3d. I can't open this as it has been sealed. This is the machine providing air. As you can see the fresh and exhaust air going outside the building are left hanging for now as I was testing before committing. Hopefully this makes more sense now.
I'll update soon with pictures of the inside of the inlet plenum.

Thank you Stuart!

minus a couple of missing pieces of duct liner.

Are you CERTAIN that is duct liner? It doesn't look like it... what brand / model of duct liner is that? Also, how thick is it? Proper duct liner only needs to be on inch thick (25mm) to be effective in studio silencer boxes: that stuff looks to be thicker than that...

- Stuart -

I don't know whether "duct liner" is a branded name (like Rockwool) or a kind of material (like mineral wool), but this is a material that seems to be good for (and whose suggested applications are) duct isolation works. I'm aware it almost looks like bubble wrap, but it's actually a highly absorptive material called Stratocell Whisper (Quash) and it is mould proof, water proof, fire proof. It doesn't disintegrate or leave fibers. It's flexible, but it has a rigidity of its own. It's made of close cells expanded polyethylene.
The green line on the bottom graph shows the acoustic properties.
I picked the 4cm after consulting with you in another topic and it is indeed very effective. I can't hear a whisper from the other side of the duct (other than the rumble with the HVAC on, that is)



Do you think it is the wrong material for the job?

If the issue is related to the cross section of the register, would removing the liner from the wooden sleeves going through the leaves help? That would mean that the cross section of the final part of the duct (the sleeves are 34cm long) would become 35x32cm, going from the current 648 cm2 to 1120 cm2.
I'm not sure if you notice it in the picture, but to go from the plenum to the sleeve there is a small "step" created by the liner.

It's made of close cells expanded polyethylene

Then it is not duct liner, and it is also of no use at all, acoustically. Only open-cell materials are any use acoustically.

Think of it this way: at the microscopic level, sound is just air particles that are vibrating back and forth in place: in other words, air that is moving. Acoustic absorption happens when a sound wave moves through a porous material, such as fiberglass or mineral wool, or open-cell foam. The material absorbs sound when the sound (vibrating air) moves through it, and interacts with the fibrous, porous structure inside. With closed-cell materials, that does not work. Since the cells are "closed", the vibrating air molecules cannot get inside it, nor move through it, and therefore cannot be properly absorbed by the material.

What you have is probably EXTERNAL insulation for ducts: it is used to wrap around the OUTSIDE of HVAC ducts, not the inside.

So this is not good news: you will have to open up both of your silencer boxes, remove that stuff, and put proper duct liner inside.

Duct liner is the description of the product itself, not a brand name. Several manufacturers make it. Here are some
examples:
It is usually black or dark grey in color, and it is always fibrous, open-cell. It usually has a tough surface layer bonded onto it, to hold the fibers in, so they cannot be eroded or blown off by the air, but that surface layer is also porous.

I'm not sure if that is the actual problem with your system right now, or if changing it will solve the problem, but it must be replaced: it is not duct liner. Duct liner is expensive, unfortunately, but there are no substitutes.

- Stuart -

Thank you Stuart.

I bought this material because on paper it seemed to be efficient, not cause I thought it was cheap (it's €30/m2. ouch!)
Your explanation makes sense, but what doesn't make sense to me is that the material actually seems to be doing its job exactly as shown on the graph and as described by the manufacturer: everything from mid low frequency above is getting absorbed and nothing is coming into the room from the outside. When the box was open and I would stick one ear inside I would get the nauseous sensation that happens when one ear is experiencing the real world and the other is experiencing a dead environment, if you know what I mean.

The only sound that is getting through the duct is low frequency rumble, sub 150Hz, which is where duct liners are less effective. That is supposed to be taken care of by jumping from a small cross section of the flex duct to a cross section doubled or more in size of the plenum, isn't it?
That makes me think that the cross section at the register needs to be larger. Would making the cross section about 75% larger during the last 34cm of the duct help? (i.e. removing the "liner" in the final run)
Would it makes sense experimenting with only having the supply or return fan on rather than both?
What about trying longer runs of flex duct before getting into the plenums? I'm thinking that the return plenum has a longer run of duct getting to it and it also happens to be the quieter than the supply plenum.