Silencer boxes

[Hub]

I am trying to work out the silencer boxes we need to make for our small studio build.

Our air conditioning man said that on his calculations that he will use 3 x 350mm diameter supply air ducts and 2 x 400mm return air duct broken down as follows:


Live Room - 1 x 350mm diameter supply air duct and 1 x 400mm return air duct.

Control room - 2 x 350mm diameter supply air ducts and 1 x 400mm return air duct.

He said the duct sizes were calculated at 50% larger than normally specified for these size rooms to allow for reduced static pressure which in turn would reduce the air movement noise.

The Control Room has a volume of 72 cubic meters and the live room is 36 cubic meters.

With all the research so far I have deducted that I need to make the boxes, internal flow path, twice the size of the duct.

So does that mean that with a 350mm diameter duct (which actually works out to be a 310mm square duct) now becomes a 438mm square duct (Which equates to double the size)?


Also I have seen a lot of different pictures of silencer boxes both here and elsewhere with varying numbers of internal baffles.
What would be a minimum amount? 2, 3, 4?

I read somewhere that each baffle takes away 'x' amount of sound. Not sure where I heard this.

Any insight would be most appreciated.

[Soundman2020]

Our air conditioning man said that on his calculations that he will use 3 x 350mm diameter supply air ducts and 2 x 400mm return air duct

That's strange. Normally, the return ducts have greater flow capacity than the supply ducts, or at the very least the same. It's unusual to have less flow in the return duct. It's a fair amount less, too. Nearly 15%. Did you ask him why he wants less capacity in the return ducts?

Live Room - 1 x 350mm diameter supply air duct and 1 x 400mm return air duct.

That's fine.

Control room - 2 x 350mm diameter supply air ducts and 1 x 400mm return air duct.

That's a problem! That's a BIG difference. The return duct has only 65% of the capacity of the supply ducts. Why?

He said the duct sizes were calculated at 50% larger than normally specified for these size rooms to allow for reduced static pressure which in turn would reduce the air movement noise.

Sorry, but there is zero relationship between static pressure and air noise. Static pressure is a measure of the resistance to air flow. Air noise arises from high air flow velocity, not high static pressure. It also arises from non-laminar flow, or in other words, places where the air does not flow smoothly, and breaks up into turbulence, such as at tight corners or kinks or protrusions inside the ducts. You can have high static pressure with slow speeds and no air noise, and you can have low static pressure with high speed, plus kinks, angles, protrusions and therefore high noise. It would be interesting to see his explanation for why he equates high static pressure with air noise...

The Control Room has a volume of 72 cubic meters and the live room is 36 cubic meters.

So the CR has twice the volume, and therefore needs twice the flow rate, yet it has the same flow capacity as the LR, which only needs half the rate? Difficult to explain....

With all the research so far I have deducted that I need to make the boxes, internal flow path, twice the size of the duct.

Not twice the size: twice the cross sectional area. And not necessarily twice: "at least twice" would be a better way of putting it.

So does that mean that with a 350mm diameter duct (which actually works out to be a 310mm square duct) now becomes a 438mm square duct (Which equates to double the size)

Ummm... something is wildly wrong with your math! The cross sectional area of a duct that has a diameter of 350mm, is 962 square cm, or 962000 mm2. Double that is 1,924 cm2 area, so the interior dimensions of your silencer box duct liner would need to be something like 40cm x 50cm. That's HUGE! Allowing for 25mm thickness of duct liner, the outside dimensions would need to be about 45cm x 55 cm. Your boxes are going to be many meters long, and enormous.

I would be VERY interested in why he wants such massively huge ducts, which forces you to have even more massively huge silencer boxes. For small rooms like yours, I normally use 150mm duct, or maybe 200mm, but there's no need to go larger than that. I'd really like to know why he wants ducts that are 4 to 8 times the size they need to be.

Also I have seen a lot of different pictures of silencer boxes both here and elsewhere with varying numbers of internal baffles.
What would be a minimum amount? 2, 3, 4?

Depends on how much isolation you need. I would go for at least 3 baffles, preferably 4. Or more, if you want extreme isolation.

I read somewhere that each baffle takes away 'x' amount of sound. Not sure where I heard this.

Not really true. There is a relationship between insertion loss and baffles, but it's not a simple linear relationship at all.

But I would go back and check the math an the reasoning, right from the start. Something doesn't sound right here.


- Stuart -

[Hub]

Thanks Stuart I appreciate your answer. I will get back with a response.

In the meantime here is a picture of a silencer box design I came up with.

ScreenGrab221 06-Jun-17.jpg

The lid is clear to show through to the inner detail. The box would be constructed with 32 mm mdf to match the approximate thickness of the studio walls.

The 32mm lid would unscrew so we could clean the unit periodically. I thought it would be better to have curved lines through the silence to reduce turbulence noise.

ScreenGrab222 06-Jun-17.jpg

ScreenGrab223 06-Jun-17.jpg

[Hub]

I am arranging a meeting with the air conditioning fellow to go over things again. I will raise these concerns you have so we can finish the correct plan.

I will sort out the discrepancy with the size & number of ducts.

For small rooms like yours, I normally use 150mm duct, or maybe 200mm, but there's no need to go larger than that.

Do you mean, say - Control Room 2 x 200mm supply air and 2 x 200mm return air?
----------------------- Live Room 1 x 200mm supply air and 1 x 200mm return air?

Would this be adequate?


Also John has a recommendation of 25-30% fresh air into the system. Would this suffice?



Anthony

[Soundman2020]

You could do it with those curved shapes, and that would certainly reduce turbulence in the airflow, but it might not be so easy to do in practice. Duct liner comes in rolls or sheets that are a specific thickness, usually 1" or 25mm, so you'd need to have some type of backing that you can cut to that curved shape to give the form to the duct liner. But it's an interesting possibility. If you do decide to do this, please document it with photos here on the forum, so others can see how you accomplished it, and what the results are.

Having said that, there are a couple of points you are missing here: One is that in order to get good insertion loss, you need to have sudden, abrupt changes in cross sectional area at both ends of the silencer. So do not put flow reducers on the ends, as you show. You do not want slow, soft transitions here, as part of the principle of operation of a silencer box, is impedance mismatch. You need sudden changes in impedance, which forces sound waves to reflect back up the duct, cancelling themselves out.

The 32mm lid would unscrew so we could clean the unit periodically.

If you have a proper system of filters in your HVAC system, and replace them regularly, that should not be necessary. And considering that your silencers will likely be in totally inaccessible locations, you likely wont be able to do that anyway!

- Control Room 2 x 200mm supply air and 2 x 200mm return air?
- Live Room 1 x 200mm supply air and 1 x 200mm return air?

Would this be adequate?

Assuming that your control room is much larger than your live room, then that sounds about right, but you do need to do the math to confirm.

Also John has a recommendation of 25-30% fresh air into the system. Would this suffice?

That sounds about right, yes.

- Stuart -

[Hub]

Hi Stuart

Our Control Room has a volume of 72 cubic meters and the live room is 36 cubic meters.

You mentioned that you usually use 150mm or 200mm air con ducting. Is this internal diameter or external with insulation built in to the outer side? This is the week I have to get these things sorted with our Air con guy.

I am thinking based on what you suggested that we have...

- 2 x 200mm(internal dimension)supply ducts and 2 x 200mm return air ducts for the control room and
- 1 x 200mm supply duct and 1 x 200mm return duct for the live room.

What do you think of this?
I will get the specifications of the 7kw Daikin premium ducted system that he is proposing this week.

If this all works I will redesign the silencer boxes with your added recommendations and post a pic for comment.

We have now installed our glass on the control room side and expect to install the Live Room side glass this Friday.
Our back diffusion wall is now completed and we are ordering our superchunk insulation for the bass traps.
I did say I would get a video to you. Sorry about that. I will en devour to get one this week for everybody to see our progress.

Cheers.

[Hub]

Here is a link to a rough studio video update.
Apologies for the poor camera handling.



https://youtu.be/6lZM6U1ClGo

[Hub]

[saemola]

I would go for at least 3 baffles, preferably 4. Or more, if you want extreme isolation.

Hey Stuart,
I'm curious: in a scenario where there is a lack of space, assuming we only care about sound not getting into the control or recording room and it doesn't matter what happens to the rooms nearby, would it be fair to assume that the exhaust duct needs less baffles compared to the supply duct since exhaust air is moving away from the room and taking more of the sound with it, or is there no significant difference?

[Soundman2020]

in a scenario where there is a lack of space, assuming we only care about sound not getting into the control or recording room and it doesn't matter what happens to the rooms nearby, would it be fair to assume that the exhaust duct needs less baffles compared to the supply duct since exhaust air is moving away from the room and taking more of the sound with it, or is there no significant difference?

There's no significant difference. The air is flowing at a speed no higher than about 1.5 m/s. Sound moves at a speed of about 343 m/s, so from that point of view, the sound is still going to get through, but will arrive about 0.4% later than it would have...

On the other hand, since one common way of designing HVAC systems is to have the fan at the far end of the exhaust duct, with no fan at all on the intake duct, from that point of view it would be possible to have less isolation on the intake than on the exhaust... assuming that your only worry is fan noise. However, if your biggest problem is the same as what most studios have, low frequency sound getting through the HVAC ducts, then it's the same in both directions.

Bottom line: build them the same for both ends of the system.


- Stuart .

[saemola]

Interesting. I thought it was the impedance mismatch between conducts that would take care of low frequencies and that baffles were pretty much only for high frequencies.
Thank you Stuart!