Acoustic Flex Duct & Baffle boxes

[Paulus87]

After extensive discussion with a few professional acousticians, including Philip Newell, I feel it is beneficial to share some information about acoustic flex duct & baffle boxes for those of us who were and are unaware of this interesting phenomena...

Providing the duct runs are long enough (But also not TOO long, let's say under 20-30m) and the penetrations are well sealed, the use of baffle boxes is not necessary unless incredibly high isolation is required.

I'll explain why:

Acoustic flex duct has very good HF & MF attenuation, manufacturers of these ducts should provide data so that you can calculate how much attenuation according to the diameter and length. The attenuation occurs because of the duct material but also because of the length. 10m (a standard size from most manufacturers) in general is more than long enough to "outrun" the HF & MF wavelengths.

What about LF?

Here is a very over-simplified explanation to a phenomena that occurs. It is true that the material that these ducts are made from is not very good at attenuating LF by itself (it essentially has very little mass) BUT (here's the clever bit) LF cannot actually leak very much at all through small penetrations (let's say diameters of 315mm and under). This is due to a large impedance mismatch between a large body of air (such as a control room) and a relatively small hole. The wave lengths are too large to fit through the holes, and as they attempt to squeeze through, so much energy is lost in the process that it becomes a non issue.

This is because air is rather springy, for example, much effort is exerted by the loudspeaker as it attempts to push the sound out, more energy is actually used moving the air out of the way and then back again. This is similar to what happens with holes and LF, you could view a hole a bit like a speaker. In fact, the hole acts a bit like an acoustic barrier and LF is reflected back into the room (rather than through the hole) due to this difference in impedance.

If a woofer was to be placed in direct contact with a penetration, and the woofer was more or less sealed to the hole, then the woofer is no longer firing into a large body of air first before hitting the hole as the speaker is already in the hole (this is what happens when we flush mount our speakers) and so the woofer and the hole would be coupled, and in this case you would indeed not have the same occurrence since there would no longer be an impedance mismatch between the large body of air and the hole. Move the woofer away from the hole out into a larger space, and then listen from behind the room with the hole in it and you will find a distinct lack of low end coming through the hole. There will be some, but it will be greatly attenuated. This attenuation will improve with distance.

Another way to look at it is what happens with earbuds. When the earbuds are pressed into your ears and more or less sealed to your ear canal you hear full bass, the tiny earbud driver is very efficient and as it moves in and out at LF the air it needs to push is small, and so we can still hear those large wavelengths. However, take the earbuds out of your ears and all low end disappears, in fact, providing the volume is loud enough only the very highest of frequencies can be heard.

So, all of this to say, providing you are careful then the use of baffle boxes is not required for your HVAC, which will save you a lot of time and money and effort.

A few important caveats:

- Daisy chaining ducting from one room to another (e.g. between control room & tracking room) should be avoided
- The use of inline duct silencers should be used if you are worried about excessive fan noise (this is because the fan will be directly coupled to the duct) Like these for example: https://www.ventilation-alnor.co.uk/ind ... stems.html
- It is not recommended to have very long runs of flex duct due to static pressure, but they also need to be long enough to achieve the desired sound attenuation
- Sharp "S" bends etc are not recommended, but gentle, gradual bends are okay (you don't want to inhibit the air flow)
- It is good practice to run the ducting "in" your treatment. In other words, surround it with fibre to increase attenuation further. If running ducting in a hall way or something where there is no treatment, then you can build a bulkhead around it if you are concerned with HF/MF leakage.

I will say that ultimately, correctly sized baffle boxes which are built properly is a no-brainer solution which will provide excellent results, especially if long straight runs of flex duct cannot be accommodated.

But for those of us that can accommodate it, I hope this has been helpful.

[gullfo]

thank for this Paul!

just as a side note, when not using baffle boxes - most times using a simple mass enhanced plenum on the inside of a room from which the interior ducts are freely routed, through soffits or behind clouds, a simple none-direct path to/from the opening is often enough to provide the needed attenuation.

and definitely pay attention to not routing directly between rooms or chaining - once you have good isolation / low noise - you'll find even a small penetration leak is enough to be clearly heard...

[Paulus87]

thank for this Paul!

just as a side note, when not using baffle boxes - most times using a simple mass enhanced plenum on the inside of a room from which the interior ducts are freely routed, through soffits or behind clouds, a simple none-direct path to/from the opening is often enough to provide the needed attenuation.

and definitely pay attention to not routing directly between rooms or chaining - once you have good isolation / low noise - you'll find even a small penetration leak is enough to be clearly heard...

No worries - could you elaborate on "mass enhanced plenum"?

Is it simply a long box made from drywall or something that goes from one wall to another (sealed to the walls) with the duct inside?

[DanDan]

Despite the fact that I concur, and perhaps helped hash out the conclusion, I remain sceptical. I will have to see and hear this tested.

[gullfo]

yes, instead of simply having a plywood or metal box, it gets wrapped with a layer of drywall, or even two so there is no direct path from the opening although its only a simple 90° turn, it help reduce the duct noises (if any - fans, creaks, etc) and any inter-mass noises coming in via the ducting.