Silencer box design

#1

I'm doing a garage-to-band-practise-room conversion. This, after years of exploring my needs/options and I think I'm all good; Concrete slab, mass-spring-mass, mini split unit aircon. Since my external leaf is almost finished (double sheets of 18 mm osb all walls and ceiling) and nearly ready for the inner leaf, I'm (again) reading up on the silencer boxes for the fresh air supply and stale air return. It's clear to me that I need silencer boxes and that they should match the density of the leaf that the air duct is penetrating. What's not clear to me is if I need to make baffles inside the box and I can't decide if I want a "straight-look-through" silencer (with no baffles inside the box) or the baffled silencer box. I know of the impedance mismatch that needs to occur in the air path but that is relevant for both silencer designs.

The baffles create a longer path through the box, and thus more attenuation of sound. The 'straight look through' box is easier to build, can have thicker duct liner, creates a lot less turbulence. I found this thread on Gearspace with quotes from Eric Desart and others https://gearspace.com/board/studio-buil ... -look.html
With some interesting statements;
"The narrower the exhaust, the better the attenuation. The thicker the wool, the better the attenuation."
"Even with all the absorptive material on one side of the silencer only, in order to maximize the absorption thickness."
There is more than enough length for your damper without the need to apply a complex duct.
Just straight look-through works as good for sound as long as you got around 40/60 air-to-absorption ratio.

Seems to me that the baffled box is best suitable when you don't have the space for a long silencer box. I made some simple drawings and with a zigzag pattern with 3 baffles, the path of the air is roughly 1.6 times as long as the straight line from entrance to exit.
If you do however have the space to fit a long silencer box, a straight-through-box is easier, faster and maybe even cheaper to build.

Any thoughts on this?

#2

depends on what you need to silence. the partitioned attenuator will have a longer path but also prevents any direct passthrough - particularly mid-range sound and transients. the straight-through when you have the length, is good for steady state like fan noise.
i tend to always use the partitioned attenuator as i don't have to be concerned if there are transients and i almost always pair that with a an in-room plenum for expansion and velocity reduction. there are a couple of spots to address -- the exterior side of the attenuator -- should for most or all of its length (and sides etc) should match the main isolation boundary of the room -- however the second spot -- the connection between the attenuator and the decoupled inner boundary. simply connecting the isolation boundary and the attenuator potentially exposes that short space to the exterior sounds, and hard connecting means a loss of the decoupling...

to solve that i usually nest a pair of duct board enclosures so they slide, then add gwb to the outside of assembly with a small gap. and finally seal the small remaining gap thoroughly to complete the "shell" and if not using the duct board for the ducting, then the airway is connected via flex duct. this pretty much eliminates the hard contact and the open exposure issue.

#3

Thanks Gullfo, sounds reasonable to me. It's just that Eric Desart was quite the authority on sound isolation. The sound passing straight through might be reduced by the narrow exhaust, he's talking about.
Maybe I'll find time to build 2 micro silencer boxes from scrap wood and measure the difference in attenuation. Off course, it won't be a scientific lab test but hopefully accurate enough to see the difference in performance between these two types of silencers.

One more thing; I know the coupling between both the inner and outer leaf attenuator should not be rigid. I have a piece of expanded polystryene (eps) duct https://www.ubbink.com/int/products/ventilation/aerfoam-insulated-mass-flow-ductwork/ducts/aerfoam-125-duct-2m/ Would that duct be flexible enough to prevent flanking noise?

#4

agreed, however you need to pay attention to speed and vent noise. even if you cannot hear moving air, your microphones can. so straight through units work when the air velocity is low. <1m/s and you have low noise registers. so regardless of the attenuator used, i would still have a plenum in the room space so you can expand the volume and reduce the velocity before venting. same on the return.

#5

gullfo wrote:Source of the post i would still have a plenum in the room space so you can expand the volume and reduce the velocity before venting. same on the return.

Do you have a picture of such a plenum? I think I understand what you mean but want to be sure.

#6

in this example - ductboard encased with plywood. if the entry levels are low enough, a simple ductboard box would suffice -- making sure that the mount for the register is properly secure

#7

Thanks! I don't think I'll have the space for a plenum; the dimmensions of my room are 4,5 x 3 x 2,2 m within the inner leaf. I'll try my luck with the 4 silencer boxes and if it's too noisy, I can add a plenum later on. It'll be most of all a practise room and the recording I'm doing (if any) is in the box -> guitar/bass into vst. I'm more concerned with sound escaping than intruding my room.

Just for reference: It seems the more common name for "silencer box" is "baffle box". This video shows a design by Rod Gervais: https://www.youtube.com/watch?v=lSQE78UScW0 The drawing certainly looks like the one that's in his book.

In the meantime I did some research for suppliers of ductliner here in the Netherlands. Luckily I found one that has the right stuff. If you're in (Western) Europe you might find it easier to look for Knauf Duct Liner, Isover Climliner 602 slab or Paroc Invent. Ductliner certainly can't be found by regular insulation shops.

#8

in commercial spaces -- HVAC Duct Noise (or Sound) Attenuator :-)

for me, a plenum is the final step to getting the air entry (or exit) positioned correctly -- centered / even placed for critical listening or maximum cross-ventilation for live spaces. and since i'm almost always going to put in a ceiling soffit for absorption (mainly) i put the various things needing routing there (ducts, conduits, etc) which keep things looking neat without taking up a lot of room. yes - duct liner (or my preference - duct board - which a thin hard surface with the duct liner attached - and easily formed via duct tape (metallised, not cloth) into ducts, plenums, attenuators etc) are hvac specific so you would need to check hvac suppliers.

#9

If I use 125 mm round duct for my ventilation and go to 16 x 16 cm square inside the baffle box, which is roughly twice the size, my attenuator/silencer/baffle box

will be 93 x 40,5 x 25 cm. This is with 3 baffles inside the box. I can't go bigger due to restricted space between the joists. Will that do enough attenuation you think? I can make the box longer but not wider.

#10

it should be. the main use of the attenuator is to reduce duct-borne sounds and in our studio designs, cover the opening to reduce sound getting in or out of the hole we cut in our isolation mass layer :-)

so, if the air velocity is low and the volume is correct, the equipment isn't noisy and the ducts are not creating noise, then most attenuators could just be a section of lined duct. if there are any of the aforementioned noise sources, then the bends in the attenuators improve the reduction by exposing more surface and eliminate a direct path. at the cost of increased flow resistance.

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