Silencer box ratio

#1

I'm working my home cinema with MVHR and need to start constructing the silencer boxes soon. I've read through all the useful posts on these forums and had decided to construct a box similar to Gregwor's silencer box plan that was posted.

However I noticed that the box X and Y dimensions seem to be multiples of each other, is this beneficial in attenuating the sound or would I be better off changing the ratio to reduce problematic standing waves inside the box?

Also I searched for months for duct liner and couldn't find anything in the UK, I made multiple calls to all the big manufactures and none could help. In the end I bought Pyrasorb-S Class 0 acoustic foam which is suitable for air-conditioning equipment and duct systems, hope this helps anyone based in the UK looking for something to use.

#2

the duct attenuator has several roles: 1) attenuate sound traversing one space to another via an opening in an isolation boundary (wall or ceiling), 2) attenuate air flow noise, and 3) in some cases, provide expansion (on supply) or contraction (on return) to reduce air speeds (and thus potential noise).
as with most things, it's part of a system. motor vibrations, motor & fan noise, air noise, duct vibrations and noises, room sounds penetrating the ducts and being carried, temperature changes causing expansion and contraction of ducts and parts (creaks), vent noise, and because microphones are objective listeners, the air flow across them can induce noise.
so: your hvac system should be stable across temperature changes, provide all necessary fresh, filtered, and regulated air whilst removing stale air and airborne particulates. no transmitted vibrations or machine noises, no induced air flow noises, and finally, something to prevent inter-room sound transfers. your post sounds like you have all the other bits covered so onto the silencer design.

not sure what "problematic standing waves" you have in the box, but it's unlikely any.
what is the air flow rate - volume and velocity needed for the room(s)?
is the silencer (as part of the system) in 2 parts and decoupled in the middle? (presumes you have 2 hard boundary to pass through)?
are the adjacent spaces prone to significant sound levels? (e.g. drum room into a control room, or control room and iso booth)
are you feeding a plenum in the room to increase space volume and reduce air speed? (correct answer is yes)?
are you using a low/no noise air vent?

#3

Hi Glenn, thanks for the response. It's a great checklist to work through!

I was thinking that if a silencer box was to resonate at a certain frequency having multiple dimensions the same could compound the problem, if this is incorrect and not going to be an issue then I'll build of Gregwor's plan.

Room volume is 46.9m3
A.C.R. No./h 0.6
8.1 l/s normal flow rate, 10.2 l/s on boost.

I plan on fitting the silencer box within the joist space as an extension to the inner leaf, it'll be connected to the inner leaf with a flexible gasket of Tecsound SY70. Possibly have the grill on an extension into the room so it's venting into open space.
The MVHR unit will need to be balanced so an adjustable grill will sit on the end of a 125mm diameter opening.

Silencer boxes are only planned for the inner leaf as the MVHR unit is in the main building at the end of a large semi rigid duct run. The 90mm semi rigid ducting (75mm inner diameter) will run through acoustic insulation in a sealed joist space.

It's a home cinema that I'm building and the adjacent rooms are entrance hall and utility room, utility appliances are on the wall furthest away from the cinema room and will be fitted on Mason Industries waffle pads to keep noise and vibration to a minimum. The cinema room is connected to the house on one side and that wall will be 547mm thick. 215mm solid block/220mm stud partition full of acoustic insulation and the inner leaf which is decoupled. Do you think I would be fine with one silencer box for each air supply/return with the duct running through so much insulated joist space? The MVHR engineer want's the least resistance possible.

https://photos.app.goo.gl/jnLPJVxqaF6mHYYJ9

https://photos.app.goo.gl/pK7be86dXuZHmdLYA

#4

i think the approach is good - what is the target velocity in the 75mm duct? 125mm opening (and presumably the equiv cross-section of the silencer air path) only yields a potential 1.6x reduction in air flow velocity. ideally to reduce air noise, you'd want this down to <70m/s and prefer down to 30m/s or less. which requires more cross-section - hence the plenum recommendation - you can also (if done around the perimeter of the room) leverage the plenum for air distribution and absorption.

#5

Having no prior experience, I had not thought about any target velocity of the airflow.
At 8.1 l/s the round 75mm supply duct should have a velocity of 1.833m/3, 125mm outlet will be 0.66m/s before the vent restricts the airflow as it needs to be balanced with the rest of the system. The plan is to have this vent in it's most open setting then balance the rest of the house from this point.
I had planned on making the silencer cross section 12,656mm2 which is 2.86 times the cross section of the supply duct and slightly larger than the open vent. As the outlet vent is unable to be changed do you think increasing the silencer box volume would make any difference?
The planned soffit will house the downlights and may be used for absorption but not planned for a plenum and it would position the vent at an additional boundary, although an additional meter of space between the listeners and vent would be gained.

#6

if you're already expanding on the silencer and there is only minimal reduction at the vent, you should be ok.

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