Designing to block sound from coming through a patio door and still allow air in/out

Lennybob

Standard Member
Hello everyone,

I pretty much understand the basics of soundproofing, mass to reflect sound, the conversion of sound energy to heat with the use of fiberglass insulation, greenglue to again convert to heat by viscoelastic damping, decoupling.

Got that more or less, but I need to design something for my situation here (details below) that allows fresh air to get in, and old air to get out (maybe 50 cfm or more both ways, the 2 fans are each rated for 150 cfm through 10 feet of 6 inch duct so I think I should be okay there) while blocking sound, my goal is to block the sound to 50 decibels or more, A weighted. But actually Low frequency noise is a problem, there are trucks going by and stuff, it's next to a busy main road, so I'm worried this might be hard....

You can see the exact situation in the diagrams, there is a glass sliding porch door, and outside there is of course the porch. I also plan on running the electrical cord for the fan inside through one of the ducting paths, which I wouldn't think should cause any problems.

The noise in my apartment is terrible, and there is no ventilation system at all. The glass sliding door for the patio is a big part of the problem, just double glazed glass. If I didn't need to be able to get fresh air in this way, I would just put rock wool, then an inner layer of MDF on the indoor side....

But my plan is to soundproof it while still allowing air in with some sort or silencer, and actually allow air out too, so there would be an in path and and out path, both silenced. Actually I was thinking 2 silencers, one built in to the inner MDF sheet, the other into the outer MDF sheet.

I guess I would integrate the 2 ventilation paths into the same silencer assemblies?

My main problem is that I don't know how to design the silencers. I remember seeing somewhere that every 90 degree bend in an insulated duct vent gives you a sound reduction of 0.4, meaning the sound energy after the bend is 60% less, or coincidentally, about a 4 decibel reduction in sound. Does anyone know of any reference for this information? I can't find the info anywhere again and want to make sure I remember it right. I know I am sort of ignoring the differences in what happens to the different frequencies here. I'm definitely worried about low frequency sound. But just running with this, 10 bends should be about 40 decibels on both the inside and outside silencers, hopefully allowing me at least my 50 decibels total for the entire assembly, as ventilation paths would not be the weak points on the assemblies. I.e. for that area of wall they should block at least as much as a solid peice of MDF.


I have looked at the wall assembly test data PDFs from NRC, and I think the main part of it, aside from the silencers should be okay, it's sort of decoupled and 6 inches separation between 2 3/4 inch MDF boards, with 3.5 inches of rock wool in between.


The plan is cracks and stuff would be sealed with ordinary caulk and with closed cell neoprene weatherstripping, would be nice to get soundproofing caulk but I'd have to order it over the internet or something....

The MDF sheet + silencer assemblies will be screwed to the wall against the door opening for the inside wall and probably glued using caulk for the outside one. So yeah it would prevent me from using the porch, but that's fine. My only concern is maintenance if the fan breaks down.

Do you think this will this work for what I want? Is there a better way?

I've read the following pages recently:
Building a Dead Vent for Soundproofing Ventilation HVAC System | Soundproofing Company
Acoustic vents - AVS Forum

A long time ago I tried to read all up on "acoustic vents" and "silencers" and stuff that can be embedded in walls, there are the hit and miss type (never figured out how those worked) and the foam type, basically foam or fiberglass the air is forced through... but they are really expensive given I need 4 and offer pretty poor airflow, unless I bought even more of them, which would be too expensive (>$500 right there).


 
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