I'll go on record as stating that there is no such thing as a soundproof room--at best, you can minimize the leakage of internal/external sound, but you can't completely rid yourself of it over the entire frequency band (especially the lower bass portion), at least not without spending yourself into the poor house or entombing yourself in a concrete bunker.
For the HVAC portion, if you use "S" runs in the ductwork coming into and out of the room, and you use heavily insulated ductwork, you can cut down on the noise *in* the HVAC system. Alas, even doing this won't kill noise transmission *through* the system. For this, you can look at air handling products from companies like IAC to minimize sound leakage, but it gets expensive very quickly. IAC specializes in both forward flow (where sound and air are moving in the same direction) and reverse flow (where sound and air are moving in the opposite direction) soundproofing technologies, and even though they were developed for things like jet test rooms, they could work very well in a home theater....assuming your name is Bill Gates. I have one of their "soundproof" doors (STC rated 51) and I will be the first to tell you that it is in no way even close to soundproof--but it works better than any door under $3,000. I did not use their HVAC-related products, though, because I have a dedicated HVAC zone for my theater, and I could care less what it sounds like in the AC/heater room.
Thanks for your reply. Alas, Den is my real name, so the "Gates"-option is out of the question for me. You're absolutely right in that no room is completely soundproof. I plan to build my HT as cheap as i possibly can (without making too many compromises, i hope), so i'm interested in how you all solved this problem. In my situation fresh air will be coming from outside, basically, and i want to minimize sound leakage through the ventilation system. The ventilation system will not be connected to any other part of the house, so that should make things considerably easier as vibrations are not transmitted throughout the house.
Soundproofing the HT-room will have to be a priority though, so any suggestions are greatly appriciated.
There are lots of things you can to cut down on the noise--I'm not an expert, though (but I hired an expert to help me!). I read up on this forum for about 1 year prior to building my new theater--things like staggered framing (2x6 end plates with staggered 2x4 framing inbetween), double layers of sheetrock (usually 1/2" layered over 5/8" with offset seams), floating walls/ceiling, massive amounts of rockwool/filgerglass insulation, USG acoustic caulk (for all the seams, corners, gaps, etc.), Johns Manville products (i.e., Theater-Shield+, Insul-Shield, etc.), etc. can make the difference between a noise controlled room and a normal room. I don't know if you have access to the same types of products we do here in the U.S., but you can find a wealth of knowledge here on this forum and simply from reading about products from companies like www.jm.com. Search for posts by Ted White to see some specifics!
As you did, i also have been reading all kinds of posts on this forum for about a year now, including the ones about room isolation using lots of rockwool, sheetrock, special kinds of building walls etc. This is what i plan to do in my HT room, but while the room may not be airtight, i should pay attention to ventilation so it doesn't become too hot (which is a side effect of insulating everything with rockwool, i presume) and fresh air is supplied to the ones inside.
The Johns Manville website has a section about sound control, which states that "air ducts are excellent pipelines for noise". That is exactly what i'm trying to block or at least reduce, if possible using readily available materials. As far as i know products like Theater-Shield+, Insul-Shield etc. are not available here, but i haven't been searching yet (don't know if i should considering budget ).
I will try to read up on posts from Ted White, thanks again,
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