So I'm setting up my basement as my record player poker lounge. It's not a finished basement, so the hvac vents run along the ceiling, exposed, and transfer all the sound of me jamming out down there to every room in the house. Sound leaking into the first floor isn't such a big deal, but if SWMBO is on second floor sleeping, the vents carry that sound up to her room like a metal drum, and if there is one thing I know it is that I don't want angry wife yelling down the stairs for us to quiet down. Is there a product that I could wrap the exposed vents with that would minimize this issue? Or is that a lot of work without much effect? Or is finishing the ceiling and covering joists, vents and all the way to go?
I've done searches for soundproofing AC vents, but most everything I can find is for recording studios to minimize sound of the ventilation system itself coming in to the recording studio. I'm just trying to prevent sound from my stereo and poker buddies from getting into the vents.
Ah I am your man here! Hah. Beyond the fact that I provide engineered solutions to clients for soundproofing, I also did this myself for my own home theater.
Soundproofing an AC vent is very possible and VERY expensive. There may be half measures that help, but a total solution is likely not what you want. It costs more than stand-alone AC.
Now is your basement already soundproofed? What is the level of sound isolation between your jamming out room and the rest of the home? While HVAC can be a direct conduit that pipes the sound around, spending heroic measures on the HVAC and not having addressed anything else may be somewhat fruitless.
In any case, let's start with the theory of what we are trying to do. You don't really want to soundproof. While a commonly used term, the reality is we can't soundproof anything. Instead, the goal is transmission loss. You want to reduce the volume of the sound that is transmitted. And ducts transmit sound like you wouldn't believe (or would you, many science museums have such communication devices to show how well a pipe can pipe sound over great distances).
Some will tell you that flex duct is a solution in home theaters, but that misses the point of the problem, its not a great solution. So instead we should think about the problem, the domains that solutions fall into, and from there you can develop a solution.
There are two types of noise escape that happens with ducts, Breakout noise and transmitted noise. Breakout noise is the noise that escapes the walls of the duct itself. It is like sound that escapes through the walls. Noise that transmits through the duct is the noise that follows the duct and comes out of openings, your floor registers. Both are a problem you have.
How do you stop breakout noise? Use a rigid, massive, and solid material. Steel ducts are villified but actually good for this. The problem is that steel ducts have a resonance that causes a narrow frequency band where high noise transmission is possible. It also increases noise transfer through the duct. So to fix that you damp the duct material itself and line it with duct liner. A fiberglass material that lines the inside to absorb sound as it bounces around. To do this and not impede the flow of air, you need to increase the duct size. In my case, all of my ducts are 2-4 inches larger than needed where I added duct liner.
To stop sound from escaping into rooms, you may also want to ensure that anywhere the sound could escape is itself sound isolated. In my case, that means ensuring that the places where ducts run into floors or ceilings has some sound isolation such as MLV, Fiberglass insulation, etc. around the outside of the duct.
Duct liner is part of how you stop sound from transmitting through the duct, but it only works so well. You need to do a few other things. First, you need to decouple the duct to stop the duct itself from conducting the sound. Think about why a pair of cups and a string held taught can transmit sound and you get the problem. Damping compound helps, but is not a complete solution. Fiberglass helps, but is not a complete solution. Instead, you want to create a break in the hard ducting using something isolating. They make flexible couplers for this purpose. Flex duct with fiberglass lining could be a solution here, it allows significant breakout, so you have to be careful where you use it, but its an option. It also can impede airflow.
Ok so what are the better ways to address this problem. You need to make it so sound doesn't have a direct path between the duct openings at each register. First, I like to use expansion chambers with indirect paths. Imagine a straight duct that is broken apart with a large box many times the volume of the primary duct path. Then create an indirect path for sound to enter and exit the expansion chamber and have the entire box lined with sound insulation. I often have steel barriers put in place to block direct sound and add absorbing triangles to help absorb and reflect sound toward the walls. Expansion chambers dramatically lower the velocity of air and help improve absorption.
Mufflers: They work, but not the cheap ones that look to just be an enlarged section of duct lined with foam or fiberglass. It isn't that those don't work at all, but you need 10 feet or more to make that work at all, and it doesn't work well. Better mufflers work like my mentioned expansion boxes, they force sound through a labrynth that absorbs sound. These are REALLY expensive to have made, and they must be custom made for your needs. They are also HUGE.
Ok so what might be an inexpensive solution that you could do? Remember that I said that creating expansion chambers and indirect paths with a labrynth was a good idea? You can do this in your room too. Build a box that does this over the register. You need to be careful, maintain appropriate aspect ratio and total area, don't make any points where the air can get trapped too much or create a lot of turbulance, but this can be done. I have designed a few of these and they were built out of ductboard. I plan to make a pair of these for my home theater when I have some time.
you need to think big, however. For an 8x14 duct opening, you need the area of the path to be at least the same, and aspect ratio shouldn't change too much from that. Ideally, the area should be larger, a lot larger. The total path length needs to be 10-20 feet.
This is what I suggest doing. However, I would do it and try to address some of the other problems I mentioned. You still likely need to damp and isolate your listening room ducts from the trunk with a flex coupler and damping compound.