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We've covered the walls and the ceiling and now there's just one larger surface to look at, and it's one you might not have thought of. Your floor can carry a surprising amount of sound, especially in configurations where your listening room is above another used room in the house. Similarly, if you've got a home theater in one part of the basement, but you still use the other half; you'll find that low frequencies travel through a concrete slab quite nicely. Carpeting is a start, but it's definitely not the end-all be-all.
It's going to sound very familiar by this point, but the most effective way to keep sound from going out of your home theater to the rooms below is to isolate the floor as much as possible. You can do this by creating a floating floor. Like the very similar concept of the drop ceiling which doesn't actually drop (hopefully!) a floating floor doesn't actually float. Instead, it stands on top of a sub-floor, creating a space for air, insulation and sound elimination.
Creating a new floor isn't for everyone, and it's not practical in every instance, but it can make a big difference. If it's not possible in your theater room but sound is leaking out through the floor, you'll want to work on the ceiling below instead. You can also help to isolate the sound coming from your speakers by separating them from the floor. Speaker stands, monitor isolation pads and spiked downriggers can make a huge difference in the transmission of vibrations.
Now that we have all the big surfaces out of the way, it's time to talk about the smaller stuff that can still make a huge difference. One of the prime ways that sound can leave the room is the same way you do. Your door can let a surprising amount of noise through.
Most doors found in homes are of the hollow variety - go ahead and give yours a knock, it'll be obvious. That hollow door does very little to keep sound from escaping. Now go check out the door at the entrance to your home. It's sturdier, heavier, and better as keeping sound in and out. The simple reason is that it provides far more mass than your interior doors. A door upgrade can make a huge difference, but just moving up to a solid door isn't always enough. You'll find that there's still plenty of sound coming though, since your door isn't exactly air-tight. If you want to step it up, you really only have two choices. The first is to install a door specifically made to eliminate sound, like the ones used in recording studios. The second is to create what's called a "sound lock room." It's essentially a very small room between the studio and the rest of your house.
Another one you may not have thought of goes back to when you were a kid. Did you ever talk on a tin can telephone? How about talking to people through your house's air vents? That's right, those air vents used for heating and cooling can carry sound with them too. Luckily, the fix to keep noise from travelling through your HVAC system is a simple one. All you need to do is install a baffle box to your ducts. The baffle box looks like the maze you used to find on the back of cereal boxes and it's maze-like properties are what we're counting on. Air can get through with no problem, but due to the sound absorbing material and the series of twists and turns, sound won't stand a chance.
If you're building a particularly impressive room, you'll also want to consider the electricity and the lighting as well. Electrical boxes, for example should be sealed and separate from boxes on the other side of the wall. Though it might look nice, recessed lighting won't help your soundproofing at all. After all, installing it requires that you - or whoever's doing your lighting work - cut through the soundproofed ceiling you just installed.
Windows can be a problem when it comes to sound proofing and there's only so much you can do about it. If your home is equipped with single-pane windows then an upgrade to double-paned windows, or those with acrylic frames can be a help. Sound dampening drapes can assist in blocking sound coming in and out, but if you want the best soundproofing and you're not too worried about aesthetics, you can order or build window-plugs that can be inserted when it's time to watch a movie and removed when you're done.
You have plenty of options when it comes to soundproofing a room, but whatever you go with, make sure it's done properly. If you're handy with a hammer, you might be able to do a lot of this yourself. If you're not sure though, call in the pros.