Keeping the Sound in the Room: Soundproofing 
Home Theater Feature Articles Audio Related Articles
Written by Dick Ward   
Monday, 13 December 2010

As home theater enthusiasts, we tend to get excited about making our home theaters bigger, better, clearer and louder.  We drool over more powerful amplifiers more capable speakers and subwoofers that provide just the right amount of boom. We want best sound possible for our home theater, but when it comes to areas outside of our viewing room - when we're reading a book in the next room or trying to sleep in our beds - we just want silence.  Hearing those massive speakers outside of the home theater is a no.

Whether you've got a dedicated theater space or a multi-purpose room that you use for your home theater, there is a wide array of options available to you, ranging from the cheap and simple to the costly and complex. To know how to block the sound that's escaping your room, it's important to know just how it's getting out in the first place.  There are two possible ways for sound to get out, and more often than not you're getting a mixture of both.

The first way  - airborne - is the simplest, both to think about and to fix.  Sound waves travel through the air to reach our ears inside our listening room, so why should it be any different outside of it? Windows, vents and walls can all be escape avenues for sound waves. To stop airborne sound, you simply need to put something in front of it.  It's not the size that matters, but the mass and therefore the density.  It makes sense of course; a sheet of lead will stop sound better than a sheet of paper.

The second way sound is getting out of your home theater is through solid objects like the floor.  Like airborne sound, structure-borne sound can travel through your walls and ceiling as well, but through different avenues. The waves travel through the object itself. Structure-borne sound requires separation.  If two objects aren't touching, the sound can't get through. It's not the most complicated concept, but executing a fix can be difficult, especially if the object your audio is travelling through is the floor.

In order to conquer the task of soundproofing your home theater, you'll need to be able to defeat both airborne and structure-born sound.  The combination of these two requires a dual pronged approach.  It requires massive objects to block the airborne sound and separation of those objects to prevent structure-born. If you've lived in a less than spectacular apartment or an old house, you've experienced the dreaded paper-thin wall that allows sound to travel wherever it pleases.  The walls inside of your home are likely superior to these, but they could be better.

You'll notice the most immediate improvement to the audio isolation of your home theater by fixing up your walls. This is where people building from scratch get lucky.  There's no need to tear down walls and you won't have to worry about losing a little bit of space in your room. Most people with drywall have a standard setup that consists of sheetrock on either side of the wall, insulation in between and a single stud that both pieces attach to. As you might imagine, this isn't the best solution. Just two pieces of sheetrock and a bit of insulation in between isn't going to do much to stop that airborne sound.  To add to the trouble, using a single stud means those structure-based vibrations can travel right through the wall into the next room.

A step up from the standard is the staggered stud technique.  Instead of using the same stud, a staggered stud drywall setup utilizes separate studs for each side of the wall.  It requires a slightly wider wall, but the additional insulation and separation of the sides makes for a much more sound-proof construction. For maximum sound isolation, you'll want to add something special into the mix as well. You can go with either soundboard or a product like Green Glue that's made specifically to keep sound from travelling through the walls.  Either way, you'll be adding mass to keep the airborne sound from getting out.

You can also decouple the drywall from the framing using resilient channels - metal strips that move the drywall an extra half inch away and add an extra bit of sound isolation.  Some experts warn against these though, as they have potential to create extreme amounts of resonance at specific frequencies.  That's definitely not something you want while enjoying a movie.  Once your walls are set there's another big surface you'll want to take a look at - the ceiling.  Ceilings can be very similar to walls if you're going with the drywall approach, but there's more you'll want to take into consideration.

The thing about ceilings - the definitive thing about ceilings - is that they're attached to the floor above them.  It's not as big a deal if there's nothing above you, but if you've got a basement viewing room or a home theater with a bedroom about it, keeping sound from going out through the ceiling will be essential. There are several methods for isolating the ceiling including suspending it from specialized spring hangers, or using resilient channels to decouple the ceiling from the floor above.  Whatever method you use, separation is key to avoid both sound going out and sound coming in. Typical drop ceilings don't do much to keep the sound isolated, but with proper soundproofing it can work.  If you've already got a grid installed, for example, you can pick up special tiles, thicker and denser than those commonly used, and with much stronger sound absorption qualities.

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.

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