Sound Treatment for Home Theater
How to get the most out of your accoustical treatments
By Mark Wainwright
Sound Proofing versus Sound Treating
There are two basic approaches to sound control in a home theater setup. Sound proofing is the term everyone is familiar with, and it can be a little over used sometimes, but for our purposes, sound proofing a home theater means that you can crank up your speakers and not disturb everyone in the house (or neighbors). To accomplish that, you are going to have to tackle it at the construction level. In new construction, you can use a variety of dampening barriers between the house structure (i.e. studs and floor joists) and the room flooring/walls ceiling. After the room is constructed, you can add a layer of dampening then a second layer of flooring, drywall or ceiling. There are mats, glues and suspension systems that can accomplish the soundproofing. You’ll have to look for an option that will work for your situation.
A lot of people think that sound foam is for sound proofing, because recording studios use it so heavily. But those rooms are also sound proofed at the construction level. The foam and panels inside the room are for sound treatment.
Sound foam has a slight sound proofing effect, but what the sound foam is really good for has less to do with keeping the sounds in or out of the room, and is more about making sure that you hear the right sounds when you are in it.
When you are listening to speakers, all you want to hear is the bit of sound that travels directly from the speaker grille to your ear.
But, since sound travels in all directions, you have to worry about ‘reflections’ that can muddy up the sound. All the reflections can give you weird dead spots and reverberations.
In the office here we have a big empty room with bare walls that gets affectionately referred to as the echo chamber. It can be almost impossible to carry on a conversation in there. I’m sure you’ve been in similar spaces.
What to do?
You’ve probably seen recording studios with massive foam spikes and wedges covering the walls like some bizarre mix of a bounce house and a torture chamber. Those setups are designed to absorb any reflection no matter where they originate in the room. It makes for really crisp recordings. But in person, the space can feel pretty dead. Luckily, that isn’t usually necessary in a home theater.
Your home theater is going to be more like a listening room or a mixing station. There is a specific spot (or couple of spots, in your home theater) that you want to optimize for the best sound. And you will notice that in the image above, there are just a few problem areas. If you address those problems, you will have a reflection free ‘sweet spot’ that sounds great.
Allowing some reflecting sound in the rest of the room can help the space feel more natural. (Some places use acoustic panels that actually disperse (refract) the sound instead of absorbing it to make the room feel bigger than it is)
Trying to decide where to start?
I have some good news and some bad news.
First, the Bad
The example I gave is a little simplistic. We are only looking at a two dimensional view. When really the sound travels in three dimensions. So you also need to consider the ceiling, the floor, and possibly some more complex angles.
Also, my example is only showing stereo speakers. Many home theater setups now have 5 or 7 speakers plus a subwoofer (or two). Each speaker will have similar reflection points, and the bass tones from a subwoofer tend to behave separately from the mids and highs you get from regular speakers.
The more spread out your seating arrangements are, the more surface area you need to cover to block all the possible reflection points. That can add up to a lot of wall treatment (maybe those covered walls are starting to seem more reasonable after all)
Now, the Good News
Acoustic treatment follows the laws of diminishing returns--you get a lot of benefit from a little bit of treatment. You can continue to add more and more to get better and better results, but if you place them right, you’ll get the biggest impact out of your first few panels.
The other bit of good news, is that how much treatment you use is entirely up to you. When it sounds good to you, you are done. In a mixing studio or a recording space, there’s an obligation to protect every nuance of sound. But in reality, everyone’s tastes are a little different, so your only obligation is to yourself and your own preferences.
You can make the spot in your favorite chair sound amazing, and leave the rest of the room at ‘OK,’ or you can opt for a more flexible seating arrangement and get really aggressive on the treatment. Maybe you are OK with boomy bass, but you want dialog to be sharp and crisp. Then you’ll opt for fewer bass traps and more wall treatment.
Sound treating isn’t as all-or-nothing as sound proofing can be, and it isn’t usually as permanent. You can even put your sound treatment on hanging or freestanding panels, or just temporarily attach them to the walls. Then you can change as you move furniture or preferences.
Where to Start
First, let’s talk about bass. Since the lower notes have the longest wavelengths, they will travel the most. (That’s why when something balanced is played in one room, in the next room all you can hear is the bass) They’ll bounce all around a room a lot before they naturally fade away. Notes can drag out much longer than intended, and since the same wave might stay in the room for a long time, the natural peaks and nulls that exist in a room can get multiplied quickly with bass notes. They don’t react to sound treatment the same way as higher notes do, so it is worth addressing them separately.
Bass traps are designed specifically for the long wavelength notes that tend to linger. Since they also tend to accumulate in corners, foam bass traps are often formed in a 90 degree wedge shape to fit the corners of your room. Most people recommend starting by putting your first bass traps in the bottom corners behind your right and left speakers, and add as necessary.
If your primary concern is just the rumble of an explosion, you might not need a lot of bass treatment. You will want enough to keep from having dead spots, but a lingering echo won’t be the end of the world. For music lovers having a crisp bass response can be a lot more important and you might want to consider starting with wall-to-ceiling traps in the front two corners, then adding along other corners as necessary.
High and Mid reflection points
Talking about sound distortion as reflections also gives us a great way to pinpoint the trouble spots. Place a mirror against the wall and sit in your favorite spot. Where you see the reflection of the speaker, that is where you’ll need to treat the walls. (having a friend help can make this testing easier) Repeat for all the seating areas you want treated, and you’ll have a good idea of the coverage you will need.
First Things First
In the picture above, the reflected sound arrives at your ear just after the ‘true’ signal. This will often have the most harm on a good listening experience. You will lose clarity and definition, and it can create a fluttering effect to sounds that should be crisp. This is usually the first place you address (Right and left side)
You also need to be aware of indirect reflections—when sounds from the right side reflect to your left side. In addition to arriving out of phase like the first example, these sounds can narrow your stereo field, bringing all of the sounds back towards the center
Off the rear wall, you can get both direct reflections (same ear), and indirect reflections (opposite ear)
Though since the sound has farther to travel, back wall reflections tend to be weaker than and not as much of a problem as the closer reflection points, so if you are prioritizing, they tend to be lower on the list.
Surround Sound Setups
If your home theater uses 5 or more speakers, typically you will still want to treat the front right and left speakers first, then work your way through the front and satellite speakers until you are happy with the room acoustics.
Other Areas to Consider
Some speakers allow a lot of sound ‘bleed’ out the back. If this is the case, you might want to treat a reflection point behind the speakers.
Every room is different. If your theater room is very wide, the rear wall reflections might matter much more than an indirect wall reflection. Doorways, furniture, and irregular room shapes can also have a significant effect on how sounds interact in a room. It’s a good idea to test out a few different layouts of your sound treatment before you commit long term.