For part 4 in our home theater design and installation series, we’ll look at more interior room acoustics and some home theater lighting design considerations. One of the main goals in home theater is to get great overall sound quality and dialog intelligibility. The sound that leaves your speakers should sound the same after it travels through the room to your ears as it did when it left the speaker. One way to ensure this happens is to treat the interior surfaces of your theater with acoustic room treatments.
Acoustically, here are three main types of room surface treatments; absorptive, reflective and diffusive. They can be used in different combinations to help achieve great sound in your home theater. One common mistake encountered when treating a room is to make it too “dead” acoustically. This occurs by using too much absorptive treatment. In the rear, especially, it is desirable to have the room a bit “live” so the sound can bounce around and create an enveloping surround field.
As a starting point, the front of the side wall can be treated to minimize the “first reflection” The first reflection is the reflection caused when the sound leaves the speaker and travels to your ear. It takes a direct path and a reflected path. The first reflection path is the one created when the sound reflects off one wall surface before reaching your ear. Because the reflected path is longer, the sound takes longer to reach your ear than the identical sound that travels directly to it. This can cause a “smearing” of the sound and a loss of intelligibility. A simple trick to find the first reflection point on the wall is to use a small mirror. Sit in your primary listening position. Place the mirror flat on the wall and then move it around until you can see the speaker in the mirror. That is the first reflection point. Use this procedure for the center channel speaker and the left and right speakers. It is sometimes beneficial to use this technique on the ceiling of the theater as well as the walls.
Treat the first reflection point for the center speaker and the wall’s corresponding speaker. For example, find the first reflection on the right wall for the center and right speaker, and on the left wall for the center and left speaker. The areas indicated as the first reflection point can be treated with absorptive wall panels to reduce the first reflection. Use at least 2″ thick panels. The most commonly used are made of packed fiberglass. The fiberglass is usually wrapped with decorative, acoustically transparent fabric.
The thicker the absorptive panel, the lower the frequency at which it will stop being an effective sound absorber. A 2″ thick panel will work down to around 500Hz or so. Acoustic panels thinner than 2″ just have the effect of reducing the high frequency energy in the room. As one esteemed acoustics expert put it; “You’re just turning down the tweeter”.
It can be effective to use panels greater than 2″ thick, but these can be unattractive when hung on the walls. In the event it is necessary to use thicker panels, the entire depth of the wall cavity can be used. To do this, remove the interior wall surface and fill the stud bay with acoustic treatment. Cover the cavity with a traditional acoustic panel or create a fabric panel to fit flush with the exterior surface of the wall and affix it to the studs. Be advised that using this technique will put a large hole in the interior sound barrier if you have installed any. This will substantially lower the Sound Transmission Class (STC) of the wall. If you wish to reclaim the lost STC, measures must be taken to restore it by restoring an effective sound barrier on the exterior surface of the wall.
The best acoustic results in a home theater are usually achieved with a combination of treatments. Diffusion is the act of scattering the sound in many different directions. Diffusive treatments can be very effective at improving the interior acoustics of your home theater. Diffusive panels are available from companies such as RPG. One popular product from RPG is the BAD (Binary Amplitude Diffsorbor™ ) panel. These diffusers are relatively thin and can be hung on the wall like absorptive panels. Because diffusion scatters the sound in many different directions, it doesn’t sound so acoustically dead like a room will with too much absorption. Diffusion is very effective at the rear of the room. A bookcase filled with books of different sizes makes a great, inexpensive (providing you already have one) diffuser. Placing one on the rear wall of the room can work wonders. Diffusive elements on the rear wall help eliminate the reverberation caused by the sound that travels directly to the rear wall.
If the layout of your home theater dictates that your front left and right speakers are placed close to the side walls, some acoustic treatment will help reduce comb filtering. Comb filtering is caused by alternate cancellation and reinforcement of certain frequencies, causing the frequency response of the affected speaker to resemble a comb. You can easily hear the effect of this by placing a speaker very close to a wall when playing pink or white noise. White noise is an audio signal that has equal energy at al audible frequencies. It sounds very much like the static you hear on an FM radio when it is tuned away from a station. Notice the difference in the sound of the pink noise when the speaker nears the wall, as opposed to when it is farther out into the room. See how the sound changes when absorptive treatment is placed on the wall immediately adjacent to the speaker. It sounds much more like the sound of the speaker when it is away from the wall.
If you have to place your front speakers inside a cabinet, you can severely alter the characteristics of the speaker, in a bad way. If you must do this there are two techniques that can help reduce the damage. First, you can stuff the entire cavity that the speaker is placed in with some acoustic absorptive material, such as fiberglass insulation or Dacron holofil, at a density of approximately 1 pound per cubic foot. A better approach is to create a baffle that extends the speakers baffle to the sides of the cavity in which the speaker is placed. Extending the baffle will cause a rise in low frequency response and may make the speaker sound a bit “tubby”. This is much preferable to having the speaker in a cavity however. In most home theater applications, the front speakers will be high pass filtered anyway, which will help reduce the low frequency enhancement.
For basic room acoustic treatment, remember the following:
* All things in moderation, you can easily over treat a room.
* Try some diffusive elements in the rear of the room.
* 2″ absorptive panels are typically much more effective than 1″ panels.
* If your left and right speakers are very close to the side walls, put some absorptive treatment on the wall to reduce comb filtering.
Basic Home Theater Lighting
When planning your lighting, remember how you will be using your theater and your desired dÃ©cor. If you will be watching a lot of sporting events and TV, it is usually more comfortable for there to be a bit of light in the room. For rooms that will be used mostly or entirely for movies, you will probably only want light at the beginning and end of the feature. You must plan your lighting accordingly. A good, basic layout is to have recessed can lights in the ceiling and sconce lights on the side, and possibly rear, walls. There a literally thousands of different sconce lights. Choose one that fits the desired dÃ©cor of your home theater.
There are many types of recessed can fixtures too. They come in 4″, 5″ and 6″ sizes. They can accommodate different types of bulbs. Bulbs are available in low voltage, line voltage halogen and standard incandescent bulbs. There are also many different types of can trims. You can use whichever combination suits your taste. Consult your lighting sales person and/or electrician, and look at the available units.
If you have can lights, and I highly recommend that you do, split the circuit so that you can control the front and rear cans separately. This will allow you to leave some light on in the rear of the room while leaving the screen in darker conditions. Recess the bulbs as far as possible in the fixtures. This will minimize your ability to see the bulbs from the side, reducing the distraction caused by the lights and its intrusion on the screen. If your home theater has stairs or seating risers, remember to provide stair lighting for safety. Rope light under the stair treads or recessed side fixtures work great for this. Put the stair lighting on a separate circuit as well.
For god’s sake, use dimmers. Dimmers are essential to create appropriate lighting ambience for a theater environment. There are several types of dimmers that can allow remote control of your lighting, either using an IR remote or a lighting control system. Remember if you are using low voltage lighting to check your lighting fixtures. There are two types of low voltage transformers, magnetic and electronic. Using a dimmer that is incompatible with the type of transformer you are using can be dangerous. Your electrician will be aware of the correct type.
You should now have enough information to begin planning the interior room acoustic treatment and lighting of your home theater. Next time, we’ll look at choosing equipment and speakers.