Where to Place Acoustic Treatment in a Home Theater

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Locate specular reflection points with a mirror and a friend

Front wall image of right surroundOnce you have carefully placed your 5.1 surround sound loudspeakers, video display, and prime listening position seating (see my article here for tips), you are ready to consider acoustic treatment for your home cinema.

Acoustically, a home theater is considered a small room (as compared to large auditoriums, concert halls, and churches). Sound behaves much differently in a small room. Low frequency (bass) response is characterized by unique modal resonances (wiki) determined by the size and proportion of the rooms length, width, and height. Above a certain range, low-mid to high frequency response is characterized by specular (mirror-like) sound reflections.

It is a common mistake to visualize sound as projecting like a laser in the direction the source points. The style of loudspeaker commonly used in home theater has very low directionality, the bass and mid-range energy disperses in all directions. The acoustic energy reflects off of hard/smooth architectural surfaces like drywall, plaster, masonry, hardwood floors, windows, etc.

These sound wave reflections arrive late in time to the listener, after the direct sound from the loudspeaker, distorting and coloring the audio quality. Even though the reflection is only delayed a fraction of a second , it can cause comb filtering (wiki), or a sense of echo. Sound that continues to bounce around within the space will be perceived as reverberance. All of this serves to degrade the natural clarity, balanced frequency response, stereo imaging, and surround localization cues possible from your sound system.

Source: Wikimedia Commons

Angle of Reflection = Angle of Incidence

So, how can we stop this echo? Specular reflections (wiki) have the most energy and follow a predictable path from a speaker, off a wall/floor/ceiling surface, and to the listener. Specifically, the angle of incidence equals the angle of reflection. The same geometry applies to the “banking” of a billiard ball on a pool table or the reflection of light off a mirror. This predictability allows us to use a simple trick to find reflection points that require sound absorption treatment. The correct placement of sound absorptive wall panels will effectively eliminate these sound reflections!

What you will need:

1) A large hand-held mirror with flat back

2) 30 Post-it page markers, flags, or notes (5 colors)

3) A -very patient- friend

First, ensure that your 5 speakers (left, right, center & surrounds) and prime listener seats are all in their final position. Changing the location of any of these will also change the reflection points on your walls, floor, and ceiling.

Next, remove all other furniture, decorations, and furnishings from the room; you will need bare walls to do this investigation.

Take one post-it note of each color and place it on a loudspeaker to identify it. Each speaker will need its own color, which will be helpful later in identifying which points correspond to each source.

Sit in your center listening position (sweet spot). I chose to perform this process for two listening positions, one on each end of the couch. But, this effectively doubles the amount of marker points you will make. So, feel free to just sit in the center seat with your head and body as you would have it while watching a movie on your TV.

Now, start with one of the side wall surfaces. Have your friend hold the mirror perfectly flat against this wall. It is important that it remain flat and exactly parallel to the wall. Slide the mirror across the wall surface until you find the reflected image of one of the loudspeakers. Mark that position on the wall (under the mirror) with the corresponding color of post-it note. Do not tilt the mirror or move your head to find the image, line the image up to the actual listener position only by moving the mirror parallel to the wall/ceiling/floor.

 

 

 

 

 

 

 

 

 

 

 

 

 

 

This will be a little tricky at first; you will have to give instructions to your friend to direct the mirror’s movement toward each speaker image. Be nice and patient to them while you both get the hang of it, or switch positions if one job is easier for you. Some reflection images will only show the back or side of the respective speaker, mark these points also. The longer wavelength energy will diffract (wiki) around the back of the loudspeaker and reflect on the wall at this point.

Each of the five speakers (Front-Left, Front-Right, Center, Surround Left, Surround Right) should have an image point on each of the four walls, the floor, and the ceiling. That means you should find 30 specular reflection points in your home theater (5 speakers x (4 walls + 1 floor + 1 ceiling)). Some of the reflection points may be blocked by the tv, light fixtures, open hallways, etc. These probably can not be marked or treated. Reflection points on windows should be marked, however.  Don’t forget the ceiling, you’ll probably need a stepladder to reach it!

Once you have marked or accounted for all of the reflection points, you will have to decide what to do with each. The reflection images from the Front-Left and Front-Right speakers are probably the most critical to address since they reproduce music and movie sound effects, followed by the Center channel (speech) and less critical are the Surrounds (ambient and behind listener effects). You will employ sound absorption materials (wiki) to eliminate these reflections.

Different sound absorbing treatments will be necessary depending on the surface material at each reflection point. My recommendations are as follows:

Surface Material                     Recommended Treatment
Hard floor (wood, tile)……………Thick rug (shag baby!) with heavy pad underneath
Carpet…………………………………..None
Lightweight Drywall………………2” thick fabric wrapped sound absorption panel (see my article here)
Masonry or Heavy Plaster………4” thick fabric wrapped sound absorption panel (see my article here)
Glass/Windows…………………….Sound absorptive curtain (see my acoustic drapery article here)

Treatment Installation Example Photos:

A big thanks to Kevin Arndt, recording engineer at The Exchange for showing me how to use a mirror to check for slap off a mixing console.

Check out our complete guide to home theater acoustics for more information on loudspeaker placement and DIY sound absorbing materials.