Where to Place Acoustic Treatment in a Home Theater

Last Updated on Saturday, 7 April 2012 05:11 Written by AcousticsFREQ Tuesday, 28 February 2012 11:50

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.

 

 


13 Comments

  1. NatureTM   |  Wednesday, 29 February 2012 at 10:32 pm

    Very clever! I saw this on hackaday.com, read the title of the article, saw the picture of the speaker in the mirror, and immediately had an epiphany. There’s something special about the simple ideas you never would have thought of yourself, but seem so obvious when someone else shows you. I live in a small apartment and worry about stuff like this. I fancy myself an audiophile who just can’t afford to be one yet. Thanks for the excellent tip!

  2. Mike   |  Thursday, 01 March 2012 at 2:59 am

    I also found you on hackaday.com. Very cool idea. Could use more lasers though ;-)

  3. n2o_skillz   |  Thursday, 01 March 2012 at 9:43 am

    I have read from these things at the Master Handbook of Acoustics (F.Alton Everest), but never got my self around applying anything. Though this example was so damn clear and straightforward I might even have to try this! :D Great and very interesting article to read. Those rock wool panels seem easy enough to build and if I can tell from the pictures that at least the ceiling ones have little gap between panel and surface which increases the absorption also if i remember correctly?

  4. AcousticsFREQ   |  Thursday, 01 March 2012 at 10:28 am

    Thanks for the encouragement! Yes, leaving a few inches of airspace behind the panel and/or using a thicker core material will improve the low frequency absorption characteristics. Some internet commenters on a few of the forums have said that a 2″ panel flat to the wall is too thin too be effective, but that that is frankly absurd. The 2″ panel is the bread and butter acoustic treatment and will be perfectly good for most home theater applications; it will yield a HUGE improvement over the existing bare drywall. If we were designing a professional recording studio or commercial theater, we would get a bit more involved, but this article is targeted at people who live in the real world!

  5. Jim   |  Thursday, 01 March 2012 at 9:03 pm

    Ha ha. Love your idea, but with all the high tech gears & you are watching BW silent movie? Just funny! ;-) Gotta go shopping 1st thing tomorrow! Good job!

  6. Kiley   |  Thursday, 16 August 2012 at 10:57 pm

    Thank you so much for the detailed DIY steps on building the soundboards. We have made many already. One question I have….. Where is the best places to place them if we live in an open ranch style house with a open floor plan- our room is 35×50 with floor to ceiling height of 17 foot. All tile and hard wood except big fluffy rug and your sound boards. Your acoustical article was for home theatres which are generally smaller. Do we need to place them on the ceilings as well? Have four children who are exacerbating the echo, echo, echo…. Any help is appreciated, and thanks again for the detailed DIY steps. I was cringing at the cost of ordering them pre-made.

  7. JHarkin   |  Tuesday, 09 October 2012 at 4:26 pm

    Hello,

    Thanks for the very useful series of articles. Like Kiley I too live in a large, open plan space which is beautiful but noisy. Any advice on the positioning of sound absorption panels in such a space would be much appreciated (on the vaulted ceiling?).

    Thanks again.

  8. AcousticsFREQ   |  Tuesday, 09 October 2012 at 7:06 pm

    Hello Jane,

    Well, the ceiling would be a good place, or any large flat wall areas. Keep in mind that for a large space like that you will need quite a few of them to make a noticeable reduction in ambient noise levels. Basically, you need to significantly change the ratio of reflective to absorptive surfaces in the space. So, one or two will not cut it. The square foot area you need depends on the cubic air volume and other materials in the space. Acoustical ceiling tile is often used for this purpose, and it typically covers the entire ceiling. Also note that you are only absorbing reflected energy and reverberation, not reducing the sound power produced by your noise sources. I hope this helps,

    -Eric

  9. tyler   |  Monday, 26 November 2012 at 11:53 am

    This is a great and extremely simple way to figure out acoustical treatment placement. Anyone can do this and achieve good results.

  10. Mark   |  Sunday, 03 March 2013 at 7:59 am

    Great article, many thanks.

    Are you moving your head to look at the mirror as your helper moves it, otherwise how do you see the mirror for surround speakers if you’re in your normal listening position and looking forward? Sorry for the silly questions just wanted to make sure as it’s not mentioned explicitly.

    Thanks :-)

  11. AcousticsFREQ   |  Sunday, 03 March 2013 at 9:23 am

    Mark,

    Thats a good question! Yes, you will have to rotate your head to see behind, but try to keep yourself in the same general position. One important not is to make sure the mirror remains perfectly flat to the wall. Do not tilt the mirror to find the position. I hope this helps! -Eric

  12. Dan   |  Thursday, 21 March 2013 at 10:48 am

    Back when only Dolby Pro Logic was around, I had gone to an acoustic class at THX and they had indicated that they wanted a fairly reflective surface in the upper areas of the room in the rear because at that time surround was not 5.1 and they wanted to disperse the surrounds so as not to sound like mono headphones.
    Seems like this idea has changed now that that there is 5.1 and 7.1
    Am I correct then in saying that we now want to prevent this “liveness” in the surround walls or is there a chance that too much acoustic treatment could deaden the room?

    Thanks in advance

  13. AcousticsFREQ   |  Thursday, 21 March 2013 at 11:17 am

    Dan,

    My opinion would be that if the room were to be used for home theater, then “dead” is the goal. Take a look at professional THX certified movie theaters; all surfaces are treated with sound absorption. This is to completely remove the room from the audio chain. This will provide a more realist soundscape during outdoor scenes, etc. However, for media production studios and music listening rooms some “room sound” or “liveness” can be necessary. Hope this helps!

    -Eric

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