How to Find and Damp Specular Mirror-Like Room Surface Reflections

tmallin

WBF Technical Expert
May 19, 2010
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I find that damping room reflections is necessary in order to maximize the imaging and staging abilities of a home audio reproduction system, as well as to eliminate spurious brightness and grunge from the reproduction. The subjective magnitude of the improvements which such damping can provide are very substantial, easily audible to even the most casual listener as an improvement. To an audiophile, they are the proverbial "night and day" improvement.

But how do you know what part of room surfaces to damp? Audiophiles are more wary of "overdamping" their listening rooms than the should be. But I think that the procedure described here will assure that the room will not sound "too dead" to most listeners, at least once one gets used to the sound of hearing the space actually recorded, as opposed to the constant overlay of spurious spatiousness caused by undamped room reflections.

The procedure described here relies on the assumption that sound travels in straight lines and reflects off room surfaces much the way light reflects off a mirror--a specular reflection in which the angle of incidence equals the angle of reflection. While this assumption is not literally true, my years of experimentation and listening convinces me that it is a close enough approximation to be useful.

One way to find the specular, mirror-like reflection points on room surfaces without test equipment is to use a small flat mirror. I use one which is about 4" x 6". It has a cardboard backing to make it a bit less fragile while still keeping it thin and light-weight. I move it around flat against the room surfaces. I use two small strips of blue painter's tape to attach the mirror very temporarily flat against walls and ceiling surfaces. No tape is needed for the floor surface, obviously.

Damp with acoustical foam or dense fiberglass any spot on walls, ceiling, or floor where, when sitting in the listening seat, you can see any part of either speaker reflected in the mirror. I find it helpful to mark the spots on the room surface where the reflection of any part of the speaker begins and ends--a reflective outline of the speaker, in other words.

I find that foam or fiberglass thicknesses of 3" or more are necessary for best results. While you might think that you only have to damp the areas where you can see the actual mid and treble drivers, I find more damping as described above provides more imaging stability and also allows a bit of movement of the listener within the listening area without getting into a brighter sounding position. One reason for this is that sound waves don't really move in straight lines or bounce as coherently off room surfaces as light rays bounce off a mirror; the specular reflection assumption is not strictly true.

That will take care of first reflections. If you also want to deal with second reflections, put pieces of contrasting tape or something else easy to see on the room surfaces where you can see the speaker reflections and then move the mirror around some more, damping the additional spots where you now see the contrasting tape.

Then use a "clap" track to test the results. Track 4 of the Sheffield/XLO Test and Burn-In CD is such a track. First listen to the track with headphones, to hear what this track should sound like with absolutely no room echo. Then listen to it through your speakers. It should sound a bit more reverberant than through headphones because you haven't killed all room reverb by damping the first and second reflections, but there should be no slap echos or long reverberant tails. If you hear problem echos, experiment with additional damping to see what room surface is causing the echo.
 

marty

Well-Known Member
Apr 20, 2010
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Tom,
That is absolutely great advice and a key for best sound! However, its hard to put that small mirror everywhere you might like to in order to get a sense of where the mirror-like reflections to the speakers occur. May I suggest an alternate approach? If you use a great hand held laser that is powerful enough, you will find that ordinary white walls and ceilings serve as pretty good "mirrors". From the listening position, you can shine a laser on the wall or ceiling an if it in the right place, you will see its image appear on the speaker drivers. Now, unlike using a mirror, it will not appear as a sharp, precise narrow pinpoint beam. Rather the wall will incur some natural diffraction and spreading of the beam so what you will see is a blotch of light on the speakers, but still, it provides the info you are seeking; namely the areas of the wall or ceiling to damp. The key is to get the right laser. My preference is the green Executive series below, specifically a P75. It also happens to be a killer laser for star-gazing (green is always preferable to red). But the laser is powerful and should be respected. (Do not aim at airplanes or neighbor's homes).
http://www.wickedlasers.com/lasers/Executive_Series-55-3.html
 

tmallin

WBF Technical Expert
May 19, 2010
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Tom,
That is absolutely great advice and a key for best sound! However, its hard to put that small mirror everywhere you might like to in order to get a sense of where the mirror-like reflections to the speakers occur. May I suggest an alternate approach? If you use a great hand held laser that is powerful enough, you will find that ordinary white walls and ceilings serve as pretty good "mirrors". From the listening position, you can shine a laser on the wall or ceiling an if it in the right place, you will see its image appear on the speaker drivers. Now, unlike using a mirror, it will not appear as a sharp, precise narrow pinpoint beam. Rather the wall will incur some natural diffraction and spreading of the beam so what you will see is a blotch of light on the speakers, but still, it provides the info you are seeking; namely the areas of the wall or ceiling to damp. The key is to get the right laser. My preference is the green Executive series below, specifically a P75. It also happens to be a killer laser for star-gazing (green is always preferable to red). But the laser is powerful and should be respected. (Do not aim at airplanes or neighbor's homes).
http://www.wickedlasers.com/lasers/Executive_Series-55-3.html

Rather than playing with wicked-powerful lasers in a room where the beam could go into my eye, I'll stick with the mirror method, Marty. If you can shine the laser on the room surface spot where the speaker's reflection would be, you can also put a small mirror there temporarily with blue painter's tape. Such tape won't even harm wallpaper.

Also, when you find the "boundary" of the speaker cabinet's reflection, it is then a simple matter to mark the wall with a bit of painter's tape. I usually define the corners of that "boundary" polygon with bits of tape. Then I apply the sound-absorbing material so as to cover up that polygon with a bit or more to spare.

For those who think they want further room damping, I also like the effect of damping the tri-corners of the room (where two walls and the floor or two walls and the ceiling meet) with small wedges of Sonex. I also apply sound-absorbing material to any additional surface as necessary to kill any remaining slap echo audible with a clap track played through the speakers and listened to at the sweet spot.

In one of my earlier posts, I noted that room treatment of this type is not really an option for most people who live with others and don't have a dedicated audio room. The room treatment ends up being applied in a most decorator-unfriendly fashion. Of course, if you spend more, you can make most anything attractive enough. Acoustically transparent attractive fabrics covering the foam or fiberglass batts are an option. But disguising the critical ceiling and floor treatments is more difficult. And if you want large-diameter round Tube Traps, well . . . you'll have to work that out with your significant others.
 

Bruce B

WBF Founding Member, Pro Audio Production Member
Apr 25, 2010
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Or you can do it correctly and hire an acoustician. Best money you'll ever spend!

Room treatments placed incorrectly is worse than none at all... you just wasted money!
 

MylesBAstor

Well-Known Member
Apr 20, 2010
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Or you can do it correctly and hire an acoustician. Best money you'll ever spend!

Room treatments placed incorrectly is worse than none at all... you just wasted money!

And the wrong room treatments placed incorrectly is even worse :)
 

vinylphilemag

WBF Founding Member
Apr 30, 2010
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A quick clarification, please. My understanding is that the "first reflection points" are those places where if one placed a mirror one would see one (or both) speakers. But what are the "second reflection points"? From Tom's initial posting, my understanding is that the second reflection points are those where if one places a mirror one would see one or more first reflection points; is that correct?

In other words, to illustrate (please excuse the ASCII art):

HTML:
+----------------------+
|                      |
|    S                 |
|                      |
|A                     |
|                      |
|                      |
|                     B|
|                      |
|           L          |

L = listener
S = speaker
A = first reflection point, where a listener at L can see the speaker S
B = second reflection point, where a listerer at L can see the first reflection point A

Is that right? (Obviously, only 1 speaker is shown for simplicity!)
 

Nyal Mellor

Industry Expert
Jul 14, 2010
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Rich, yes that is correct. Although just to confuse you there is also a first reflection point for that speaker on the right wall :)
 

vinylphilemag

WBF Founding Member
Apr 30, 2010
810
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www.vinylphilemag.com
Rich, yes that is correct. Although just to confuse you there is also a first reflection point for that speaker on the right wall :)

Thanks for the clarification. (I'm aware of the left speaker's 1st reflection point on the right wall and, uh, on reflection, I should've added it to my pic!)
 

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