Finding reflection points in room

Discussion in 'Home Theater Projects' started by Thi Them, Aug 17, 2004.

  1. Thi Them

    Thi Them Producer

    Joined:
    Apr 20, 1999
    Messages:
    3,649
    Likes Received:
    0
    I have a few questions about using the mirror to find the reflection points in my room.

    1. If I have the mirror against the left side of the room, do I consider the left speaker's appearance (likewise for the right side)? If so, then the first reflection point on the left wall would be the left speaker's reflection near the front of the room, correct?

    2. Do I find the reflection points of the center speaker?

    3. When looking in the mirror, where exactly is the reflection point? Is it the middle of the face of the speaker?

    4. How much of the reflection point should I cover with absorption material?

    Thanks.

    ~T
     
  2. Wayne A. Pflughaupt

    Joined:
    Aug 5, 1999
    Messages:
    6,098
    Likes Received:
    33
    Location:
    Katy, TX
    Real Name:
    Wayne
    Thi,

    I’ve never particularly cared for this method of taming reflections, for all the reasons you’re asking questions about. The problem is that sound doesn’t behave like a beam of light (which is essentially what you’re dealing with, using mirrors). Light is narrowly focused and spreads relatively little with distance compared to sound, which behaves more like a pebble being dropped in a still pond, with waves radiating out in a wide pattern. Of course, that pattern can be somewhat controlled by various means, especially in the high frequencies, but hopefully you get the idea.

    As far as the amount of treatment to use, I’d say that depends on how much you already have. If your room has carpeted floors, draperies, bookcases and overstuffed furniture, you probably don’t need much more treatment – if any. If it’s a bare room with a concrete floor, that’s another matter.

    If you still feel you need treatment at the “reflection” points, then it only makes sense that the more wall you cover around that nebulous “target point,” the better.
    If by that you mean how do you find the reflection point of the center speaker, that would be the wall behind the viewing area, and the floor, if it’s tilted downward.

    Regards,
    Wayne A. Pflughaupt
     
  3. Ethan Winer

    Ethan Winer Stunt Coordinator

    Joined:
    Apr 6, 2004
    Messages:
    95
    Likes Received:
    0
    Thi,

    > If I have the mirror against the left side of the room, do I consider the left speaker's appearance ... <

    Here's how that works: You sit in the listening position while someone else places a mirror flat along the side walls. You look at the mirror from your seat, and any place you can see any speaker should be treated with absorption.

    Note that the ceiling is also a valid place to absorb first reflections. It's more difficult to get a mirror up there, but it can be done. I have one of those long "suction cup on a stick" things meant to change light bulbs on the ceiling. You can also use a broom stick. If you have a hand mirror you can attach the handle to the stick with rubber bands. I've done that in my recording studio and also in my home theater, and it worked well.

    For the complete story see the Acoustics FAQ, second in the list on my Articles page:

    www.ethanwiner.com/articles.html

    See the sidebar "Creating an RFZ" (Reflection Free Zone).

    --Ethan
     
  4. Thi Them

    Thi Them Producer

    Joined:
    Apr 20, 1999
    Messages:
    3,649
    Likes Received:
    0
    Ok, I've done the mirror trick. I've identified 3 reflection points (left, center, and right speakers) on each of the side walls, and 3 on the ceiling (left, center, and right speakers).

    I have the left and right front speakers set up at ear level, so the reflection points are also at that level. However, the center speaker is about a foot lower than they are. Should I place the absorption material around the actual reflection point of the center speaker or should I place it at ear level above the point?

    ~T
     
  5. Ethan Winer

    Ethan Winer Stunt Coordinator

    Joined:
    Apr 6, 2004
    Messages:
    95
    Likes Received:
    0
    Thi,

    > Should I place the absorption material around the actual reflection point of the center speaker or should I place it at ear level above the point <

    The correct place is wherever you see the speakers in the mirror. I'm not suggesting you mark the walls and ceiling with a crayon, but if you did you'd have an outline of the surfaces that need treatment. Again, anywhere you can see any speaker is where absorption should be placed.

    I'll also mention that this is very worthwhile to do, but you have to do it completely. Once the last reflection point is covered, all of a sudden the sound will open up and seem very wide - wider even than you'd expect from the speaker placement. It's similar, though maybe not quite as dramatic, to the magical huge sound you get with headphones.

    --Ethan
     
  6. Thi Them

    Thi Them Producer

    Joined:
    Apr 20, 1999
    Messages:
    3,649
    Likes Received:
    0
    Thanks again, Ethan and Wayne.

    ~Thi
     

Share This Page