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Tweakers' Asylum Tweaks for systems, rooms and Do It Yourself (DIY) help. FAQ. |
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Tweakers' Asylum Tweaks for systems, rooms and Do It Yourself (DIY) help. FAQ. |
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In Reply to: Miror method and proposed string method posted by Chris_F on December 23, 2004 at 04:52:13:
What the string method does is identify surfaces where the distance from speaker to surface to listening position in total is less than the length of the string. All of the surface identified will not be an early reflection point.Your method of doing the string technique, however, is wrong. Simply using a 20' string ignores the fact that the direct path is x feet long and that path takes sound roughly x seconds to travel. The areas mapped by a 20' string will identify potential reflection areas for reflections arriving (20 - x) ms after the direct sound arrives. If you're going to worry about reflections arriving within 20 ms or so, make the string 20 feet longer than the length of the direct path from the speaker to you.
Sound largely reflects from a flat surface at an angle equal to the angle of incidence, the same as light reflects in a mirror. That's why the mirror trick works for identifying early reflection points. Yes, there is some scatter but not a great deal, and not as much as I think you're worrying about. If there was a reasonable amount of scatter then a single reflection would produce diffuse sound which it doesn't. It takes many reflections over time to produce a diffuse sound field. The angle of scatter from a single reflection is relatively small - much smaller than you're worrying about when you mark out an area using the string technique and a reflection path 20 ms longer than the direct sound path. In a relatively small room the string technique based on a 20 ms time will identify a quite large part of the wall if the speakers are relatively close to the wall, but most of that area will definitely not be an issue for early reflections for a single listening position.
Additionally, while all of the room surfaces are reflection points, most points don't reflect to the ear immediately. They reflect to second, third, fourth and even later reflection points which then reflect to the ear. Those later reflection points usually aren't as much of a problem for 2 reasons: first, the total reflection path is longer so you have the standard loss of intensity with increase in distance, and second, because there is an energy loss at each reflection point. Early reflections - which are the only reflections with a single reflection point - are much stronger because their reflection path is shorter, much closer in length to the direct path, and there is only one reflection so energy losses at reflection are least.
Yes, treating areas identified using the string technique will ensure that a large area around the listening position is free of strong early reflections, but I think you'll end up treating a much larger area than you really need to treat using that technique.
David Aiken
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Follow Ups
- Re: Miror method and proposed string method - David Aiken 16:05:12 12/23/04 (1)
- Re: Miror method and proposed string method - Chris_F 04:41:48 12/24/04 (0)
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