There's a lot of talk in the hobby about "first reflection" (also often referred to as "early reflection") when discussing room acoustics and acoustic treatments. Getting a grasp of this fundamental acoustic concept is the best place to get started when considering the implementation of acoustic treatments for your listening space. Though acoustic treatments are certainly not a "cure-all", in most every case (when done properly) they will provide a noticeable improvement in the sonic performance of your system. I'd like to offer a simplified tutorial that explains the concept as the improvements I've realized through the appropriate use of proper acoustic treatments has been nothing short of profound and I now consider room acoustics (and to a slightly lesser degree, speaker/listener positioning) the most important factor in the performance of any audio rig.
In a nutshell, first reflection is the location on the side walls, ceiling, front wall (wall behind speakers), floor and rear wall of the listening space where the sound waves from the speakers reflect and create the shortest indirect path to your ears. Think of it like a bank shot in billiards where the speaker is the point where the ball starts and your listening position is the pocket. That point on the rail where the ball must make contact so it goes into the pocket is the early reflection location.
Why is it important? Because these reflected sound waves reach your ears within fractions of a second after the direct sound wave from the speakers, the result will be the development of an echo that will detract from your ability to hear and discern the sonic content contained in the direct sound wave clearly and accurately. The consequence of this typically manifests in the form of poor/shifted imaging, blurred sound stage resolution and generally poor overall fidelity. You don't actually hear it as an echo the way you hear an echo in a big auditorium, canyon, tunnel, etc., but it's enough to play hell with your speaker's sonic capabilities and your ability to perceive the content in the recording accurately. And keep in mind the sound waves don't necessarily stop once they reach you. They will continue bouncing around the room off of any hard, reflective surface they come in contact with. This is why a room with hardwood floors, bare walls and few absorptive furnishings will actually have audible echos compared to a carpeted room with drapes/curtains and absorptive furnishings. But don't think for a second if you have carpet, curtains and a cushy sectional sofa you shouldn't be concerned with taming these out of control reflections. Although you're better off than "hardwood floor/no curtains guy", you still don't have the sort of control over the reflections/echoes that's needed to realize optimum (or near optimum) fidelity and having proper absorption devices in the proper locations on the walls/ceiling is the only way to remedy this.
What do you do about it? Simply place an acoustic absorption panel at that location on the wall or ceiling where an early reflection is generated. These panels effectively eliminate or sufficiently weaken the strength of the reflection so the echo is eliminated or reduced to the point it no longer has an adverse effect on fidelity. There are tutorials here in the Listening Spaces forum on DIY absorption panel construction or you can purchase commercially available units (you'll save mucho dinero building them yourself).
How does one locate first reflection point? It's pretty easy, really. There's a good chance you've heard of the "mirror" method. All it amounts to is sitting in your listening position and moving a mirror along the wall until you can see the reflection of the speaker (that's on the same side of the room as the wall you're moving the mirror along) in the mirror.
Looks like EmmyLou found first reflection, too!
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So, this is where you want to place your absorption panel. Usually the optimum height above the floor puts the middle of the panel even with your noggin (the one pictured in my room could probably stand to be lowered about one foot).
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Additionally, if you want to tame the reflection from the speaker on the opposite side of the room you continue to move the mirror along the wall until you catch its reflection in the mirror. I've seen this referred to as second(ary) reflection , but really it's just another early reflection point for the speaker and wall opposite its position. In this picture I'm experimenting with diffusion/scattering , but I've since located another absorption panel to that location as I didn't like the results I got from the pictured device (so I don't encourage anyone to copy it!).
[/IMG]
To locate ceiling first reflection you draw an imaginary line between the listening position and the speaker, stand at the mid-way point between the two, look up at the ceiling directly above that spot and that is where you want to locate the middle of the panel. Panels in this location are often referred to as ceiling "clouds".
For the front wall it's usually preferred to have a few or more panels in that location (one panel directly behind each speaker and one panel at the point on the wall mid-way between the speakers has proven to work nicely for me). The thicker the panel the better as the goal here is to absorb as low in the spectral range as you possibly can...particularly mid and upper bass content as it often has a tendency to migrate to this location and it takes thickness and density to trap these lower frequencies. If it's not practical for you to have panels on the side walls (for any number of reasons including resistance from the significant other or obstructions created by decor/furnishings/structural items) getting panels up behind the speakers only should yield a noticeable improvement in mid and upper bass reproduction...which tends to bolster and improve the fidelity of mid range content. It won't do much to help image resolution (that happens with absorption at the side walls and rear wall), but some panels up on the front wall will often get you headed in the right direction and generally aren't as intrusive to the decor as they can kind of be hidden behind the speakers in many situations. And if you can provide some airspace between the panel and the wall the absorption panel's performance in terms of absorbing lower frequencies will improve. Keep in mind not all loudspeaker types benefit from having an absorptive surface behind them. If the design is such that it propagates a rear wave of sonic content...planar, ESL, and open baffle designs (all dipoles), as well as bipolar and omnipolars...it's that rear wave of sonic energy reflecting from the front wall and coming back into the room that gives them their unique (and often celebrated) sonic personality. I will add here that my experience has been having a hard, flat surface behind dipole designs is better than having absorption behind them (which kills that rear wave energy) but not necessarily ideal. I've found the performance of dipole designs actually improves if the surface behind them is diffuse, but diffusion is a complex (and fascinating) topic worthy of its own discussion. Since individual tastes vary, experimentation is encouraged as the sound you may prefer and how the room is responding may dictate absorption behind these designs is the preference.
Absorption on the rear wall (behind you) is very important as well...especially if the distance between the rear wall and the listening position is roughly eight feet or less. When this is the case I consider treating this wall just as important as treating side wall first reflection points. You can also use the mirror method for this location to find the early reflection points. A third panel located directly behind the listening position pays additional dividends.
A carpeted floor will generally offer sufficient absorption at the lower boundary surface. A hard, reflective floor will need a rug and the thicker the better. A coffee table located in front of the listening position generally creates reflections that can be tamed with throw pillows, a blanket or the like.
Have a look at this link for more information regarding treating your listening space for optimum acoustic response....
http://www.realtraps.com/art_room-setup.htm
And here's a build for DIY absorption panels....
http://www.audiokarma.org/forums/showthread.php?t=445703
- Michael
In a nutshell, first reflection is the location on the side walls, ceiling, front wall (wall behind speakers), floor and rear wall of the listening space where the sound waves from the speakers reflect and create the shortest indirect path to your ears. Think of it like a bank shot in billiards where the speaker is the point where the ball starts and your listening position is the pocket. That point on the rail where the ball must make contact so it goes into the pocket is the early reflection location.
Why is it important? Because these reflected sound waves reach your ears within fractions of a second after the direct sound wave from the speakers, the result will be the development of an echo that will detract from your ability to hear and discern the sonic content contained in the direct sound wave clearly and accurately. The consequence of this typically manifests in the form of poor/shifted imaging, blurred sound stage resolution and generally poor overall fidelity. You don't actually hear it as an echo the way you hear an echo in a big auditorium, canyon, tunnel, etc., but it's enough to play hell with your speaker's sonic capabilities and your ability to perceive the content in the recording accurately. And keep in mind the sound waves don't necessarily stop once they reach you. They will continue bouncing around the room off of any hard, reflective surface they come in contact with. This is why a room with hardwood floors, bare walls and few absorptive furnishings will actually have audible echos compared to a carpeted room with drapes/curtains and absorptive furnishings. But don't think for a second if you have carpet, curtains and a cushy sectional sofa you shouldn't be concerned with taming these out of control reflections. Although you're better off than "hardwood floor/no curtains guy", you still don't have the sort of control over the reflections/echoes that's needed to realize optimum (or near optimum) fidelity and having proper absorption devices in the proper locations on the walls/ceiling is the only way to remedy this.
What do you do about it? Simply place an acoustic absorption panel at that location on the wall or ceiling where an early reflection is generated. These panels effectively eliminate or sufficiently weaken the strength of the reflection so the echo is eliminated or reduced to the point it no longer has an adverse effect on fidelity. There are tutorials here in the Listening Spaces forum on DIY absorption panel construction or you can purchase commercially available units (you'll save mucho dinero building them yourself).
How does one locate first reflection point? It's pretty easy, really. There's a good chance you've heard of the "mirror" method. All it amounts to is sitting in your listening position and moving a mirror along the wall until you can see the reflection of the speaker (that's on the same side of the room as the wall you're moving the mirror along) in the mirror.
Looks like EmmyLou found first reflection, too!
So, this is where you want to place your absorption panel. Usually the optimum height above the floor puts the middle of the panel even with your noggin (the one pictured in my room could probably stand to be lowered about one foot).
Additionally, if you want to tame the reflection from the speaker on the opposite side of the room you continue to move the mirror along the wall until you catch its reflection in the mirror. I've seen this referred to as second(ary) reflection , but really it's just another early reflection point for the speaker and wall opposite its position. In this picture I'm experimenting with diffusion/scattering , but I've since located another absorption panel to that location as I didn't like the results I got from the pictured device (so I don't encourage anyone to copy it!).
To locate ceiling first reflection you draw an imaginary line between the listening position and the speaker, stand at the mid-way point between the two, look up at the ceiling directly above that spot and that is where you want to locate the middle of the panel. Panels in this location are often referred to as ceiling "clouds".
For the front wall it's usually preferred to have a few or more panels in that location (one panel directly behind each speaker and one panel at the point on the wall mid-way between the speakers has proven to work nicely for me). The thicker the panel the better as the goal here is to absorb as low in the spectral range as you possibly can...particularly mid and upper bass content as it often has a tendency to migrate to this location and it takes thickness and density to trap these lower frequencies. If it's not practical for you to have panels on the side walls (for any number of reasons including resistance from the significant other or obstructions created by decor/furnishings/structural items) getting panels up behind the speakers only should yield a noticeable improvement in mid and upper bass reproduction...which tends to bolster and improve the fidelity of mid range content. It won't do much to help image resolution (that happens with absorption at the side walls and rear wall), but some panels up on the front wall will often get you headed in the right direction and generally aren't as intrusive to the decor as they can kind of be hidden behind the speakers in many situations. And if you can provide some airspace between the panel and the wall the absorption panel's performance in terms of absorbing lower frequencies will improve. Keep in mind not all loudspeaker types benefit from having an absorptive surface behind them. If the design is such that it propagates a rear wave of sonic content...planar, ESL, and open baffle designs (all dipoles), as well as bipolar and omnipolars...it's that rear wave of sonic energy reflecting from the front wall and coming back into the room that gives them their unique (and often celebrated) sonic personality. I will add here that my experience has been having a hard, flat surface behind dipole designs is better than having absorption behind them (which kills that rear wave energy) but not necessarily ideal. I've found the performance of dipole designs actually improves if the surface behind them is diffuse, but diffusion is a complex (and fascinating) topic worthy of its own discussion. Since individual tastes vary, experimentation is encouraged as the sound you may prefer and how the room is responding may dictate absorption behind these designs is the preference.
Absorption on the rear wall (behind you) is very important as well...especially if the distance between the rear wall and the listening position is roughly eight feet or less. When this is the case I consider treating this wall just as important as treating side wall first reflection points. You can also use the mirror method for this location to find the early reflection points. A third panel located directly behind the listening position pays additional dividends.
A carpeted floor will generally offer sufficient absorption at the lower boundary surface. A hard, reflective floor will need a rug and the thicker the better. A coffee table located in front of the listening position generally creates reflections that can be tamed with throw pillows, a blanket or the like.
Have a look at this link for more information regarding treating your listening space for optimum acoustic response....
http://www.realtraps.com/art_room-setup.htm
And here's a build for DIY absorption panels....
http://www.audiokarma.org/forums/showthread.php?t=445703
- Michael
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), and well-presented. Well done! :thmbsp:
