Panel Placement (Cutting Corners)

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Let me confess that my very first lesson on sound absorption came from an Auralex catalogue. (Mea culpa, mea culpa.) I did, however, learn something useful from that catalogue, which is that bass waves, like dust bunnies, collect in the corners.

Later, when the good people of this forum enlightened me on the advantages of rigid fiberglass, I added that to my previous knowledge and assumed that I should place my fiberglass panels in the trihedral corners of my room, in the same position as one would place Auralex's LENRD foam wedges. (See Pathetic Sketch #1 in the attached drawing.)

However, as I have perused this site and all the many links herein, I haven't seen anyone putting their panels there. All the panels I've seen have straddled dihedral corners, as in Pathetic Sketch #2.

What's up with that?
 

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Wow! That was fast.

Mind Riot said:
Actually, neither of those configurations will work as well as putting the fberglass in such a way as it forms a triangle with the two walls. Read this, it'll tell you all you need to know.

http://www.ethanwiner.com/acoustics.html
Click to expand...
Thanks for the swift reply, Mind Riot. I've already read Ethan's site, and I've also seen some of his videos at RealTraps.com. Although the link you gave me does show a diagram of a bass trap in the corner between two walls (as in Pathetic Sketch #3 below), it doesn't explain why that's an especially good placement. (I'm guessing that it's because it puts the traps at ear level?)

However, I've also seen many pictures of studios with traps mounted up in the corners between the walls and the ceiling, and I don't know why they're up there. (Take a look at the pics along the top of RealTraps.com's home page, for instance.) I also don't understand why foam traps belong in the trihedral corners but fiberglass traps belong in the dihedral corners.

I need some rules of thumb to help me determine the best placement for traps around my own little studio. It may also help people to know that I'll be recording and mixing in the same 12' by 14' room.
 

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That's right, but usually you see it slid up to meet the ceiling.
 
Innovations said:
That's right, but usually you see it slid up to meet the ceiling.
Click to expand...

Is it more important the this reach the ceiling, or meet the floor?

And would the same be true for foam bass traps (that is, is it more important to reach the ceiling than reach the floor?).
 
HapiCmpur said:
Thanks for the swift reply, Mind Riot. I've already read Ethan's site, and I've also seen some of his videos at RealTraps.com. Although the link you gave me does show a diagram of a bass trap in the corner between two walls (as in Pathetic Sketch #3 below), it doesn't explain why that's an especially good placement. (I'm guessing that it's because it puts the traps at ear level?)

However, I've also seen many pictures of studios with traps mounted up in the corners between the walls and the ceiling, and I don't know why they're up there. (Take a look at the pics along the top of RealTraps.com's home page, for instance.) I also don't understand why foam traps belong in the trihedral corners but fiberglass traps belong in the dihedral corners.

I need some rules of thumb to help me determine the best placement for traps around my own little studio. It may also help people to know that I'll be recording and mixing in the same 12' by 14' room.
Click to expand...

The reason why the fiberglass would be mounted up at the ceiling, forming the triangle with the walls is that all the bass frequencies that bounce around the room build up more in the corners, where three surfaces meet. Two surfaces are good (as in your sketch) but three are better. Fiberglass placed there will essentially 'trap more bass' so to speak. And if you neglect the corners and put treatment elsewhere it will pretty much be self defeating as the corners have the most bass buildup.

Whether foam or fiberglass, absorbing bass frequencies is best done in the corners of a room. Bass wavelengths are pretty long, and as you may know, sound travels by compressing and rarefacting air molecules. But when a bass frequency hits the wall, the air is not moving there at all. It takes a few inches (again depending on bass frequency) away from the walls after the wave bounces off for the air to start moving. To steal one of Ethan's illustrations, think of a pool ball that is banked off the side rail. It may be moving at a hundred miles an hour, but at the exact point where it strikes the rail, it's not moving at all. This is why you can't make bass traps out of thin material attached to all your walls.

One of the reasons putting them in the corners is so effective is that the fiberglass is effectively at varying distances from the wall. At the center of the fiberglass sheet, where it's directly opposite the corner, it may be more than two feet from the wall. But an inch from where it meets the wall, it may be only an inch or so away. This allows the fiberglass to absorb a range of frequencies, since many different frequencies' wavelengths will be within those distances.

As these waves are bouncing off all the surfaces in a room, naturally the most effective way to absorb them is to place your absorption (fiberglass or foam) where multiple surfaces meet. In a rectangular room that so many of us have, this is the corners at the floor and the ceiling.

Foam bass traps along the wall/ceiling juncture do work and can augment the bass trapping of a room, but the best place to START is the corners.

Put fiberglass four inches thick across as many corners of your room as you can, and that'll be a VERY good start. Not just on the floor, but on the ceiling too. You can build a simple frame for them to hang them, or make a cloth covering for them and hang them with hammer in grommets or whatever you can manage. This is by far the most cost effective way to do it, and it's way more effective than foam bass traps.
 
Mind Riot said:
The reason why the fiberglass would be mounted up at the ceiling, forming the triangle with the walls is that all the bass frequencies that bounce around the room build up more in the corners, where three surfaces meet. Two surfaces are good (as in your sketch) but three are better. Fiberglass placed there will essentially 'trap more bass' so to speak. And if you neglect the corners and put treatment elsewhere it will pretty much be self defeating as the corners have the most bass buildup.
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Okay. I'm with ya. I can totally see the logic of treating corners before walls.

But riddle me this, Batman: If fiberglass panels are better than foam wedges (which I'm convinced is true), and if treating points where three surfaces meet is better than treating points where two surfaces meet (which I'm also convinced is true), then wouldn't Figure 4 (below) be a better placement of fiberglass panels than Figure 5, especially if I jam a foam wedge up into that trihedral corner behind the top panel? (And assuming that one of the panels is hung at the same level as my near-field monitors.)
 

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fraserhutch said:
Is it more important the this reach the ceiling, or meet the floor?

And would the same be true for foam bass traps (that is, is it more important to reach the ceiling than reach the floor?).
Click to expand...
in a perfectly empty room it is equally important. However from a practical standpoint at the floor level there are chairs, tables, etc, which take up space and, as a side benefit, diffuse sound.
 
HapiCmpur said:
Okay. I'm with ya. I can totally see the logic of treating corners before walls.

But riddle me this, Batman: If fiberglass panels are better than foam wedges (which I'm convinced is true), and if treating points where three surfaces meet is better than treating points where two surfaces meet (which I'm also convinced is true), then wouldn't Figure 4 (below) be a better placement of fiberglass panels than Figure 5, especially if I jam a foam wedge up into that trihedral corner behind the top panel? (And assuming that one of the panels is hung at the same level as my near-field monitors.)
Click to expand...
I guess you might consider figure 4 marginally better, but it is so much harder to actually mount the foam that way most folks just do figure 5.

In my opinion it is not necessary to wedge the foam behind, let the sound get around behind the trap and it will be absorbed by the backside, which is good.
 
HapiCmpur said:
Okay. I'm with ya. I can totally see the logic of treating corners before walls.

But riddle me this, Batman: If fiberglass panels are better than foam wedges (which I'm convinced is true), and if treating points where three surfaces meet is better than treating points where two surfaces meet (which I'm also convinced is true), then wouldn't Figure 4 (below) be a better placement of fiberglass panels than Figure 5, especially if I jam a foam wedge up into that trihedral corner behind the top panel? (And assuming that one of the panels is hung at the same level as my near-field monitors.)
Click to expand...

Figure 4 would be a better placement than Figure 5. Putting foam behind the fiberglass would help simply because foam is more absorbent than the air that would otherwise be behind there, but you'd be better off spending that money treating more corners with the fiberglass method. One really super duper awesome bass trap is not as good as five or six good ones. To quote Ethan, it's all about coverage.

One thing I feel I should mention is that bass frequencies are less directional than higher frequencies, so putting the bass traps at the level of your monitors is not really necessary. It'll be more effective to put them in the corners where three surfaces meet.
 
Innovations said:
I guess you might consider figure 4 marginally better, but it is so much harder to actually mount the foam that way most folks just do figure 5.
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Mind Riot said:
One thing I feel I should mention is that bass frequencies are less directional than higher frequencies, so putting the bass traps at the level of your monitors is not really necessary. It'll be more effective to put them in the corners where three surfaces meet.
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Cool. That's very helpful info, and I thank you for it. One more question, though: Once I've treated the four main corners of the room with 4-inch thick fiberglass panels (as in Figure 5), would it make sense to deal next with first refelections by using 4-inch fiberglass panels, thinner fiberglass panels, or simple foam patches? I'm refering here to the spots on the ceiling above my listening position and on the walls to the left and right of it.
 
HapiCmpur said:
Cool. That's very helpful info, and I thank you for it. One more question, though: Once I've treated the four main corners of the room with 4-inch thick fiberglass panels (as in Figure 5), would it make sense to deal next with first refelections by using 4-inch fiberglass panels, thinner fiberglass panels, or simple foam patches? I'm refering here to the spots on the ceiling above my listening position and on the walls to the left and right of it.
Click to expand...

Actually, there's eight corners of the room that you can (and should if space permits) treat, four on the floor and four on the ceiling. While treating mids and highs can be taken too far, you can never have too much bass trapping. If you make the room too dead for mids and highs it'll sound like a stuffed closet and it'll be uncomfortable to play and listen in, but the more bass traps you get the tighter, cleaner and more accurate the low end gets.

I'm actually in a similar boat as far as first reflections go. I got my bass traps all up and now I just finished putting burlap on my first reflection panels. I asked Ethan that very same question, what should I use for first reflection panels, and he recommended 2 inch thick fiberglass panels. 2 inches of fiberglass will extend their absorption all the way down into the mid bass region.
 
my question is this - once i go around my room and place rigid fiberglass panels on all of the corners from floor to ceiling, like in figures 4/5, wouldnt it also be a good idea to place even more along the the walls and ceiling like in figure 2? i would think the more the better.
 
stayouttamalibu said:
my question is this - once i go around my room and place rigid fiberglass panels on all of the corners from floor to ceiling, like in figures 4/5, wouldnt it also be a good idea to place even more along the the walls and ceiling like in figure 2? i would think the more the better.
Click to expand...

Yes, it would be good, if you can afford it. The more bass trapping, the better. The trihedral corners are the best place to start, but once you've got them covered the regular wall ceiling joints and floor wall joints are the next best places.
 
Mind Riot said:
Actually, neither of those configurations will work as well as putting the fberglass in such a way as it forms a triangle with the two walls. Read this, it'll tell you all you need to know.

http://www.ethanwiner.com/acoustics.html
Click to expand...


you were talking about figures 1 and 2 with that statement, but you just told me to start with the trihedral corners because those are the most important places. isnt that exactly what is happening in figure 1?
 
stayouttamalibu said:
you were talking about figures 1 and 2 with that statement, but you just told me to start with the trihedral corners because those are the most important places. isnt that exactly what is happening in figure 1?
Click to expand...

Almost, but while it should straddle the corner like that, it should hang straight down and not start out far at the top and then angle back in as in Figure 1. There's no benefit to angling it like that, in fact you lose some low end absorption because you're reducing the air space behind the panel. Hang them straddling the corner, but hang them straight down.
 
so hang it touching the ceiling as in figure 1, but just straight down, perpendicular to the ceiling? and then go all the way down to the ground? it seems to me that would be placing it just like in figure 5, which was said to not be as good as figure 4.
 
stayouttamalibu said:
so hang it touching the ceiling as in figure 1, but just straight down, perpendicular to the ceiling? and then go all the way down to the ground? it seems to me that would be placing it just like in figure 5, which was said to not be as good as figure 4.
Click to expand...

Well yeah, pretty much, except in figure 5 (and I don't know if this was deliberate or not) it appears that the panels stop short of the roof, which you wouldn't want to do. You'd want them to go all the way to the ceiling to cover that trihedral corner. So yeah, like figure 5 basically, except all the way up to the ceiling and down to the floor.

EDIT: Oh, and you would ideally not have the panels spaced like in figure 5, but all together from ceiling to floor.
 
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