rectangular bass traps in tight corners?
Posted: Mon Mar 03, 2014 4:14 am
Hey all;
Location: Hamilton, Ontario, Canada (near Toronto)
Description of my space:
My room is 8' by 9'6" by 6'10". It is in a basement, so one wall (the one I face as I mix) is concrete block covered by rigid foam insulation, furring strips and drywall. Another wall is wooden slats that represent a partition wall between the studio and fruit cellar (to my left). The other two walls are drywall over 2x4 framing with Roxul insulation. On the wall to my right are two closets that go basically the length of the room, each covered with hollow-core bi-fold doors. It has occurred to me that, despite the doors to the closets and fruit cellar being closed virtually all the time, that they may be acting as resonating chambers of some sort and impacting the room in some way, but I won't pretend to have a clue as to how. For what it is worth, the fruit cellar is empty (below grade, concrete block), and the closets are full of stuff. Also, being a basement floor, it slopes slightly (a couple of inches is really about all... maybe 3") from the outer wall where the window is to the entry door behind me.
I have built and installed acoustic panels based on input from these very forums. Panels are on 1x3 furring strip frames. Semi-rigid fiberglass (similar to OC703) 2" thick in two layers for a total of 4" of fiberglass. Wrapped one layer with 6 mil vapour barrier (sides and front, but not underneath), and wrapped with a breathable cloth (cotton/poly blend). They are stood out from the wall using 3.5" lengths of ABS drain pipe so that the fiberglass is out 4" from the wall.
Three panels across the front wall. 2x2, 1x2, and 2x2.
A pair of panels on either side. 1x3
I built a 4x3 cloud by adding 4" of semi-rigid fiberglass onto a sheet of 1/2" plywood, covered in vapour barrier and then covered in cloth. The front is lower than the back, so it slopes upwards and away from the mix position, ending directly above the mix position, at an angle of 11 degrees.
Behind me are two panels (18" x 24"), one that is 12" x 12", and one that is 12" x 24".
Here is a 360-degree "tour" of my studio (with commentary) that is 1:26 and shows a lot more than pictures can, possibly. http://www.youtube.com/watch?v=GaB3dPs83EE
My "corners" issue:
SE corner - The distance between the rear-most wall and the front of the build out (hides the water meter, etc.) that contains the built-in shelves is about 15 inches. I could put something in there. There is only about five inches at best between the shelving and the other wall, though. (pic was taken before I added the acoustic panels as shown below)
NW corner - The closet door opening is 7 inches from the corner, all the way up... and then there's the window...
NE corner - the swing door opening is about 3 inches at best from the corner.
SW corner - could use this corner. There is about 20" from the corner to the fruit cellar door (the first reflection absorbers are mounted to the door).
Thinking of placing traps horizontally:
Across the west wall there is the window in the corner. I suppose I could mount something in front of it, though I would prefer not to.
Across the east wall is the sloped 45 degree bevel that comes down to cover the beam across our basement. Below that leaves about 2 inches to the top of the door.
Across the north wall is the closets. I have no more than about 10 inches at the deepest above either of those closets.
Across the south wall is the top of the fruit cellar door, and then the build-out for the duct work and stuff. Not much room there either. There is only 6" clearance between the top of the door and the ceiling.
Here is a Google Drawing of the front of my studio - the top representing the floor and the bottom drawing representing the ceiling:
https://docs.google.com/drawings/d/1O1B ... ygku4/edit
Some info I have gleaned from my last thread where I was asking about adding a subwoofer (started a new thread, because now this is no longer about subwoofers). I did buy one, btw.
All quotes from soundman2020 unless otherwise identified: (hopefully nothing gets distorted as I draw these out of context. Thread here: http://www.johnlsayers.com/phpBB2/viewt ... 12&t=18682
"10" is way better than 0"!!!! It's not huge, but pretty much anything would be better than what you have now." (I had asked about bass traps only 10" in the corners)
"Symmetry isn't critical for bass trapping, especially if is behind you. It would be good to keep things symmetrical at the front of the room, though, if possible."
"You have a big modal dip at just under 96 Hz, and a peak at 148? They were nowhere near as obvious as they are now, and what looked like a single peak at around 60Hz is now clearly a few different things, at 40, 46 and 65 Hz.
You also have a problem with grounding: There are two "tails" in the waterfall plot, at 60 Hz.and 120 Hz. that do not decay over time. They are at a constant level, so they are not acoustic issues. That is "mains hum" getting into your system somewhere. Probably lack of grounding, or a faulty mic cable, or some such. Worth trying to find and fix that.
Now take a look at the RT60 graph, and you'll see that you have a bass problem in your room. The decay time rises from less than 200 ms for the rest of the spectrum, to over 400 ms below 90 Hz.
Also take a look at the IR graph: You have some pretty large reflections hitting your listening position within 2 ms of the direct sound, so those are most likely coming off your desk or your video screens. To check that, pile up a few thick towels on your desk and repeat the test: I'm betting those will go away, and the mid range will smooth out a bit...." (I posted some REW graphs. I'll re-do them, though, as I have the sub all hooked up now, and I intend for it to be a part of my monitoring setup going forward.)
"Right now, anything at all will be useful to some extent. It's not going to tame your modes entirely, or course, but it will make some difference." (when asked about adding a corner bass trap on the floor behind my desk and breaking it up into 3 sections - wall to side of desk; under desk; side of desk to other wall. Note that, according to the google drawing, having anything bigger than 10" would require moving my speakers forward. I'm not sure if that's a good idea or not. Even then... I don't want to push my mix position back too much farther, than it is, because it's not far from where I am to the dead center of the room. (currently, my ears are about 30" away from each speaker and about 38-40" from the wall... or 33-35% mark. The dead center of the room would be the 57" mark.) I could go much further UP the wall... just not out. )
"First is symmetry, and I suspect the "nook" over on the right (referring to the closet). If you look at just the left and right speakers, for both sets (with and without towels), you'll see that in both cases there's a large dip at 95 Hz for the left speaker, which just isn't there for the right speaker, bot for both cases of the right speaker there's a large dip at 300 Hz that isn't there for the left speaker. So something in the room is sucking out energy asymmetrically, with the frequency depending on which speaker is firing. It could be two entirely different things, or it could be two aspects of the same basic problem. I'm inclined to go with the second option: two aspects of those "nooks" over on the right wall. One of the reasons I'm thinking of those is because in the "both speakers" graphs, those artifacts disappear, making me think that they cancel each other out perfectly on the room center-line, but since your ears are not on the center line (even though the mic is), your ears will be hearing the individual un-balanced signals. So it would be good to try to trace that issue and fix it.
Next, is the effect that just a few towels is having on the entire room: Those are actually reducing your overal decay time, believe it or not, and are even affecting the modal response: acting like a bass trap, and fairly effective, too, give the small volume. So imagine how effective true bass traps would be, located correctly...
" (I added towels to the top of my desk and it reduced one of the reflection points quite a lot)
So what I'm wondering....
I could build much larger bass traps if I made them rectangular than if I made them triangular. For instance, a triangular bass trap that is 18x18" would have the same area as one that is rectangular and 18x9". Would that be a useful consideration, or do they need to be triangular? I can fit a rectangular 9x18" trap in each of the front corners... even 18x10". If they have to be triangular, the best I can do is 10x10"
And of course... any other help in getting my room improved is most welcome.
Thanks for your ongoing help and future help, and thanks for enduring my lengthy post.
Chris
Location: Hamilton, Ontario, Canada (near Toronto)
Description of my space:
My room is 8' by 9'6" by 6'10". It is in a basement, so one wall (the one I face as I mix) is concrete block covered by rigid foam insulation, furring strips and drywall. Another wall is wooden slats that represent a partition wall between the studio and fruit cellar (to my left). The other two walls are drywall over 2x4 framing with Roxul insulation. On the wall to my right are two closets that go basically the length of the room, each covered with hollow-core bi-fold doors. It has occurred to me that, despite the doors to the closets and fruit cellar being closed virtually all the time, that they may be acting as resonating chambers of some sort and impacting the room in some way, but I won't pretend to have a clue as to how. For what it is worth, the fruit cellar is empty (below grade, concrete block), and the closets are full of stuff. Also, being a basement floor, it slopes slightly (a couple of inches is really about all... maybe 3") from the outer wall where the window is to the entry door behind me.
I have built and installed acoustic panels based on input from these very forums. Panels are on 1x3 furring strip frames. Semi-rigid fiberglass (similar to OC703) 2" thick in two layers for a total of 4" of fiberglass. Wrapped one layer with 6 mil vapour barrier (sides and front, but not underneath), and wrapped with a breathable cloth (cotton/poly blend). They are stood out from the wall using 3.5" lengths of ABS drain pipe so that the fiberglass is out 4" from the wall.
Three panels across the front wall. 2x2, 1x2, and 2x2.
A pair of panels on either side. 1x3
I built a 4x3 cloud by adding 4" of semi-rigid fiberglass onto a sheet of 1/2" plywood, covered in vapour barrier and then covered in cloth. The front is lower than the back, so it slopes upwards and away from the mix position, ending directly above the mix position, at an angle of 11 degrees.
Behind me are two panels (18" x 24"), one that is 12" x 12", and one that is 12" x 24".
Here is a 360-degree "tour" of my studio (with commentary) that is 1:26 and shows a lot more than pictures can, possibly. http://www.youtube.com/watch?v=GaB3dPs83EE
My "corners" issue:
SE corner - The distance between the rear-most wall and the front of the build out (hides the water meter, etc.) that contains the built-in shelves is about 15 inches. I could put something in there. There is only about five inches at best between the shelving and the other wall, though. (pic was taken before I added the acoustic panels as shown below)
NW corner - The closet door opening is 7 inches from the corner, all the way up... and then there's the window...
NE corner - the swing door opening is about 3 inches at best from the corner.
SW corner - could use this corner. There is about 20" from the corner to the fruit cellar door (the first reflection absorbers are mounted to the door).
Thinking of placing traps horizontally:
Across the west wall there is the window in the corner. I suppose I could mount something in front of it, though I would prefer not to.
Across the east wall is the sloped 45 degree bevel that comes down to cover the beam across our basement. Below that leaves about 2 inches to the top of the door.
Across the north wall is the closets. I have no more than about 10 inches at the deepest above either of those closets.
Across the south wall is the top of the fruit cellar door, and then the build-out for the duct work and stuff. Not much room there either. There is only 6" clearance between the top of the door and the ceiling.
Here is a Google Drawing of the front of my studio - the top representing the floor and the bottom drawing representing the ceiling:
https://docs.google.com/drawings/d/1O1B ... ygku4/edit
Some info I have gleaned from my last thread where I was asking about adding a subwoofer (started a new thread, because now this is no longer about subwoofers). I did buy one, btw.
All quotes from soundman2020 unless otherwise identified: (hopefully nothing gets distorted as I draw these out of context. Thread here: http://www.johnlsayers.com/phpBB2/viewt ... 12&t=18682
"10" is way better than 0"!!!! It's not huge, but pretty much anything would be better than what you have now." (I had asked about bass traps only 10" in the corners)
"Symmetry isn't critical for bass trapping, especially if is behind you. It would be good to keep things symmetrical at the front of the room, though, if possible."
"You have a big modal dip at just under 96 Hz, and a peak at 148? They were nowhere near as obvious as they are now, and what looked like a single peak at around 60Hz is now clearly a few different things, at 40, 46 and 65 Hz.
You also have a problem with grounding: There are two "tails" in the waterfall plot, at 60 Hz.and 120 Hz. that do not decay over time. They are at a constant level, so they are not acoustic issues. That is "mains hum" getting into your system somewhere. Probably lack of grounding, or a faulty mic cable, or some such. Worth trying to find and fix that.
Now take a look at the RT60 graph, and you'll see that you have a bass problem in your room. The decay time rises from less than 200 ms for the rest of the spectrum, to over 400 ms below 90 Hz.
Also take a look at the IR graph: You have some pretty large reflections hitting your listening position within 2 ms of the direct sound, so those are most likely coming off your desk or your video screens. To check that, pile up a few thick towels on your desk and repeat the test: I'm betting those will go away, and the mid range will smooth out a bit...." (I posted some REW graphs. I'll re-do them, though, as I have the sub all hooked up now, and I intend for it to be a part of my monitoring setup going forward.)
"Right now, anything at all will be useful to some extent. It's not going to tame your modes entirely, or course, but it will make some difference." (when asked about adding a corner bass trap on the floor behind my desk and breaking it up into 3 sections - wall to side of desk; under desk; side of desk to other wall. Note that, according to the google drawing, having anything bigger than 10" would require moving my speakers forward. I'm not sure if that's a good idea or not. Even then... I don't want to push my mix position back too much farther, than it is, because it's not far from where I am to the dead center of the room. (currently, my ears are about 30" away from each speaker and about 38-40" from the wall... or 33-35% mark. The dead center of the room would be the 57" mark.) I could go much further UP the wall... just not out. )
"First is symmetry, and I suspect the "nook" over on the right (referring to the closet). If you look at just the left and right speakers, for both sets (with and without towels), you'll see that in both cases there's a large dip at 95 Hz for the left speaker, which just isn't there for the right speaker, bot for both cases of the right speaker there's a large dip at 300 Hz that isn't there for the left speaker. So something in the room is sucking out energy asymmetrically, with the frequency depending on which speaker is firing. It could be two entirely different things, or it could be two aspects of the same basic problem. I'm inclined to go with the second option: two aspects of those "nooks" over on the right wall. One of the reasons I'm thinking of those is because in the "both speakers" graphs, those artifacts disappear, making me think that they cancel each other out perfectly on the room center-line, but since your ears are not on the center line (even though the mic is), your ears will be hearing the individual un-balanced signals. So it would be good to try to trace that issue and fix it.
Next, is the effect that just a few towels is having on the entire room: Those are actually reducing your overal decay time, believe it or not, and are even affecting the modal response: acting like a bass trap, and fairly effective, too, give the small volume. So imagine how effective true bass traps would be, located correctly...
" (I added towels to the top of my desk and it reduced one of the reflection points quite a lot)So what I'm wondering....
I could build much larger bass traps if I made them rectangular than if I made them triangular. For instance, a triangular bass trap that is 18x18" would have the same area as one that is rectangular and 18x9". Would that be a useful consideration, or do they need to be triangular? I can fit a rectangular 9x18" trap in each of the front corners... even 18x10". If they have to be triangular, the best I can do is 10x10"
And of course... any other help in getting my room improved is most welcome.
Thanks for your ongoing help and future help, and thanks for enduring my lengthy post.
Chris