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Bass/Lower Midrange issues. Completely in the dark
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djinfuze
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#1
23rd March 2011
Old 23rd March 2011
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Bass/Lower Midrange issues. Completely in the dark

So here is the deal. I am working with limited resources and knowledge here, so bear with me.

My room is basically a rectangle and it is 9 feet wide and 10 feet long. Ceilings are about 9 feet high. None of this is ideal, I realize, but it is what it is. Construction is drywall for the walls, wood floor, and strange, spongey ceiling tiles.

I have traps that made out of owens corning 703 2" that I built to try to make the room better. While the room sounds much nicer with the traps in place (front corners, floor to ceiling and side walls), the problem I am having is that when I do a sine wave sweep from 20-200hz, I am finding problems at 169hz and at 80-90hz. The issue is that at these frequencies, especially at 169hz, my sine wave almost completely disappears! I don't know what's causing this or what I can do to help it, but what I do know is that I am gonna have a hell of time making my mixes translate if I can't hear what I am doing in the lower midrange/upper bass region.

Also, I have Focal Solo6BE monitors, so I know it's not the speaker.

Please let me know your thoughts on this puzzling issue. I am sure those of you with a lot of acoustic experience have some idea of what's going on.
#2
24th March 2011
Old 24th March 2011
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SAC
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24th March 2011
Old 24th March 2011
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What you have discovered are what are commonly referred to as room modes. Frequencies where the energy is either constructively or destructively reinforced created by energy whose wavelength is larger than the room's dimension and which when reflected establishes standing waves rather than being reflected specularly - which happens when the wavelength is smaller than the room boundary dimension.

Typically these are addressed by what are commonly referred to as bass traps which may be in the form of large, thick porous velocity based absorbers or through the use of tuned resonant pressure based absorbers.

As you have discovered, these regions of high and low pressure are distributed throughout the space and can be 'discovered' by use of the playing of sympathetic sine waves or by measurement techniques such as the waterfall, cumulative spectral decay, or spectrograph displays.


Edit: I have no idea what the weight of speaker stands has to do with this issue!
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24th March 2011
Old 24th March 2011
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Lightbulb

Quote:
Originally Posted by djinfuze View Post
I have traps that made out of owens corning 703 2" that I built to try to make the room better.
SAC sent you on the right path. Bass traps made from 703 need to be much thicker than two inches. Four inches is a good minimum, and thicker is even better. You can place two of the panels you already have adjacent, no glue needed.

--Ethan

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27th March 2011
Old 27th March 2011
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djinfuze,

Room modes, standing waves, resonances. That's what you've got. It is common for all rooms and room sizes.

Your room is very small and it shares common dimensions (height and width). This causes double/triple modal incidences. (This is where modal resonances of different dimensions are at the same frequency.)

Room volume = 810 cubic feet - This is about half the minimum recommended volume. Therefore, you will need a very large amount of trapping.

Double incidences:
62.78 Hz
84.46 Hz
125.56 Hz
129.27 Hz
137.68 Hz
140.38 Hz
151.32 Hz
168.92 Hz *** Here's the one you really noticed.
180.21 Hz
180.75 Hz
188.33 Hz
196.63 Hz
198.52 Hz
206.4 Hz
210.94 Hz
219.63 Hz .... you get the idea.

These can only be eliminated by moving walls. or changing rooms. Due to these doubles you will need even more trapping.

Substantial trapping will smooth these out and lower their 'Q' (bandwidth) but will not eliminate them. With proper trapping (That means a REALLY lot!), you can make this room workable, but it will entail considerable 'work' from you.

Cheers,
John
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djinfuze
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21st April 2011
Old 21st April 2011
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Thank you so much for your answers. Would corner trapping be the way to go in this case? I don't want to sacrifice too much of my usable space in the room, but I am going to do what I have to do to make it workable. Right now I have these 2 inch thick OC 703 panels (12 of them). I could rip some of them apart to make thicker traps. Would it make more sense to put some of that auralex corner trapping stuff in the corners and use the panels I have for broadband absorption? Would it make sense to put anything on the ceiling? Sorry for 20 questions, but I am really desperate to make this space sound a bit better. I am pretty sure I have about 75 percent of the materials on hand to do it really easily. Let me know what you all think. Sorry for the late reply, my email didn't let me know that you guys had posted these replies!
#7
21st April 2011
Old 21st April 2011
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djinfuze,

Julio's control room was about the size of yours. Check out his build diary for some ideas. If you would like something specifically designed for you, PM me.

Cheers,
John
djinfuze
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21st April 2011
Old 21st April 2011
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Where do I find Julio's build diary? I'm having trouble finding it with the search function.
#9
21st April 2011
Old 21st April 2011
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Quote:
Originally Posted by djinfuze View Post
Where do I find Julio's build diary? I'm having trouble finding it with the search function.
There is a link to it in John's post....it's the underlined bit
djinfuze
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#10
22nd April 2011
Old 22nd April 2011
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Haha sorry about that. Stupid me.
#11
22nd April 2011
Old 22nd April 2011
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Quote:
Originally Posted by djinfuze View Post
Thank you so much for your answers. Would corner trapping be the way to go in this case? I don't want to sacrifice too much of my usable space in the room, but I am going to do what I have to do to make it workable. Right now I have these 2 inch thick OC 703 panels (12 of them). I could rip some of them apart to make thicker traps. Would it make more sense to put some of that auralex corner trapping stuff in the corners and use the panels I have for broadband absorption? Would it make sense to put anything on the ceiling? Sorry for 20 questions, but I am really desperate to make this space sound a bit better. I am pretty sure I have about 75 percent of the materials on hand to do it really easily. Let me know what you all think. Sorry for the late reply, my email didn't let me know that you guys had posted these replies!
If you have 12 pieces I would make 6 4" panels and straddle floor to ceiling in the front corners and put one in each back corner. The problem is this really is still not enough as you have not straddled enough corners and have not put thick absorption on the back wall. Also you have not covered early reflection points on sides walls and ceiling. See the following
GIK Acoustics: Room Setup
What Are Early Reflection Points. Spotlight on Bob Ebeling - Reflection Studio. GIK Acoustics Articles and Newsletters.

One other thing to do is start with facing the short wall in the room and sit 38% from the wall. You would then test the room and move forward and or back to find the best result. Set up alone can be half the battle.
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