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Discussion Starter #1
My room has a strong room mode(+12dB) around 90-100Hz. It is impractical to position main speakers/seating to truely minimize its effect.


What do you think the best/most realistic way would be to treat it?:


1)Cross the subwoofer over at 100Hz (as opposed to 80Hz or lower) and position the subwoofer appropriately (ie. in the width/length null spots for the mode). My concern is that the subwoofer bass will be directional at this high crossover.


2)Room treatments - obviously I will do as much as I reasonably can with room treatments but it just doesnt seem feasible to take off 9dB, even using bass traps specifically tuned to 100Hz. Can it be done without covering every single wall surface with panel traps?


3)EQ the main speakers/center channel. Obviously not the most desirable option.


Thoughts? Opinions?


Andy K.
 

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I don't know about the other options, but I don't think you'll localize the 100hz xover unless you have extremely sensitive hearing. I've tried both 80 and 100 and could not "localize" my sub in either case. I use 80 because it gives me a flatter FR in my room.
 

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Hi Andy,


I think that 1 is your easiest and cheapest option, so it's worth a try. 80 hz is not a magic threshold for non-localization of bass. In fact, there is recent research that indicates that localization can occur at much lower frequencies.


If you can localize the bass, PM me. I've got some ideas. (Your profile says you are an ASIC Designer. Ever build a Hilbert transformer? :) )


Regards,

Terry
 

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Discussion Starter #4
Thanks for the feedback. Issue #1 has another current roadblock. My receiver crossover is fixed at 80Hz. I'm reluctant to buy another one now when receivers that support HD-DVD are just around the corner.


Do any subwoofers take in high-level inputs for L,R, and C? Most take high-level inputs for L/R but I'm not sure if any take in the center channel signal? I'll have to poke around...


Never heard of a hilbert transform - looks like it generates orthogonal signals. You want me to monkey around with the phase, huh :)


Andy K.
 

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Andy,


Looks like your current post on the subwoofer forum is a good strategy.


Is the 100 Hz mode excited by the center speaker as well, or just the L & R? If not, you may be able take C out the equation, leaving just L & R, which would expand your subwoofer choices.


Regards,

Terry
 

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I've got a similar situation and have tackled it via careful localizing of sub, some room treatments, and EQ on the sub channel only. Even if the x-over frequency of the sub is 80Hz and you want to reduce a peak at 100Hz, you can simply increase the EQ filter amount at 100Hz by around double the amount if it were below the x-over frequency and you will accomplish the same thing. This works because there is still significant sound coming from Sub at 100 with an 80 Hz crossover (it's only down a max of about 6-8 dB at that point relative to flat response). You do not need to introduce EQ on the L-C-R channels.


Dave
 

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Discussion Starter #7
Hi Terry,


I cant say with 100% certainty. What I can say is that, before I finished building the room (concrete walls), I did measure the center as exciting the mode. Now that the room is built I still hear a mode, deep male voices sound extremely 'chesty'. The unfortunate thing is, due to my ceiling height and screen size, the center channel is very close to the ceiling (4 inches away) angled down to the seating position. I have yet to treat the ceiling around the center (and will do so very soon), but I dont think this will help that much. I'll try to find time to take some ETF graphs shortly for your amusement :)


Dave, thanks for the feedback. I had considered that but didnt think it would work very effectively. If you have experience to the contrary that is great to hear and I will definitely give it a try.


Thanks!


Andy K.
 

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Quote:
Originally Posted by kromkamp
1)Cross the subwoofer over at 100Hz (as opposed to 80Hz or lower) and position the subwoofer appropriately (ie. in the width/length null spots for the mode). My concern is that the subwoofer bass will be directional at this high crossover.
Your main speakers will be at most 6dB down at their cross-over frequency, so the higher cross-over will leave you with at least a 6dB peak.
 

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Quote:
Originally Posted by kromkamp
I cant say with 100% certainty. What I can say is that, before I finished building the room (concrete walls), I did measure the center as exciting the mode. Now that the room is built I still hear a mode, deep male voices sound extremely 'chesty'. The unfortunate thing is, due to my ceiling height and screen size, the center channel is very close to the ceiling (4 inches away) angled down to the seating position. .
Much (if not all) of your problems come from the center channel's proximity to the ceiling as opposed to a discrete room mode.
 

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Discussion Starter #10
Well, we are both right I think.. The proximity to the ceiling is exciting the first fundamental mode of the height of the room which is around 100Hz. I dont think this is related to SBIR-type interference - firstly it would be a null and secondly the frequency would be higher.


Dave, I've been thinking about your suggestion some more - my center channel is already set to 'small' in the receiver, and its still exhibiting the problem. So can I truely compensate with only the subwoofer?
 

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Quote:
Originally Posted by Drew Eckhardt
Much (if not all) of your problems come from the center channel's proximity to the ceiling as opposed to a discrete room mode.
Hi Drew,


Could you please elaborate.


Thanks.


Larry
 

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Quote:
Originally Posted by LarryChanin
Could you please elaborate.
You have a 6dB SPL increase each time you halve the acoustic space a speaker is operating into. Getting within 1/8 wavelength of a wall, ceiling, or floor almost halves the acoustic space (some sound leaks or is absorbed). Assuming the center channel within 4" of the ceiling is about 8" high we might get a 5dB boost at 200Hz and below, 3dB at 300Hz, and loose the effect by 400Hz. Speech starts arround 200-250 Hz. The expected bass boost from this center channel placement will definately give male actors chest colds.


The 100Hz room mode will only be a problem when listening to male baritone and bass singers hitting their lowest notes.


Sub-woofer equalization will not fix the placement and room mode problems.


Shelving high-pass and notch filters on the center channel will.
 

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Discussion Starter #13
Well, theres only one way to prove or disprove this - I need to sit down and run a quick ETF test. Will try to do so this weekend.


I've never heard of that theory but it sounds like its typically more used in speaker design not room design?


If you hear someone like James Earl Jones speaking, I'm sure the lowest fundamentals of his voice go as low as 100Hz.
 

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If it is feasible for you, I'd get some insulation or sound board from home depot/etc


shouldn't cost too much, use the panel at an angle in a corner, make it 4" thick (double up)


worth a try anyway...


you can get more expen$ive, good looking, etc if things work out. Just stick a couple together and try it, as long as you don't cut it or glue it, etc Home Depot will take a return back...so you are out nothing but time.


Be thankful your problem is at the higher end of bass...the lower you go the more extreme the room treatments need to be.


Sorry I missed that this was caused by the center channel near the ceiling...you can still take or leave my idea but you may want to try some other and alternative placements for the treatment
 

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Hello Drew,

Quote:
Originally Posted by Drew Eckhardt
You have a 6dB SPL increase each time you halve the acoustic space a speaker is operating into. Getting within 1/8 wavelength of a wall, ceiling, or floor almost halves the acoustic space (some sound leaks or is absorbed). Assuming the center channel within 4" of the ceiling is about 8" high we might get a 5dB boost at 200Hz and below, 3dB at 300Hz, and loose the effect by 400Hz. Speech starts arround 200-250 Hz. The expected bass boost from this center channel placement will definately give male actors chest colds.


The 100Hz room mode will only be a problem when listening to male baritone and bass singers hitting their lowest notes.
But this room boundary effect will not be specific to 100 Hz. It will not create a peak.

Quote:
Sub-woofer equalization will not fix the placement and room mode problems.

Shelving high-pass and notch filters on the center channel will.
Andy was talking about the posibility of EQing the center channel, not the sub-woofer.


Regards,

Terry
 

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Discussion Starter #16
Paul, I have plenty of 2" 3pcf fiberglass for room treatments and I intend to place them up there, but I dont know if it will help that much @ 100Hz. Its still pretty low.


Terry, Dave originally posited that, even though the problem frequency is above the subwoofer crossover point, its close enough that some component of the sound will be coming from the subwoofer (more or less depending on the crossover slope). So, if you over-compensate on the sub side you may approximate a flat response.


However, since you can already hear this tremendous peak when the center is set to small already, I'm not sure theres much left to work with on the subwoofer channel. Its certainly worth a try though.


Andy K.
 

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Quote:
Originally Posted by kromkamp
Paul, I have plenty of 2" 3pcf fiberglass for room treatments and I intend to place them up there, but I dont know if it will help that much @ 100Hz. Its still pretty low.


Terry, Dave originally posited that, even though the problem frequency is above the subwoofer crossover point, its close enough that some component of the sound will be coming from the subwoofer (more or less depending on the crossover slope). So, if you over-compensate on the sub side you may approximate a flat response.


However, since you can already hear this tremendous peak when the center is set to small already, I'm not sure theres much left to work with on the subwoofer channel. Its certainly worth a try though.


Andy K.
It is still pretty low but since you have some to experiment I would try...double or triple thick and at angles (increasing effectve thicknesses) and try a few positions that make sense.


People might shred me for this one...and it will be limited to helping at one narrow band and will may just shift your problem to a different freq but....


Mess with your phase dial on the sub with an SPL meter at your LP.... you may just be able to create destructive interference between the two and make at least the LP (don't move your head too much lol) a little smoother. This is really a more for fun thing but it could be fun to try and MAY just MAY impove things...


typically that kind of stunt works better at lower freqs and multiple subs (you can really mess with your response... he he)
 
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