# Room mode nightmare (graphs/pics)...help!



## yuri_rage (Jun 22, 2012)

*Sorry, bit of a long post. Skip to my bolded question and the pics below, then refer back to the post text if you want/need more info.*

This is my first full fledged home theater. By some standards, it's a budget build, but my overall investment is substantial (at least according to my budget!). As listed in my profile, here is my A/V equipment:

Epson 3010e Projector
Yamaha RX-V671 Receiver
Sony SS-F6000 Front Speakers
Polk CS-20 Center Speaker
Sony SS-B3000 Surround/Rear Surround Speakers
2 BIC F12 Subwoofers
2 Buttkicker LFE Transducers w/1kW amp
FBQ2496 Parametric EQ
(no room treatment...yet)

I'm measuring using a self-calibrated Soundblaster Live! USB card (loopback in place), Xenyx 802, and ECM 8000 with the stock REW cal file.

In my quest to create the ultimate man cave, I've had to make some compromises. The homebrew (beer) equipment on the left side of the room stays. I know it may contribute some ringing and is acoustically poor. The fridge on the right stays - despite the fact that it creates two more tall corners. The tile border is permanent - designed by the previous owner for a pool table - not in the budget to replace it.

So, I have an acoustic nightmare on my hands. There is a room mode (correct terminology, I think) at 45Hz and all of its harmonics (I think). There are significant multi-frequency nulls all over my desired listening position(s). No amount of sub placement experimentation fixed those (bass crawl, test tones, measurement, etc). I tried the recommended mid wall, two corner, stacked, and front wall placements for my two BIC F12s. I also tried each sub individually, as well as phase adjustments. The best results (both heard and measured) were from the present configuration (front wall, under the mains), with both phase switches set to 0. I'll include a graph of my right seat listening position, as the nulls seem most pronounced in that location with all sub placement options. If I could fix those deep troughs and ringing, I think I'd have a decent house curve on my hands.

The overall room dimensions are 23' wide x 21' deep, with an 8.5' flat ceiling. On the right side, there are 3 doors, one covered by the blackout curtain seen in the picture (apologies for the graininess...iPhone/low light pics were the best I could manage tonight). In the back right, there is a 6'x6' alcove for the entry door from the house. In the back left, there is a 6'x3' alcove for the bar/closet area. The speakers are laid out symmetrically, with the surrounds hanging from a lateral 16" mid-wall beam, and the rears on top of wall cabinets over the bar.

*My overarching question is: How would you treat this room?* I have roughly 10" of space behind the screen, and I can treat at least three vertical corners from the floor to ceiling (front left, front right, rear right). I assume I probably want to put some foam in the space over the wall cabinets and against that mid-wall beam. In the interest of budget constraints, I'd like to use Roxul Safe and Sound. I can buy 12 packs of 3.5" thick sheets from Lowe's for around $40. Two thicknesses of that should be effective...I think(?).

On to the pics and graphs - in addition to the ugly room mode(s), I have a spike at 50-60Hz, and another at 95. The room seems to resonate badly between 50-70Hz. I tried both subs individually and both 0 and 180 phase settings. The 0 phase setting seemed best, so that is what I'm including in this post. Individually, the subs had very similar response curves to the one measured with both, so I know they're not canceling one another.

Room front:









Room left rear:









Room right rear:









Both subs, no EQ:









Waterfall, both subs no EQ (500ms time scale):









Both subs, 3 parametric filters active in an effort to reduce the 50-60 and 95Hz spikes:









Waterfall for both subs with the 3 EQ filters active (500ms time scale):









7.2 channels, no EQ:









7.2 channels, same 3 EQ filters:









7.2 channels, EQd, measured from the right seat (the worst in the house by my perception):


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## Wayne A. Pflughaupt (Apr 13, 2006)

Sure wish you had shown us a picture of the left- ront. Is there a corner over there? 

Did you try the right front corner? In over 10 years looking at people’s response, I can count on one hand (with digits left over) the number of people who got a useless measurement from a corner. With corner placement you effectively minimize the number of boundaries that can cause interference. 

For instance, in your current arrangement with the subs under the speakers: on a horizontal plane to the sides of the subs, each is getting interference from two boundaries (left and right walls) and the other sub (IIR, a sub "sees" another sub as a boundary). 

So, each sub is seeing three sources of interference bouncing sound back out of phase and causing nulls. If they were say, both in the same corner, that figure would be reduced to one. Of course, other boundaries are contributing to response as well (e.g. the ceiling), but hopefully you get the idea.

Regards,
Wayne


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## yuri_rage (Jun 22, 2012)

Yes, there is a corner in the left front (sorry, I thought I captured that in the pic), and I did try placing one sub there while experimenting with the other sub in several locations throughout the room, including the right front corner. The problem with the right front corner is that there is a door behind that curtain, so a sub there renders the door useless.

I'll try the left front corner again when I get a chance, with the other sub placed in several locations along the front wall (which is pretty much where it will be stuck) and take more careful note of the nulls. As I recall, the response graph didn't look significantly different with corner placement, but I'll post the results once I take the measurement again. It's entirely possible that I glossed over something significant.

Any suggestions for bass traps/acoustic panels?


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## Wayne A. Pflughaupt (Apr 13, 2006)

Check our Acoustics Forum for recommendations for treatments. :T

Make sure and take a corner-only measurement. Doesn’t sound like you’ve done that yet?

You might also take a measurement a the sub between the two doors. Sometimes moving the sub a few feet from the corner gets better results than directly in the corner.

Regards,
Wayne


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## EarlK (Jan 1, 2010)

I agree that you will be better served asking those questions ( about room treatment options ) within * the HTS Home-Audio-Acoustics Forum . * 

It is moderated by a rep from GIK ( who is also an  *Acoustician* ) .

:sn:


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## yuri_rage (Jun 22, 2012)

I wanted to convince myself that front wall placement was as good as corner placement in my theater because the front wall is the most convenient option. However, if I'd taken the time to analyze the results properly, I'd have seen what you're about to see. Corner placement made a MASSIVE improvement. Thank you for the suggestions. We never use that door behind the curtains anyway, so I put the right sub in front of it. The left sub is in the front left corner (the corner just out of the picture above).

Measurements taken with the subs any appreciable distance from the corners made those sharp valleys reappear. Side wall placement yielded similar results as front wall placement.

With corner placement, the bad low frequency resonance is greatly reduced, and the deep valleys are all but gone. A walk/crawl around the room while playing sine waves yields better results as well - still a few null spots, especially in the 40-60 Hz range, but they are much smaller and fewer in number. I'm very pleased not only with the test results, but also the actual results when playing test scenes - the bass response is FAR better. There's still some ringing in the 50-60 Hz range, so I may well continue the discussion about room treatment in the other subforum.

In the interest of saving a little screen space, here is an overlayed graph:
Left sub only, corner placement (green)
Right sub only, corner placement(red)
Combined response, corner placement (purple)









Both subs, no EQ:









Both subs, no EQ:









Both subs, three filters active in the 50-70 Hz range:









Both subs, three filters:


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## HopefulFred (Jan 20, 2011)

Hope I'm not barging in (in a rude way), but shouldn't it be possible to get close to that final response with only one filter? If so, are there advantages (headroom?) to using fewer filters?

Fred


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## yuri_rage (Jun 22, 2012)

Not rude at all - you're probably right. I'm using somewhat narrow filters where a wider one may do the trick. The filter I added around 70Hz is probably entirely unnecessary, in any case.

I'm also wondering if that 50-60Hz resonance is actually background noise. There is almost ALWAYS a fan running in that room. I can never seem to get it completely quiet.


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## Wayne A. Pflughaupt (Apr 13, 2006)

yuri_rage said:


> The filter I added around 70Hz is probably entirely unnecessary, in any case.


I usually don’t bother to EQ a sub up that high. It’s so close to the crossover frequency that any equalizing is often blown out by the main speakers. However, comparing mains /subs measurements can confirm if this is the case or not.




yuri_rage said:


> I'm also wondering if that 50-60Hz resonance is actually background noise.


I’ll be shocked if it isn’t! It certainly isn’t an audio signal…

Regards,
Wayne


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## yuri_rage (Jun 22, 2012)

Thanks much, Wayne. I'm absolutely floored at the difference in response based solely on speaker placement. Seems my next move should be to make an effort to kill the background noise for a truer measurement. I can kill some circuit breakers to silence the noise from multiple appliance and HVAC fans.


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## PuckDaddy (Apr 21, 2012)

Wayne A. Pflughaupt said:


> I’ll be shocked if it isn’t! It certainly isn’t an audio signal…
> 
> Regards,
> Wayne


Sorry to barge in, but how can one tell, within REW, whether something is an audio signal or background noise? Is it apparent from the waterfall graph? I've seen similar bumps in my sweeps, but just assumed they were part of the subwoofer's response. I'm now thinking that they could be background noise from my HVAC system. Is there any way to differentiate, other than shutting off the source of the background noise? Thanks!


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## JohnM (Apr 11, 2006)

PuckDaddy said:


> Is there any way to differentiate, other than shutting off the source of the background noise?


One way is to see if the level flattens off in the waterfall, i.e. it stops decaying. That would indicate a constant background signal rather than a resonance. Another is to use the RTA with no signal playing and see what it picks up.


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## Wayne A. Pflughaupt (Apr 13, 2006)

PuckDaddy said:


> how can one tell, within REW, whether something is an audio signal or background noise? Is it apparent from the waterfall graph?


Yes, it’s easily apparent in a waterfall graph: The audio signal fades away (decays), but the noise is a steady signal that does not decay. You can see the noise in yuri’s graph from the Post #6. If you changed the graph’s time window to 1000 ms, it would be even more apparent. As John said, using the RTA feature with no signal is another way to “see”noise.










Regards, 
Wayne


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## HopefulFred (Jan 20, 2011)

Does the acoustic energy of the background noise contribute to the appearance of a peak at that frequency, or does the higher gain signal simply lay over the background noise?

I'm not sure I've phrased that correctly, and I think I already know the answer... but let's try again:
If the background noise has a 15 dB peak at 55 Hz and sine sweep were played (in an anechoic environment) would the waterfall show the frequency characteristics of the playback system or would the background noise influence the overall character (above the noise floor) of the display?

Fred


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## Wayne A. Pflughaupt (Apr 13, 2006)

HopefulFred said:


> Does the acoustic energy of the background noise contribute to the appearance of a peak at that frequency, or does the higher gain signal simply lay over the background noise?


The music (or measurement) signal swamps the noise, the same way that you might hear a cricket chirping in your room once the music stopped, but not while it was playing. The background noise has no influence on a measurement signal as long it is substantially lower in amplitude.

Regards,
Wayne


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## yuri_rage (Jun 22, 2012)

Another cue is the frequency itself. A 60Hz hum is very apparent in many appliances, due to the 120VAC/60Hz power used in American homes. Because the waterfall stopped decreasing and held constant at the peak close to that frequency, I figured it was background noise. Since I'm VERY new to this game, I'm glad my assumption was corroborated by someone with a bit (lot) more experience.


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