# Problem with sub-room interaction...



## Vader (Jul 8, 2006)

I am confused (and more than a little irritated)...

Here is my room response at my listening position, using the old Rat Shack meter (I have not repeated it using the Galaxy - I'm waiting for the cal file - but speaker levels were still pretty accurate):









To me, it doesn't look that bad considering that the room is not treated, nor is the FR equalized (until I get a laptop, a BFD really isn't in the cards...). And here is my room setup:









Here is the problem: While watching "Spider-man" the other night, I got up to check on something. When I moved from my listening position (I) toward the door on the right wall, the bass output improved immensely. For the time being, moving the subs is also not an option, and judging from the above FR, I am not exactly sitting in a null (or am I?). I think I remember reading something about sitting midway in a rectangular room being "bad", due to the nulls established by the odd-numbered harmonics. Is there any validity to this? Would room treatments not help in this case? I'm also somewhat suspicious of the angled wall protrusion to the left of the subs (sealed gas fireplace). Is it possible that this is "semi-corner-loading" working against me (reflecting bass away from the listening position)? A friend told me that a bass trap can be made out of a large cylinder filled with sand and capped at both ends. Is this true? Would it help my situation any? Am I asking too many dumb questions?


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

> Here is the problem: While watching "Spider-man" the other night, I got up to check on something. When I moved from my listening position (I) toward the door on the right wall, the bass response improved immensely.


Bass _response_ improved, or bass _output?_ I doubt you were able to determine the former that easily. I assume it’s actually the latter, and it’s common knowledge that boundaries reinforce bass output. So – no mystery there at all.



> (until I get a laptop, a BFD really isn't in the cards...


Well, you managed to take this measurement without a laptop. If you can do that, no reason not to get a BFD, although it could be argued that you don’t really need one.



> judging from the above FR, I am not exactly sitting in a null (or am I?).


The problem with rectangular rooms is that they have a bass “hole” in the dead center of the room. Move from that point towards any boundary, and bass output increases. You can test it yourself with some pink noise and a SPL meter (although the meter will probably only be confirming what your ears are telling you).



> I think I remember reading something about sitting midway in a rectangular room being "bad", due to the nulls established by the odd-numbered harmonics. Is there any validity to this?


It might not be the optimal situation, but it’s greatly preferable to say, sitting up against a wall. There are other things to be considered besides the bass, like good placement of the rear speakers in relation to the seating position. If it were my room, I’d probably have the couch right where you do.



> Would room treatments not help in this case?


Assuming you mean bass traps, their primary function is to reduce bass decay or “reverb.” The audible benefit of this is that it tightens up the bass. However, trap proponents claim that they can reduce the center null/hot boundary situation as well. To some extent at least. Traps typically don’t work well at the lowest frequencies – say below 40-50 Hz, I think (I’m sure they’ll correct me if I got the exact number wrong there).



> A friend told me that a bass trap can be made out of a large cylinder filled with sand and capped at both ends. Is this true? Would it help my situation any?





> I'm also somewhat suspicious of the angled wall protrusion to the left of the subs (sealed gas fireplace). Is it possible that this is "semi-corner-loading" working against me (reflecting bass away from the listening position)?


Questions most likely best answered at our Acoustics Forum.



> Am I asking too many dumb questions?


I saw a lot of questions but no dumb ones. 

Regards,
Wayne


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## Vader (Jul 8, 2006)

Wayne,

As always, your answer is detailed and very informative! You are correct (bad phrasing on my part): I did mean _bass output_...



> Well, you managed to take this measurement without a laptop. If you can do that, no reason not to get a BFD, although it could be argued that you don’t really need one.


My computer is about 75' away in a different room, and is not easily moved. I utilize REW by running a couple of audio RCA cables to and from my AVR and SPL meter, respectively, but I'm told that a like length USB or MIDI run is a pipe dream (something about a USB or MIDI signal being very low power to begin with). So, unless I program the BFD manually, I'm kinda stuck waiting for a laptop (now that I think of it, my wife's computer is more easily moved...hmmmmm).


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## Ed Mullen (Jul 28, 2006)

You have a pretty large null at 40 Hz (typical in this size room). Could you try the subs on the front stage, one next to (or behind) each speaker? Rerun the FR and see how that looks.


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## brucek (Apr 11, 2006)

> So, unless I program the BFD manually


That's not difficult. I've never done anything else. By the time you hook your midi cable up and figure out how to get the BFD in the midi mode, I would have the filters entered and have sat back down to enjoy the music. I've never understood the attraction of this midi programming of the filters.......

brucek


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## Vader (Jul 8, 2006)

Bruce,

Can I use REW to determine the filters needed, and then use those for programming the BFD, or does REW use its own "filter language" (obviously, I am completely new to room EQ)?


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## brucek (Apr 11, 2006)

> Can I use REW to determine the filters needed, and then use those for programming the BFD


Of course, that's what REW does... The midi is a convenience for people who don't like getting off the couch.

The filter panel tells you what to enter into the front panel. I overlayed text in the picture below to show the four entries. You select the equalizer type with REW and it tells you what to enter in the front panel.

have you read this?









brucek


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## Vader (Jul 8, 2006)

> have you read this?


Part of it. I am slowly working my way through it as I learn how to use REW; To date, I know precisely 1.0E-42% of what I need to learn.... and that's not counting what I need to learn about REW....


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## Vader (Jul 8, 2006)

> Could you try the subs on the front stage, one next to (or behind) each speaker?


Ed, thank you for your reply...

I may very well try that, my only concern is by splitting he subs up (they are currently co-located), wont the headroom gain be around +3.5dB instead of the full +6 dB? Also, What about opposing standing waves produced by each sub seperately - can't that also produce massive cancelation? I know that many here can successfully get a smoother response using the phase controls, varying the crossover on each sub, and EQ (based on Bruce's response above, a BFD is back on the radar - whoo hoo!). My knowledge of phase setting is limited to "playing a bass heavy sequence, and finding the two places where the bass output is at a minimum, then taking the spot half-way between. Or, switching the phase of the mains, finding the minimum, then switching them back. In other words, I would probably do more harm than good...?


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## brucek (Apr 11, 2006)

> my only concern is by splitting he subs up (they are currently co-located), wont the headroom gain be around +3.5dB instead of the full +6 dB?


No, splitting a high impedance connection has no effect on headroom or signal level.



> Also, What about opposing standing waves produced by each sub seperately - can't that also produce massive cancelation?


Yes, big problem. It takes a bit of work with REW to get it right. And it may be a problem getting it right over a large seating area also. Co-locating is the easiest, but multiple subs can be used in lieu of equalization with a little work.



> a BFD is back on the radar - whoo hoo


As Wayne says, it can easily be argued that you don't require one with that fairly good response you have...



> My knowledge of phase setting is limited to "playing a bass heavy sequence, and finding the two places where the bass output is at a minimum


Now you have REW to make it all easy...

brucek


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## Ed Mullen (Jul 28, 2006)

Vader said:


> Ed, thank you for your reply...
> 
> I may very well try that, my only concern is by splitting he subs up (they are currently co-located), wont the headroom gain be around +3.5dB instead of the full +6 dB? Also, What about opposing standing waves produced by each sub seperately - can't that also produce massive cancelation?


If you keep the two subwoofers within a 1/4 wavelength (use the XO frequency as the controlling upper limit), then they will still acoustically couple and you will still retain the 6 dB of headroom gain.

Placing them to the inside of the mains, flanking the center channel, should be close to 1/4 wavelength. If you are using an 80 Hz XO, the wavelength is ~14 feet. Keep the subs about 4 feet apart or less and you should be fine. Set both subwoofers to 0 degrees on the phase control and adjust overall subwoofer phase with the subwoofer distance control. 

This can be verified with an FR sweep of one subwoofer and then the other, and then both together. You should see a 6 dB gain with both subs running (provided they are set to the same gain level). 

Or you can check/verify acoustic coupling with discrete sine waves with one sub and then both subs playing. 

My dual Ultra/2 are about 4 feet apart and I'm getting + 6 dB right up to 90 Hz or so. 

Despite the fact that they remain acoustically coupled, they do indeed load the room differently from their respective locations and this resulted (in my case) of killing a 40 Hz null at the listening position that I could not eliminate with the subwoofers side-by-side.

In addition, placement near the mains will allow easier phasing between the subwoofer and the mains over the XO bandwidth, possibly improving the bass coherence and the combined FR in the 70-100 Hz region. 

Adjust the subwoofer distance control until the speaker/sub FR is optimized over this bandwidth. The optimal subwoofer distance setting may be farther than actual in order to compensate for the group delay (latency) introduced by the 4th order low pass filter in the digital bass management circuit.

Give it a try and see how everything goes - nothing to lose at this point - you've got a large null at 40 Hz that you could potentially reduce by moving the subs to the front stage.


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## Vader (Jul 8, 2006)

Ed,

Are the 20-39 PC+ cylinders magnetically shielded?


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## Ed Mullen (Jul 28, 2006)

Vader said:


> Ed,
> 
> Are the 20-39 PC+ cylinders magnetically shielded?


None of our subwoofers are magnetically shielded. The safe subwoofer distance all depends on the type of TV. Older tube style TVs are susceptible to EMI, and the subwoofer must be placed several feet away. The newer rear projection CRTs are much less susceptible (usually placement near the unit has no effect), and the latest digital flat panel LCDs and DLPs are virtually immune to EMI.


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## Vader (Jul 8, 2006)

My display is a Mitsubishi WS65-511: circa 2003. Certainly not new, but not ancient, either. (just old enough to not have DVI.... grrrrr......:rant. I'm gonna also try moving one sub nearfield, and see what that does. As soon as Sonnie posts the Galaxy calibration file, I will try both configs and post the results.


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## Ed Mullen (Jul 28, 2006)

Vader said:


> My display is a Mitsubishi WS65-511: circa 2003. Certainly not new, but not ancient, either. (just old enough to not have DVI.... grrrrr......:rant:


It will probably be fine; I had my Ultra/2s flanking an older Hitachi RPCRT and there was no EMI. I always made sure to reconverge the color guns though, whenever I moved a subwoofer near or away from the set. 

The only way to tell is to move them over and see what happens. Make sure the set has been running for at least 30 minutes, and then do an auto-reconvergence of the color guns after you move the subwoofers in place.


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

Vader said:


> Ed, thank you for your reply...
> 
> I may very well try that, my only concern is by splitting he subs up (they are currently co-located), wont the headroom gain be around +3.5dB instead of the full +6 dB? Also, What about opposing standing waves produced by each sub seperately - can't that also produce massive cancelation? I know that many here can successfully get a smoother response using the phase controls, varying the crossover on each sub


I was going to tell you that in a symmetrical room like yours it’s often easier to pull off separating the subs, but Ed beat me to it (and did a much better job explaining it than I could have). It’ll be interesting to see what your new graphs look like. 

For one thing, I think you’ll find having the subs up front will make for a more coherent presentation. I had my subs set up to the side a while back (like yours are), and I could never escape the “feeling” (for lack of a better word) that the bass was separated from the front soundstage. When I re-located the subs to a front corner, beside the right speaker, I got a seamless blend between the subs and mains. Now sounds like my little bookshelf speakers are full range!

Regards,
Wayne


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## Ed Mullen (Jul 28, 2006)

Wayne A. Pflughaupt said:


> I was going to tell you that in a symmetrical room like yours it’s often easier to pull off separating the subs, but Ed beat me to it (and did a much better job explaining it than I could have). It’ll be interesting to see what your new graphs look like.
> 
> For one thing, I think you’ll find having the subs up front will make for a more coherent presentation. I had my subs set up to the side a while back (like yours are), and I could never escape the “feeling” (for lack of a better word) that the bass was separated from the front soundstage. When I re-located the subs to a front corner, beside the right speaker, I got a seamless blend between the subs and mains. Now sounds like my little bookshelf speakers are full range!
> 
> ...


Yes, exactly. With an 80 Hz XO, subwoofer localization can still be an issue with people who are sensitive to it. The whole goal is for the system to sound like a single full range speaker. 

With the subs on the front stage near the mains, this goal is often easier to accomplish because it allows for point source phasing between the subs and mains with the subwoofer distance control.


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## Vader (Jul 8, 2006)

One last question, and this is probably extremely paranoid/naive: I just measured the available space along the front channel, and found that my only option is for the subs to flank the mains (behind and to the outside of each) as opposed to the center (the door is in a most unfortunate position). That would place them about 7' from each other (My XO is indeed 80Hz), so I am hoping to not lose to much SPL. My question is this: Several cables (component video, speaker cables, composite video, S-Video) will be running just past the base of the left sub, and a Monster power center will be about 1' away. I know that strong magnetic fields can induce counter-currents in nearby conductors, so I am wondering whether I have anything to worry about here...


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## Ed Mullen (Jul 28, 2006)

You could lose SPL in the upper bass regions with the subs 7 feet apart - you'll need to run some sweeps or test with discrete sines to see if you are getting +6 dB all the way to 80 Hz - my guess would be not.

I wouldn't worry about running your interconnects past the subs, but try everything afterward for proper functionality and PQ/SQ to be sure.


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## Vader (Jul 8, 2006)

Another possibility would be for me to switch the equipment rack with the subs. While this would require extensive re-wiring, it would keep both of them co-located _and_ corner load them. The disadvantage is that they would not be symetrically placed relative to the mains (as opposed to flanking them) - would that even be an issue?. The Mitsu cabinet is 5' wide, so they would be a minimum of 6' apart anyway if I went that route. Also, I could use my own home-brew decoupling platform if I went with the "colocation in corner" idea....

EDIT: I guess I lied. That _wasn't_ the last question.... ya know, before I got into the whole sub thing, I knew absolutely nothing about any of this stuff (like I know anything now). I guess ignorance really is bliss (but not as much fun!)..... Was it Einstein that said "as a circle of light increases, so does the circumferance of darkness around it"? Thanks a lot, guys....


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## Vader (Jul 8, 2006)

OK, with the subs flanking the mains about 8' apart, here are my preliminary results (again, using the Rat Shack Meter), and my layman's interpretation of them:









The big thing is that I lost the 40Hz null altogether! I only really have two peaks to deal with (20Hz & 45Hz), whereas previously I had a _plateau_ extending from 20Hz to almost 40Hz. A BFD will make short work of the peaks... As for the headroom gain, Judging from the LFE pink noise from my AVR, there is almost exactly 6bB difference between one and both subs, and I needed to boost the AVR volume by about 6dB when taking the FR of a single sub in order to match the REW target levels. I know this does not necessarily mean that the headroom gain is constant across an entire range, but it is encouraging nonetheless. Further, the AVR used to be set at +1.0 (calibrated at RL), and now it is at -2.0 (I would imagine a result of not having the null). 

Just for grins, I also tested the sub in the left corner where I was thinking about co-locating them (my reasoning being that since they would be completely coupled and acting as one, this would be a good approximation of the FR). The response was not that much different, _except_ a rather nasty dip around 80Hz (not good, as this is my XO!). I also do not notice as severe a difference in bass output when moving out of the sweet spot, so I guess the FR was also significantly smoothed out across the room. All-in-all, I owe you guys a Duff! :bigsmile:

EDIT: I just thought of the fact that I was probably running them +3dB hotter than I should have been (I already run them +2 to +3 dB hot anyway), and they _still_ did not break a sweat, much less bottom out (even on War of the Worlds!)... says a lot about SVS...:bigsmile::bigsmile:


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## Ed Mullen (Jul 28, 2006)

Glad this placement method worked well in your room. Clearly the 30 Hz is a relative low and not a null. You've got the potential for a ruler flat FR after killing those two peaks (easy with the BFD). 

Map out the the foot print of those peaks by plotting the response a few feet in every direction and see if it changes much.

This can make a difference if you are shooting for a perfect response at one sweet spot or better bass at a few listening positions in the general vicinity.


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## brucek (Apr 11, 2006)

> Map out the the foot print of those peaks by plotting the response a few feet in every direction and see if it changes much.
> 
> This can make a difference if you are shooting for a perfect response at one sweet spot or better bass at a few listening positions in the general vicinity.


Yeah, the AVERAGING function in REW V4 works great for this. I took about 6 readings from my couch and chairs and then pressed the AVERAGE button to get an overall new averaged response. Then I assigned filters for this new average plot. It worked quite well to make the entire area acceptable rather than just my sweet spot. 

In fact you could have a couple programs in the BFD. One for quests and one for critical listening...... 

brucek


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## Vader (Jul 8, 2006)

Bruce,

When reading the BFD manual (for about the tenth time), I found that the bandwidth is 20Hz - 20kHz. Does this mean that anything below (or above) this range is lost? If that is the case, is there any reason to tune a sub lower than a TF of 20Hz? Thanks!


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## Guest (Jun 3, 2007)

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Should I feel plenty of Sound Preasure with my Room Deminsions and Sound Proofing. 99% H/T Use. JL-13 on its way to me now.


My Room 16-Long , 
12-Wide , 8-Feet High Soundproofed.

I


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