A Sound Renovation for the Green Room - Page 4 - Home Theater Forum and Systems - HomeTheaterShack.com

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post #31 of 68 Old 03-06-17, 09:09 AM
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Re: A Sound Renovation for the Green Room

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This is interesting. Notice the dips at 100Hz, 200Hz, and 400Hz? Are they related, other than harmonically? I do think the 100Hz dip may be due to SBIR, since the center's baffle is about 2.5ft from the front wall.
Attachment 137722
The 100 Hz dip could be from a reflection / cancellation, as you suggest. The 2.5 ft path for the delayed wave would result in a cancellation at 100 Hz, and higher frequency dips could also result from that same delayed path, but higher frequencies would be at 300 Hz, 500 Hz, etc.
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post #32 of 68 Old 03-06-17, 09:20 AM Thread Starter
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Re: A Sound Renovation for the Green Room

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To my eye, the sub only FR looks nice. The weird thing is mains and sub rises from 20hz to about 70 where it falls down. I agree that, big gully is bad XO support, and I would start with the distance setting in the avr, or phase on the sub. You might want raise the xo on the center just to see what it does.
Troublemaker!
Thanks for pointing that out. But did you mean to say "center and sub" for the 20-70Hz rise/drop? The response for L+R+Sub seems to have good fill through the xover region, correct? I agree there's a big gully around 70Hz, but it applies to C+Sub. My AVR has distance settings, but only for manual EQ. I plan to use Dirac Live, which has none. I think I'll need to experiment with sub phase and/or center location to iron out that gap. I imagine the phase setting can satisy one or the other, not both; so any phase manipulation is going to be a compromise.

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As far as the FR, I would prefer it rises, as it descends from 200 down to 20. Yep, house curve. Yours seems to be the opposite by about 10(on the center). Do you have the lfe trim bumped or flat(75). What happens if you raise it by say 5db and sweep? IMO 10db roll off from 70 on down isn't going to sound good.
Agreed again. I forgot to mention earlier that these graphs represent measurements taken without EQ, so what we're seeing is natural speaker/room interaction. I've always wondered about the slightly rising response, but never got serious about it until now. Might that be a type of room-gain?

.
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Last edited by Lumen; 03-06-17 at 11:50 AM. Reason: Changed first paragraph.
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post #33 of 68 Old 03-06-17, 10:00 AM Thread Starter
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Re: A Sound Renovation for the Green Room

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The 100 Hz dip could be from a reflection / cancellation, as you suggest. The 2.5 ft path for the delayed wave would result in a cancellation at 100 Hz, and higher frequency dips could also result from that same delayed path, but higher frequencies would be at 300 Hz, 500 Hz, etc.
Being that low, the 100Hz null may respond well to bass trapping on the front wall behind the center channel. A trap effective at 100Hz is going to be too large to fit between the wall and the rack, so I'm hoping traps placed in the nearby corners may have some "residual" effect. Another option would be to move the center, which would change the cancellation frequency according to the SBIR formula: fc = c / 4dfwall, where c=1125ft/sec and dwall is the distance between the speaker and the front wall. It would be ideal to move the center away from the wall, which would drive the cancellation frequency down - preferably below the speaker's low frequency cutoff. But since the rack stays where it is, the speaker can only be moved back about a foot. That would push the quarter wavelength cancellation frequency up to 187.5 Hz, still too low to be treated with traditional acoustic panels. I think a frequency-specific panel/trap might be needed.

I don't understand how you arrived at 300 and 500Hz for higher cancellation frequencies. Is it just a matter of odd-order harmonics? Why not even order? Please explain.

.
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post #34 of 68 Old 03-06-17, 01:36 PM
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Re: A Sound Renovation for the Green Room

With a delayed signal path and a reflection that re-combines with the original signal and causes cancellation, the frequencies at which that occurs are defined as:


F1 = 1 / (2 * D)
F2 = 3 / (2 * D)
F3 = 5 / (2 * D)
F4 = 7 / (2 * D)

A 5 ft delay path will give (roughly)

F1 = 1 / (2 * 5ms) = 100 Hz
F2 = 3 / (2 * 5ms) = 300 Hz
F3 = 5 / (2 * 5ms) = 500 Hz
etc
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post #35 of 68 Old 03-06-17, 10:34 PM
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A Sound Renovation for the Green Room

Wow Lou! I can hardly keep up with you. Sheesh! Lol.
Sorry, I don't know how to multi quote your post without butchering it.
Yes I was referring to center and subs for the rise, but the mains and sub exhibit a similar rise. The XO on the mains looks excellent. I am surprised that Dirac locks out the manual delay thing. At least you have the phase knob on the sub. You asked(Wayne?) about good for the goose and gander lol. I think you'll end up with a balance in the middle somewhere. Interested to see what Dirac does. Especially with that bottom end. If I'm not mistaken(very easily I may be), room gain(or vessel pressure) starts with the longest room dim /565. If your ceiling is 8' that is 70hz, but also your shortest length so...idk! Also that these graphs are without EQ is really good. I also don't think the waterfalls are too bad either, but I've spent the least amount of time with them. Glad Wayne's here.
This link references rhythmik subs, which stands to reason since it's from their site. However, the principles apply. Just some more to stuff in that big brain.
http://www.rythmikaudio.com/phase1.html

Last edited by willis7469; 03-06-17 at 10:43 PM. Reason: Added link
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post #36 of 68 Old 03-07-17, 10:30 AM Thread Starter
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Re: A Sound Renovation for the Green Room

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Yes I was referring to center and subs for the rise, but the mains and sub exhibit a similar rise. The XO on the mains looks excellent.
I'm curious about the rise. It's present in the mains, the center, and the sub. The room is the common denominator, so if no one checks-in I'll just accept it. A house-curve with an inverse slope should make short work of it!

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I am surprised that Dirac locks out the manual delay thing.
You and at least one other person - though I've also heard it elsewhere (you may want to check out Posts #8, 14, and 15 here).

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At least you have the phase knob on the sub. ... <snip> ... I think you'll end up with a balance in the middle somewhere. Interested to see what Dirac does. Especially with that bottom end.
Yes, and there's fortunately a wide range of phase angles from which to choose that work well for the mains. I'm also considering reversing the direction of the sub's port so that it's "loaded" by a nearby wall, rather than firing into an open corner. Failing that, I'm also considering swapping it with the album crates on the side wall. That might help to restore symmetry and so raise the dip seen for the left speaker.

.
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post #37 of 68 Old 03-07-17, 10:43 AM
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Re: A Sound Renovation for the Green Room

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I'm curious about the rise. It's present in the mains, the center, and the sub. The room is the common denominator, so if no one checks-in I'll just accept it. A house-curve with an inverse slope should make short work of it!
I noticed in the center+mains thread, you mentioned to Brian that you reduced the sub level. Just grasping, but could that be what we're seeing? Yes a house curve would take care of that!





You and at least one other person - though I've also heard it elsewhere (you may want to check out Posts #8, 14, and 15 here).



I'll look. I believe you, but it just seems odd.



Yes, and there's fortunately a wide range of phase angles from which to choose that work well for the mains. I'm also considering reversing the direction of the sub's port so that it's "loaded" by a nearby wall, rather than firing into an open corner. Failing that, I'm also considering swapping it with the album crates on the side wall. That might help to restore symmetry and so raise the dip seen for the left speaker.


At those wavelengths, I'd be amazed if the port orientation would matter. Wait, where is your port located?
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post #38 of 68 Old 03-07-17, 11:41 AM Thread Starter
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Re: A Sound Renovation for the Green Room

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At those wavelengths, I'd be amazed if the port orientation would matter. Wait, where is your port located?
See how much I know! Side-firing port on bottom-firing sub. I think you mentioned sub orientation to me a while back. When would port location make a difference, if at all?

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post #39 of 68 Old 03-07-17, 11:45 AM
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Re: A Sound Renovation for the Green Room

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To identify all reflections, you'd have to cover the whole room in reflective tape (lol)!
No need. Human beings have evolved in reflective spaces for millennia. I've posted this before, but I'll oblige again: http://www.audioholics.com/room-acou...man-adaptation
and http://www.audioholics.com/room-acou...steners-prefer
Reflections aren't create spontaneously, they are created by your loudspeakers. The exact pattern is determined wholly by the source(s) of their creation. So you can't look at "room" reflections independent of what is creating them...or you may miss half the picture ;-).
Your loudspeakers have reasonably smooth off axis responses http://www.stereophile.com/content/r...kfuuYfoYxb1.97
Our brains constantly compare the "direct" onset sounds coming from the speaker and the reflections coming from all around. The summation affects the perceived timbre and of course spatial rendition of the sound. When the off axis is a mangled mess and the reflections contain little DNA of the sound origin, it become more difficult if not impossible for the brain to ignore them. Not good.
You should not have such issues. If this is primarily a MCH HT setup, by all means turn it more into an iso-ward and allow the various channels to do their thing. However, if you're trying to strike a balance and want good 2CH front sterophonic sound, be very wary of this approach. There is of course a matter of taste. One mans visually over reflective room, may be another mans lifeless studio stereo construct....to the ears.

cheers

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post #40 of 68 Old 03-08-17, 02:18 PM Thread Starter
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Re: A Sound Renovation for the Green Room

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Glad you figured out center position. Nice looking setup.
You have very good speakers, do not touch anything above 400hz or so, unless you find the overall balance a bit bright or dull.
Do not try to fill sharp nulls in the bass. Concentrate mainly on bass peak issues if they are audibly offensive.
Thank you for the advice and kind words!
The restriction over 400Hz you mention has to do with the Schroeder frequency, yes? Is it unwise to manipulate the upper frequencies because unequal EQ in L/R channels might damage SS&I in some way? And is the reason for not trying to fill sharp LF nulls that they're regions of destructive interference (room modes), which can only be tamed with acoustic traps? I like your last qualifier about leaving bass peaks alone unless they're audibly offensive. Human hearing is not all that reliable at low frequencies, so good SQ can easily be ruined with heavy-handed EQ.


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No need. Human beings have evolved in reflective spaces for millennia. I've posted this before, but I'll oblige again: http://www.audioholics.com/room-acou...man-adaptation
and http://www.audioholics.com/room-acou...steners-prefer
Interesting reading! I especially like the section on Toole's discussion of bass buildup being mistaken for treble rolloff where he says, "The key is to have the right amount of broadband absorption, and find ways to tame frequency-specific resonances without making the room overly dead." It's of special interest to me because that is my new goal. EQing and trapping can easily become an exercise for their own sake. For instance, conventional wisdom has it that first reflections should always be treated to improve SS&I, and that bass traps should always be added in the corners on the front wall to help tame bass resonances. How many of us blindly follow that advice as "given" before the first measurement is even considered? As one of your referenced articles states: "The widespread belief that first reflections in listening rooms are bad originated in the recording industry, where many mixers felt that they were better able to do their jobs when they were in a strong direct sound field; reflections attenuated." Some people may need a lot of persuading to overcome that deeply entrenched notion.


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Our brains constantly compare the "direct" onset sounds coming from the speaker and the reflections coming from all around. The summation affects the perceived timbre and of course spatial rendition of the sound. When the off axis is a mangled mess and the reflections contain little DNA of the sound origin, it become more difficult if not impossible for the brain to ignore them. Not good.
Coherent reflections, uncorrelated packets, incident waves - all very cerebral! And all very necessary for Toole's research: "If the spectra of the direct and reflected sounds are significantly different, the reflections are likely to be more noticeable, from subtle timbral effects up to a premature breakdown of the precedence effect, at which point listeners may be aware of two simultaneous sound images, one located at the loudspeaker and one located at the point of reflection." But I'm unsure I'm totally convinced, or even concerned. Perhaps I should be if I'm ever to reap the benefits of taming frequency-specific resonances. I already know too well how easily my room becomes overdamped using my current acoustic treatments. What worked well in my old listening room does not translate well into my currently smaller space. A different approach is obviously needed, and I thank you for making me question the validity of my past practices. I now prefer to fix the problems than to let the room/speaker/treatment system editorialize all content played through it. That's easier said than done and I may not reach my goal in this room, but the journey will be fun.


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Your loudspeakers have reasonably smooth off axis responses ... <snip> ... You should not have such issues. If this is primarily a MCH HT setup, by all means turn it more into an iso-ward and allow the various channels to do their thing. However, if you're trying to strike a balance and want good 2CH front sterophonic sound, be very wary of this approach. There is of course a matter of taste. One mans visually over reflective room, may be another mans lifeless studio stereo construct....to the ears.
Yes, I'll be trying for good balance with an emphasis toward music. I was pleasantly surprised at the level of performance afforded by the new setup in the untreated room. Not apples-to-apples since the center channel came up off the floor onto its own stand. And not apples-to-apples since I secretly (...ooops, cat's outta the bag) brought in a different complement of acoustic treatments for comparison. But I do think there are issues. The trouble comes in properly identifying them as reflection-based or as something else.

EDIT: Since posting the above I've done a bit more digging and came across some good explanations (in posts 24-27 here) of why a room treated for HT will not perform well for stereo. HT needs an absorbent front-wall to keep the ambient info from creeping forward, whereas stereo needs the right balance and type of reflections to preserve spatial cues for robust SS&I. Both benefit from diffusion on the back wall.

.
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Last edited by Lumen; 03-10-17 at 10:00 AM. Reason: Added HT vs stereo comments
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