Front wall-mounted 170s... to be or not to be...

[propdrc]

Hello all,
Another Newb' researching a HT system... that isn't yet... 'been there, eh? Step one is to put a 5.2 system together with five 170s and two L-12s with a 975/5000 combo and a Mini-DSP-2x4hd in between. After reading Dave's honorable mention of Cotytech, I'm imagining putting three SP-OS-08s across for the front stage under the TV (in let's say a 13'x15'x8'). Scanning the web (in the short run) does not offer much for the necessary speaker-to-wall space for rear-ported bookshelf speakers such as the 170s. The Cotytechs, once mounted and cradling the 170s will only offer an 8" clearance from the wall. Question: At this clearance, would it be best to cross them over at 60hz or 80hz? I would really like to hear from Ascend and other rear-ported book shelf owners so we stay on topic.
Thank you all in advance,
Donald

[Johnny_Mac_III]

8 inches behind the speaker is plenty. Generally a good rule of thumb is at least the port diameter away from the wall. When we are talking speaker distance away from walls, a bigger culprit is the cancelation frequency due to the reflection off that back wall. The closer you can get the speaker to the wall the lower the cancelation frequency is. And if you cross you speaker over to the sub above that cancelation frequency, it doesn't exist, because the sub ends up playing the frequency instead of the speaker. The other way to do it is to get the speaker away from the wall as far as possible. This raises the cancelation frequency giving you a better chance to treat it with absorption behind the speaker.

I wouldn't cross the 170s over any less than 80hz. They can play lower, but why when you have the L12s? Also if you are able to place the L12s strategically in your room to kill standing waves that would be all the better to cross them over at the highest frequency that isn't localizable to you. For instance, my 340s can play down to 40hz in my room. However, where they are placed, I have a null at 80 hz. So, I cross them over at 90 hz, because my sub has a linear response out to 100 HZ. In the end, this allows for a more linear response from the speaker and sub working together.

[propdrc]

J-Mac, Thanks for getting back to me, I appreciate it. More questions: :) Where did you learn about combining port diameter and wall distance? If I understand you correctly, 2" from the wall would be better than 8" to get the most from the 170s on the low end, right? From what I've read, learning REW will teach me about and finding nulls (relative to sweet spot) but what about taming cancellation frequencies? Would 2" thick 12x12 suffice behind the 170s work for this? What is the rule for absorption behind bookshelf speakers? I do have a absorption/dispersion plan. Thankfully the L12 is generous above 80hz too and yes, two are for the standing wave issue (although the thought of four...:cool: with two more 170s and a couple of boxes from miniDSP...:rolleyes:) Tweaker's delight, eh?

[sludgeogre]

J-Mac, Thanks for getting back to me, I appreciate it. More questions: :) Where did you learn about combining port diameter and wall distance? If I understand you correctly, 2" from the wall would be better than 8" to get the most from the 170s on the low end, right? From what I've read, learning REW will teach me about and finding nulls (relative to sweet spot) but what about taming cancellation frequencies? Would 2" thick 12x12 suffice behind the 170s work for this? What is the rule for absorption behind bookshelf speakers? I do have a absorption/dispersion plan. Thankfully the L12 is generous above 80hz too and yes, two are for the standing wave issue (although the thought of four...:cool: with two more 170s and a couple of boxes from miniDSP...:rolleyes:) Tweaker's delight, eh?

From what I have learned so far, it seems to me that placing absorptive material behind a bookshelf speaker will not do you any good if the cancellation frequency is below 100 Hz. The best absorptive materials don't even start absorbing until 100 Hz or so and fall off rapidly after 350 to 450 or so, so like Johnny Mac said, you need to raise the cancellation frequency in order to treat it with absorption, and this absorption would need to be placed both behind the speaker and at the first reflection points.

From what I have set up, I would agree that crossing over higher and using your subwoofers to take control of content below the cancellation frequency really does sound the best. It allows your speakers to play the mids with far less distortion and gives you a much more linear in-room response.

[Johnny_Mac_III]

No problem!

The closer you get to the wall, the more gain you will get in the low end, yes. And those peaks can be tamed by EQ. But it also activates stronger nulls. However, if you sit in neutral spots in the room (80%, 68%, or 55% of room length) the modes will have less of an affect.

As far as SBIR goes, unless you are siting in the front half of your room, having your speakers about 3 inches from the wall will shift that cancelation frequency below 80 Hz. If you move them away from the wall, I would look at 4" absorption. And like sludgeogre said, the plan is to get that cancelation to wher it can be treated. Even better with a 2 inch air gap. 2" inch absorption only works for more directional frequencies not the the lower frequencies that will wrap around your speakers and bounce off the front wall.

If you are looking to get your 170s closer to the wall, these will give you more than enough clearance for the port.
http://www.amazon.com/gp/aw/d/B000X9O8SI/ref=mp_s_a_1_1?qid=1461588891&sr=8-1&pi=SY200_QL40&keywords=speaker+wall+mount&dpPl=1&dpID=41RvsFiIitL&ref=plSrch

Of course the other mounts will work great too, just place some absorption behind them.

Here is a good read on SBIR on how to calculate your cancelation frequencies.
http://www.acousticfrontiers.com/what-is-speaker-boundary-interference/

Those L12s with CBM-170s are going to sound amazing!

[propdrc]

Thank you for getting back to me on my adventure to wall-mount land...
J-Mac, 3" from the wall, check. I like that mount too, similar in style/price to the Cotytech I mentioned above and thanks for the Math homework... lol.
S-Orge, I get what you're saying however what about applying absorption behind/around wall-mounted bookshelf speakers on the L/R? My center, near the TV, I'm thinking diffusion there... Because the bookshelves are 9" wide, would you go with 12" or 24" wide behind (or in my case above/below) the L/R speakers?

Research shows that my first room treatment plan is to DIY some 4"x24"x48" w/2" gap at 1st reflection walls and ceiling followed by 2" and diffusion at 2nd reflections and then some GIK range-limiter traps in the back corners. I'm also toying with the idea of placing Auralex T-fusers flanking my wall-mounted bookshelf center. (I wonder if I could cheat and just use the Prime Accoustics London 12? ) After all this, I begin my 3-way with REW and miniDSP in search of... (how did one of you guys put it out here in Forum Land... oh yeah) Ear-gasim... ROFLOL!

http://www.auralex.com/product/tfusor-3d-sound-diffusor/

http://www.primacoustic.com/london12.htm

[sludgeogre]

S-Orge, I get what you're saying however what about applying absorption behind/around wall-mounted bookshelf speakers on the L/R? My center, near the TV, I'm thinking diffusion there... Because the bookshelves are 9" wide, would you go with 12" or 24" wide behind (or in my case above/below) the L/R speakers?


I'm not really experienced in this area so I was just recommending what I have heard worked for other people, especially on AVS Forum. I have seen many setups that include absorption behind the speakers, especially if they are rear ported. I can't say what kind of impact this has, but some people have said it really improved overall accuracy and intelligibility of their system. YMMV.

I have only seen diffusion applied to the back wall, and I think that is the only area where it is appropriate, since it is reducing backwave radiation, but I don't believe you want to eliminate that from your front stage as that can be beneficial at the front from what I understand. My vote would be to put as much absorption as you can behind your speakers and at the first reflection points and just put diffusers on the back wall.

Also, Acoustimac has the best prices of anyone I've looked at. I haven't purchased from them yet, but will when I build my new home.

http://www.acoustimac.com/home-theater-room-package-2

[Johnny_Mac_III]

Propdrc, sounds like you've got a good plan. And go with 24 inch wide.

Since you are getting REW, I would first lay the corner traps, a thick carpet/rug between you and your front soundstage, and some bass traps behind any LCRs if they are moved away from the front wall. then run REW, read the t60 measurement, and lay the rest of your acoustical treatment in steps. The goal is to have an even decay time at least from 250 Hz to 4KHz in the .2 - .5 second range. Let those measurements dictate what you lay from hear on out.

So let's say now you want to focus on your side wall 1st reflections. Take a measurement. If both your higher frequencies (500 hz and above) and lower frequencies (500 hz and below) need to decay faster you will lay 4 inch absorption. If your lower frequencies are already decaying fast enough, but your higher frequencies are still out of control then lay some 2 inch absorption. If your lower frequencies aren't decaying fast enough; but your higher frequencies are good, then lay a hybrid absorber/diffuser that will absorb and diffuse at the frequency you need them to. If you have even decay times somewhere between .2 - .5 seconds, but you still need to treat the first reflection, then use diffusion.

Next, treat your rear wall and ceiling in the same manner. Keep in mind that if you have more than one row or your seats are spread wide appart you may need to go back and do the same procedure for those seats as well.

Keep in mind that there are many different strategies to treat rooms. This is what I've found sounds good with HT and multichannel audio. Some acousticians recommend simply covering a certain % of your wall with absorption and a certain percentage with diffusion. Others say don't treat first reflections. Others say treat all first reflection points with absorption. So, YMMV with any of these methods.

[sludgeogre]

Propdrc, sounds like you've got a good plan. And go with 24 inch wide.

Since you are getting REW, I would first lay the corner traps, a thick carpet/rug between you and your front soundstage, and some bass traps behind any LCRs if they are moved away from the front wall. then run REW, read the t60 measurement, and lay the rest of your acoustical treatment in steps. The goal is to have an even decay time at least from 250 Hz to 4KHz in the .2 - .5 second range. Let those measurements dictate what you lay from hear on out.

So let's say now you want to focus on your side wall 1st reflections. Take a measurement. If both your higher frequencies and lower frequencies need to decay faster you will lay 4 inch absorption. If your lower frequencies are already decaying fast enough, but your higher frequencies are still out of control then lay some 2 inch absorption. If your lower frequencies aren't decaying fast enough; but your higher frequencies are good, then lay a hybrid absorber/diffuser that will absorb and diffuse at the frequency you need them to. If you have even decay times somewhere between .2 - .5 seconds, but you still need to treat the first reflection point, then use diffusion.

Next, treat your rear wall and ceiling in the same manner. Keep in mind that if you have more than one row or your seats are spread wide appart you may need to go back and do the same procedure for those seats as well.

Keep in mind that their are many different strategies to treat rooms. This is what I've found sounds good with HT and multichannel audio. Some acousticians recommend simply covering a certain % of your wall with absorption and a certain percentage with diffusion. Others say don't treat first reflections. Others say treat all first reflection points with absorption. So YMMV with any of these methods.

Very well written post. Extremely sensible. Thanks a lot for properly explaining the methodology here instead of getting hung up on minor details as we often do.

[propdrc]

S-Orge, I really like the Acoustimac room treatment set you showed me and the price is nice... It looks like they use 2" for walls and ceiling and 4" for traps but I'll read-up on them, thanks! They definitely have attractive prices on panels. I'll be aiming for the less is more approach when the day comes.
J-Mac, Wow. You had to drop the Science bomb... Holy-put-away-your-credit-card, Batman... I had no idea that I'd be getting into Decay Times... Seriously, that's the best REW Cliff's Notes I've seen around... I will certainly print those up for setup day... 'Looks like first you crawl, then you REW, then you run... But if you're ever in San Diego (once the equipment arrives)... I could get to running sooner and the beer's on me... but it's porters and stouts here or you're out.
Thank you both so much!
Donald
And now there's new Emotiva on Sound and Vision...
http://www.soundandvision.com/content/emotiva-announces-%E2%80%9Caffordable-high-end%E2%80%9D-electronics-line#vqjBotmuk81A8AgE.97
There's nothing else I need. There's nothing else I need. There's nothing... Oh, look over there...

[Johnny_Mac_III]

Haha! Welcome to the hobby of home theater. Not all of us can have Dennis Erskine come build our home theater. We all have to actually learn stuff :p

[propdrc]

J-Mac,
Thanks: you sent me back to school with that T60 line... now REW is playing hard to get before I even get her home... apparently she wants an edjamakated man... luckily at the rate of my present economy, I'll be able to woo her by the time my equipment arrives...

BTW: I saw this quote below on AVS Forum, is it true? I am still yet to downgrade my ChromeBook to a PC... so I cannot see this for myself...

"If I'm not mistaken the eq tool in REW has a whole tab for modal resonances and you can input room dimensions and it will calculate room modes and T60 times."

Thanks!

[Johnny_Mac_III]

It does have a feature called "Room Sim." You input your dimensions of your room, place your speakers, give your seating position, crossovers, ect. and it will calculate room modes and give you an estimation of frequency response based on those room modes. It's actually been pretty accurate for me. So, I would assume the same for you if you have a rectangular room. As far as T60 goes -- it does not estimate that. The only type of time domain estimation is in the EQ feature. It will allow you to apply EQ and estimate what the effect would be in the time domain if the EQ was applied.

[propdrc]

J-Mac, You have 2 Mother Earth Imperial Stouts on account next time you're in San Diego (about 30min south of Ascend). How long have you been tweakin' on REW anyway?
Donald

[propdrc]

J-Mac, S-Orge, EVERYBODY!!!,
For those about to R... Absorb, GIK solutes you! DIY absorb/trap frames: $26 ea... for 4"x24"x48"
http://www.gikacoustics.com/product/diy-bass-trap-acoustic-panel-frames/

<img src="http://www.gikacoustics.com/wp-content/uploads/2016/04/GIK-Acoustics-4in-DIY-Frame-backside-510x600.jpg" alt="GIK Acoustics 4in DIY Frame backside"/>

[Johnny_Mac_III]

San Diego? Your livin the life!

[propdrc]

Raining today... Although surfing with Cameron Diaz yesterday didn't suck... Just kidding, it was her little sister... :cool: LOL... I'd still rather have my system up and going...