# REW Front and Back Row Differences



## circularduck (Jan 3, 2009)

I used REW with the BFD to get a "reasonable" response in my front row centre seat. When I checked the back row, it was way different. I'm just looking for suggestions. Pictures of my response graph (front row green, back row blue) and some pics of my theater below. The theater is 23'6" L x 12'5" W x 7'4" tall.

      

I have tried moving the subs around with only minor changes to the response. The dip at 22Hz can be boosted, though I don't want to add to much since it may affect my sub amps. I have tried playing with the phase and distance measurements to try and correct the issues in the back row around the crossover frequencies with little change. Any suggestions?


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## tonyvdb (Sep 5, 2007)

Nice looking room Mike, I think part of the problem is the rear wall being covered with the brick. if you were to place some acoustic panels on it you would improve the sound for the back row. Sometimes sacrifices need to be made for the rear row and just live with it.


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## circularduck (Jan 3, 2009)

My idea with the brick was to add some diffusion to the room. The bottom 2/3rds of the side walls are 1" thick insulation. I also have all 4 corners trapped, and the centre of my front wall (behind the speakers) has 2" insulation on it. I didn't want it to be overly absorptive (it may even be now, but it doesn't bother me). I fully expect a comprimise in the back, I'm just trying to minimize it.

Thanks for the response.


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## glaufman (Nov 25, 2007)

circularduck said:


> My idea with the brick was to add some diffusion to the room. The bottom 2/3rds of the side walls are 1" thick insulation. I also have all 4 corners trapped, and the centre of my front wall (behind the speakers) has 2" insulation on it. I didn't want it to be overly absorptive (it may even be now, but it doesn't bother me). I fully expect a comprimise in the back, I'm just trying to minimize it.
> 
> Thanks for the response.


Keep in mind that the 4 wall-wall corners only represent 33% of the corners in the room... there are also 4 wall-floor corners, and 4 wall-ceiling corners.

Your 1" and 2" insulation may not do very much at these frequencies. In fact, corner traps may not do very much at the lowest of these frequencies. (depends on their material/design/thickness and whether the corners they're placed in will ehlp the specific modes in question...

For evenning out the response in multiple locations, the importan things are (in no particular order), treatments, sub position, listening positions, additional subs.


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## t6902wf (Nov 14, 2008)

SVS EQ1 is supposed to be the best at evening out the response over a large area. If that is your goal it is likely the best solution. Or add more subs.


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## blahblahguy (Jan 18, 2010)

circularduck said:


> My idea with the brick was to add some diffusion to the room. The bottom 2/3rds of the side walls are 1" thick insulation. I also have all 4 corners trapped, and the centre of my front wall (behind the speakers) has 2" insulation on it. I didn't want it to be overly absorptive (it may even be now, but it doesn't bother me). I fully expect a comprimise in the back, I'm just trying to minimize it.
> 
> Thanks for the response.


 Very nice room. I'd point you over to Ethan Winer at Realtraps.com the guy is basically one of the best room treatment guys out there and has a great amount of product. Brick facing will not create diffusion. Not in any meaningful amount. Aside from my REW issues I do have a background in recording and some formal accoustics training. You need to look into cohesively treating the room as a package. got any waterfall plots ?


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## circularduck (Jan 3, 2009)

I grabbed the waterfall plots from REW. I'll be honest, I don't really know how to read these well. The first plot is the back, the second is the front.

  

I would love to add more subs, but esthetic reasons prevent me from adding more. I have moved the subs all around the front without too much effect on the response. I suspect the response across a row can be helped by 2 subs in the front, but the issues I'm having are probably related to the length dimension of my room. I have very little room to move the subs forward and back while still keeping them behind the screen.


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## lsiberian (Mar 24, 2009)

I think the solution is sub placement related. And any adjustment makes a different. Ultimately you will never have perfect sound so get it good enough for everyone or tuned for a spot.


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## blahblahguy (Jan 18, 2010)

circularduck said:


> I grabbed the waterfall plots from REW. I'll be honest, I don't really know how to read these well. The first plot is the back, the second is the front.
> 
> 
> 
> I would love to add more subs, but esthetic reasons prevent me from adding more. I have moved the subs all around the front without too much effect on the response. I suspect the response across a row can be helped by 2 subs in the front, but the issues I'm having are probably related to the length dimension of my room. I have very little room to move the subs forward and back while still keeping them behind the screen.


 bass is very omnidirectional. so moving the subs will have liite to no effect what so ever. I think you need to contact a professional room treatment company and get some assistance. That room has some horrific room peaks and modal ringing problems. looking at the plot. Moving speakers is not going to help you much.


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## circularduck (Jan 3, 2009)

I'm always looking to learn more. Can you explain the modal ringing? Is that the low frequency parts (<25Hz) that have a "deep" waterfall? Is there a way to figure out what peaks are due to room or SBIR? Since I am using a BFD, can these peaks be tamed? I've already put in some filters to reduce a huge peak I had at 45 Hz, and to try and reduce some of the larger peaks I had around 20 and 25 Hz, but I wanted to limit the use of filters somewhat.

When I was building the theater, I obviously could have hired someone to design it, but I was very, very limited with what I could change with the position of the walls. I also input the dimensions in any mode calculator I could find, and it wasn't that bad. So I did what many people probably do and used rules of thumb for the room treatments. The room doesn't actually sound that bad. I think it's because a lot of the bad parts are at the really low frequencies. I just need some more time to experiment to find what is causing the issues, room or equipment.


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## glaufman (Nov 25, 2007)

blahblahguy said:


> bass is very omnidirectional. so moving the subs will have liite to no effect what so ever. I think you need to contact a professional room treatment company and get some assistance. That room has some horrific room peaks and modal ringing problems. looking at the plot. Moving speakers is not going to help you much.


I disagree. Placing the sub in different locatoins changes how it excites modes, and potentially which modes it excites. The same is true of different seating locations.

Getting advice from an expert acoustician is never a bad idea. That being said, we have a great acoustics expert over in our Home Acoustics sub-forum.


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## glaufman (Nov 25, 2007)

Lots of great questions here... I'll try to step through one at a time...


circularduck said:


> I'm always looking to learn more. Can you explain the modal ringing? Is that the low frequency parts (<25Hz) that have a "deep" waterfall?


In short, yes. Basically, modes are standing waves, at certain frequencies the construction of the room is such that sound can bounce back and forth over again, and each reflection sort of reinforces the previous. 


> Is there a way to figure out what peaks are due to room or SBIR?


Generally speaking, if a specific frequency "rings" it's a mode. If it peaks or dips without showing a different decay slope to the frequencies around, chances are it's some other reflective effect.


> Since I am using a BFD, can these peaks be tamed?


Yes, depending a little on the specific frequencies, your room, your goals...


> I've already put in some filters to reduce a huge peak I had at 45 Hz, and to try and reduce some of the larger peaks I had around 20 and 25 Hz, but I wanted to limit the use of filters somewhat.
> 
> When I was building the theater, I obviously could have hired someone to design it, but I was very, very l
> 
> ...


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## blahblahguy (Jan 18, 2010)

glaufman said:


> *I disagree. Placing the sub in different locatoins changes how it excites modes, and potentially which modes it excites. * The same is true of different seating locations.
> 
> Getting advice from an expert acoustician is never a bad idea. That being said, we have a great acoustics expert over in our Home Acoustics sub-forum.



XXX Room size is the key element in modal ringing. There has been a myth in the HI Fi community for years that if you just move the speakers it'll fix low frequency ringing. Not so. At those frequencys the source is not the issue. Its the room. If he had comb filter issues " which I can't see right now becuase of the 1/3 octave peaks" those could be adressed with speaker positioning but at best it would be a very bad fix. 

He needs to consult with a professional room treatment company. I already suggested Ethan Winer at www.realtraps.com and I will suggest him again. Solid,no approach with proven products.Lots of good How To articles on his website BTW so make sure to give it a good look. 

there are some great articles on Gearslutz.com to about how to build and calculate proper bass trapping. this room needs alot of bass trapping. corners,cieling,wall to floor wall to celing. Wall corners. It needs lotsa of trapping. Then you can build you own diffusers. the QRD designs are a bit complicated to design but lots of help is available on the web to build such items. you can build you own bass traps to. Stacking 703 in the corner just won't cut it. BTW for a room this bad i'd get some 705 or mineralwool sheets to. 

But first. 

Bass Traps. Get those rings and peak and nulls under control. The room will improve 100x what it is now just doing so.

One other thing he can try is a EQ or shelf filter to cutoff say everything below 40hz. Its mostly all rumble anyways. That might clean the room up significantly all on its own. But I doubt it since it peaks at 100hz by the graph.


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## glaufman (Nov 25, 2007)

I'm always a big proponent of more bass trapping. I don't claim to be an expert in acoustics, and I don't claim that moving the sub will kill ALL the ringing, BUT it is a fact that sub placement, just like listener placement, affects the response of the room in the modal range. There's ample evidence proving this in the OP's own waterfalls he posted.

Ethan and his company are a great resource for acoustics issues. So is Bryan from GIK Acoustics in our Home Acoustics forum.


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## circularduck (Jan 3, 2009)

What would be the best way to ask questions in the Home Acoustics forum? I don't want to double post. Should I just start a new thread and refer to this one for the background info?


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## glaufman (Nov 25, 2007)

Yes, in this case I think that would be appropriate.
As an alternative, if you wish I could move this thread there.


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## JohnM (Apr 11, 2006)

blahblahguy said:


> I don't know what world you live in. but its not the same one I live in where room size is the key element in modal ringing. There has been a myth in the HI Fi community for years that if you just move the speakers it'll fix low frequency ringing. Not so. At those frequencys the source is not the issue. Its the room. If he had comb filter issues " which I can't see right now becuase of the 1/3 octave peaks" those could be adressed with speaker positioning but at best it would be a very bad fix.


That's a rather strange statement to make. Whilst the modal behaviour of the room is governed by the space itself, the location of the source has a dramatic effect on the room's response. A source at a nodal point for a resonance cannot excite that resonance, and a listener at a nodal point would not hear any resonance whether it had been excited or not. Speaker and listener positioning make a dramatic difference and are the first thing to experiment with.


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## circularduck (Jan 3, 2009)

glaufman said:


> Yes, in this case I think that would be appropriate.
> As an alternative, if you wish I could move this thread there.


If you could move it, that would be great.

Thanks.


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## glaufman (Nov 25, 2007)

So moved!


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## circularduck (Jan 3, 2009)

I've asked Greg to move my thread here so I can get some more input on my room measurements. I'm specifically looking for advice on what might be causing the response I'm getting, room, SBIR, etc. I'm limited in what I can do, that is, I don't want anything to appear any different then it is now, so my changes will be limited to hidden areas, like behind the screen. I am using a BFD to try and even out the response as well. Thanks in advance for your help.


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