# large null in theater room...help



## bcarver0267 (Nov 27, 2015)

So, I have a large null in my theater room. I spent all weekend doing the sub crawl with an SPL meter and got both my sub's in the bester location. What else can I do? I ordered a umik1 and it is on the way. I have no room room treatment as of yet. I will post measurement when it arrives...is there anything I can do in the meantime?


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## bpape (Sep 14, 2006)

Have you moved the mic front to back or left to right to see if you can determine where the null is coming from?


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## bcarver0267 (Nov 27, 2015)

The mic has not arrived yet. I have only used an SPL meter. 

Do you mean, move the mic around or change its orientation?


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## bpape (Sep 14, 2006)

Understood. Just walk with the meter front to back and side to side and see where the null changes. Try to keep it at seated head level.


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## bcarver0267 (Nov 27, 2015)

Ok...i walked the slp meter around the room to diff spots. I am down around 7 to 10 db in about a 6ft oval around the mlp. There are a couple of other spots but nobody sit in them. My room is 23d 15w 8 ft ceiling. Mlp is about 15 ft off the front wall on the long side. I don't know what to do next?


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## NBPk402 (Feb 21, 2012)

How many subs do you have? I am thinking that if you were to have 2 or 4 subs (whatever you can handle) in the room you could change the null. I have 2 subs (centered on the front wall and rear wall), and have thought about getting 2 more subs to center on the side walls to get more bass to the front row. I know that having multiple subs will even out the bass, just not sure if it will solve your problem. Bryan will know if it is possible or might have another suggestion.


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## bcarver0267 (Nov 27, 2015)

I have 2 18s...they are in the ft corners. I have moved them to every poss placement option available. They are in the best place for output at the mlp. I plan on getting 2 more in the future. I just wasn't sure if there was anything I needed to do or add to help. I have no room treatment or eq-ing of any kind until my new marantz arrives. I am changing almost my entire system and wanted to "do it right" this time.


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## NBPk402 (Feb 21, 2012)

bcarver0267 said:


> I have 2 18s...they are in the ft corners. I have moved them to every poss placement option available. They are in the best place for output at the mlp. I plan on getting 2 more in the future. I just wasn't sure if there was anything I needed to do or add to help. I have no room treatment or eq-ing of any kind until my new marantz arrives. I am changing almost my entire system and wanted to "do it right" this time.


Gotcha. 
Check out the download graphs here... http://www.harman.com/innovation


Are your subs facing the MLP or aimed toward the center?

You are def on the right track trying to get the sound as good as possible before running Audyssey, as it will have an easier time getting it perfect for you.


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## bcarver0267 (Nov 27, 2015)

I'm gonna read it right now...thanks


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## NBPk402 (Feb 21, 2012)

bcarver0267 said:


> I'm gonna read it right now...thanks


Here is the image of the sub placements... I used (a), but I will go to (c) if I get 2 more subs.


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## bcarver0267 (Nov 27, 2015)

Both of mine are in the front corners. One is angle toward the mlp the other is straight. This is what I found yeilds the best result and gives best imaging with front and rears. 

I don't have rew setup yet, but have played with sub distance on my pre/pro. I read that phase can be adjusted that way. Something like 12 ft is 90 degree. Again it all by ear and SPL meter.

The suns sound a lot better now. But I plan on builds some corner trap this weekend. Lowes has some mineral wool insulation I'm going to try. Not sure if I should just order a kit from gik tho. I have read that the custom made corner kit don't do a lot in every room.


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## bcarver0267 (Nov 27, 2015)

I have to place them I'm the corner until the projector and screen get put in later this year. This will bey first attempt at prof room treatment


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## bpape (Sep 14, 2006)

If the null is in an area that large then yes, you'll need to play with sub position. Did it change a lot side to side?


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## bcarver0267 (Nov 27, 2015)

More ft to back...about 3 ft in ft and back of me. I have moved them everywhere along the walls. I can't put them in the middle of the room so I didn't try. I may just have to live with it


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## bpape (Sep 14, 2006)

It could be a non-modal null off the wall behind you. Thicker treatment in the rear would help if that's the case.


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## Tonto (Jun 30, 2007)

Have you tried turning the all the way around (facing the wall behind them). I have seen this work very good.


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## willis7469 (Jan 31, 2014)

Try moving the LP forward. The room length divided by 1/5 is 4.6'. 1/5ths and 1/3rds are a good place to start for avoiding nulls. Assuming I read your post correctly about being 3' off the wall. Are you using the built in test tone to measure?


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Also, I'm pretty sure move the sub distance in the avr will be a "by guess, by golly", since the lfe test tone will only activate the subs alone, and won't do anything for a null, since it's really just a delay. You could adjust and listen, but REW will definitely show you in living color. It will help with impulse response, and smoothing the crossover region, but you'll need to play a tone at the crossover frequency(i.e.: 80hz). This will activate the subs and the mains at the same time, and at that point you'll look for the most output on the spl meter. YouTube has test tones, or the App Store has some nice tone generators.(android might too). 
AƒG - Audio Function Generator PRO by Thomas Gruber
https://appsto.re/us/pgCw1.i
This is the one I use. AirPlay, or mini to rca.


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## willis7469 (Jan 31, 2014)

Pics?


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## bcarver0267 (Nov 27, 2015)

My decrease in SPL is about 3ft in front and behind my mlp. Each sub is in the front corners. 
The left is about 8" off the front wall and maybe 4" off the side. It points almost directly facing the mlp. Maybe 25 degree angle.
The right is about 20" off the front wall and maybe 2" from the side. It faces parallel to the side wall.
This is the best I can get them to sound. 

When you say the 1/5th thing. Is that from both ft and side wall? Cause that will be in the middle of the room. Putting the sub in front of the mains.


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## bcarver0267 (Nov 27, 2015)

I will post some pics when I get home. I am at work now


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## willis7469 (Jan 31, 2014)

I was meaning the LP being at 1/5 since the subs are tucked in the corners. How far are your mains off the front wall?


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## bcarver0267 (Nov 27, 2015)

3 ft from side and about 2 ft from ft. 

Yes I'm using the built in test tone on my lexicon pre/pro. 

I can try moving the listening position forward some. You will see when I post pics tonight. It is a very large L shaped couch. I'm not sure if I can get 3 more feet or not. Bit I will try.


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## bcarver0267 (Nov 27, 2015)

Here are soon pics as promised....pardon the mess, it is a work in progress for the next month.


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## willis7469 (Jan 31, 2014)

Time is short at this moment but how about a shot of the back? 2/3 of my subs are behind the LP in corners. Just a quick idea...


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## bcarver0267 (Nov 27, 2015)

I tried moving the subs to the back, every combo I could come up with. I just have a huge null


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## bcarver0267 (Nov 27, 2015)

Here is a better shot of the subs up front on either side

There is no rush. This will be a month or 2 long project to get it the way I want it. I appreciate everybody's help and input. As I said before, I want to do it right this time and not just throw it together to make it work.


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## NBPk402 (Feb 21, 2012)

bcarver0267 said:


> View attachment 112866
> 
> 
> 
> ...


Have you tried the subs behind the couch facing the wall


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## bcarver0267 (Nov 27, 2015)

No not facing the wall. I had them behind the couch on the side wall face toward the center of the room behind my head at the mlp. I will try that this weekend. My umik should be here fridayish and I can fire up rew. I know this is a lot of guess work at this point. 

I do have a question about listening position and the 1/5th or 1/3rd the length of the room. If I understand this correctly. I'm going to try to move the couch back to both location and run some tests with few. 

Thanks again everyone for the help and input. I will need a lot more in the coming months.

Well I'm off to Lowes to get materials for bass traps and rear wall treatment. I will post pics when they are up.


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## bcarver0267 (Nov 27, 2015)

Well diy corner bass traps on finished and in. I notice a diff in sound in the room. Not sure how to describe it yet tho. Will put up measurements soon.


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## bcarver0267 (Nov 27, 2015)




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## Babak (Mar 20, 2009)

Hello, 

Just one question.

What is bad with a null? 

Basically the human ear works differently than the microphone that is used for measurements. 

The microphone just records sound levels when such measurements are being made , independent of the time structure of the sound.
It does not distinguish between the first wavefront and reflection that arrive at the same time as the most recent wavefront but originated with a different wavefront much earlier. 

The microphone just sums up the sound pressures and thus also detects when those sounds cancel reach others out (nulls)

The ear does does distinguish between the first wavefront and reflections because it analyzes the time structure of arriving sound patterns. 

That's why our ears are also not really sensitive to comb filter effects.

A null is just a position, the direct sound, the first wavefront is not being disturbed by room resonances. 
One can still hear the respective frequencies very well. Just try it out with sine waves with the respective frequencies. 

I would rather be worried about the resonances that increase the sound level at certain frequencies. 

Cheers 
Babak


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## willis7469 (Jan 31, 2014)

Babak said:


> One can still hear the respective frequencies very well. Just try it out with sine waves with the respective frequencies.
> 
> 
> Cheers
> Babak


Nope. Not true. I've tried this many times before I had REW. Manually implemented sine waves, and an spl meter can show in real time which frequencies are cancelled, or exaggerated by the room. Time doesn't alter this. The LP position, or wave propagation location does. That's why moving speakers/subs, or the LP alters the effect. 



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## Babak (Mar 20, 2009)

Well, an SPL meter is also a microphone and does not resemble the human hearing.they both only measure sound pressure.

It doesn't make sense to conclude an audible effect based on a measurement with devices and methods that work completely different than the human auditive system. 

The question is if it's really a null (of a room mode) or some cancellation at that frequency (a low frequency comb filter effect).

If it is a real null, than great!
Put the speakers or subs right on the null. There they can't excite any room resonances at the respective frequencies. 
Also put the listening position onto a null. The resonances with the respective frequencies will have the least sound pressure and hence the least audible effect there. 
That's also in line with recommendations by Floyd Toole. 

Of it is a comb filter effect at a low frequency - just forget them. 
The human ear is not sensitive to comb filter effects (proven in studies).
A comb filter effect is just something that looks ugly on a measurement graph because - yes microphones vs ears. 

What else can that null be that needs treatment?


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## willis7469 (Jan 31, 2014)

I guess it depends on what the null is from.(I think that's also what you're saying). Fwiw, when manually running and measuring tones, my ears heard the differences in level. If a null is created by the room, the only help is relocating equipment or LP. If a null is a phase issue then that can be addressed. I also still say nulls need to be addressed whether there created by the room, or equipment. Do I understand you to mean that nulls from phase, or comb filtering are just measurement anomalies that should be left alone, as the ears are not effected the same way a mic is?


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## bcarver0267 (Nov 27, 2015)

Well here is my waterfall. what now?


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## Babak (Mar 20, 2009)

willis7469 said:


> I guess it depends on what the null is from.(I think that's also what you're saying).


That's right 



willis7469 said:


> Fwiw, when manually running and measuring tones, my ears heard the differences in level.


Of course.
Can you tell whether that is because the respective frequencies are reduced or those frequencies are at normal level an others around them are increased in level due to resonances?

Here one should also keep in mind that due to the loudness cuves of the human ear we perceive low frequency sound with a lower loudness anyway.




willis7469 said:


> If a null is created by the room, the only help is relocating equipment or LP. If a null is a phase issue then that can be addressed. I also still say nulls need to be addressed whether there created by the room, or equipment.


Of course fluctuations in the frequency response need to be addressed. 
I personally would rather focus on the peaks and resonances rather than the nulls and dips.




willis7469 said:


> Do I understand you to mean that nulls from phase, or comb filtering are just measurement anomalies that should be left alone, as the ears are not effected the same way a mic is?


Yes and no. 

I wouldn't say that they are abnormalities because they can be clearly measured - repeatedly and consistently.

And yes, the ears are not effected the same way as a microphone is that one users for measurements. 

Cheers from Vienna


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## Babak (Mar 20, 2009)

bcarver0267 said:


> Well here is my waterfall. what now?


Do you also have a RT 60 or RT30 measurement? 

The decay times at certain frequencies would tell more. 

Cheers


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## Tonto (Jun 30, 2007)

It looks like you are about a foot off the from wall. Spin them 180 degrees & measure again. That will effectively change the null ~2 feet. If still bad can you then move them out 1 more foot from the front wall.


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## bcarver0267 (Nov 27, 2015)

Right now they are 6' off the front wall. Both subs are pointing inward toward the center of the room. I can spin them 90' and have them face toward to back of the room.

I will post rt30 and rt60 later today.


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## willis7469 (Jan 31, 2014)

BC, wish I had something to offer on your waterfall. Looks like extra ringing between 35 and 55hz. Traps? That's all's i got...


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## bcarver0267 (Nov 27, 2015)

I built 2 large corner traps in the rear corners. Can't do the front corners. Where else should I built some? Ceiling to wall corners around the tops of the side or front walls?


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## willis7469 (Jan 31, 2014)

I would try Bryan from GIK. He chimes in regularly, and I've seen him support the diy enthusiast. He's also quite brilliant. 


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## bcarver0267 (Nov 27, 2015)

I taken to Bryan. He said more low end bass traps. I guess I have a weekend project. Will post new waterfall soon


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## willis7469 (Jan 31, 2014)

I did see the thread you started. I was hoping he would drop in. Looks like you're going the right way. 


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## bcarver0267 (Nov 27, 2015)

It is more difficult than I thought it would be. I figured you put in some corner trap and panel at reflection point and then you are good to go. I was wrong...


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## willis7469 (Jan 31, 2014)

Same here. It's why I don't make much money in acoustic treatments...


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## bcarver0267 (Nov 27, 2015)

Panels are up, I hope it isn't overkill.


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## willis7469 (Jan 31, 2014)

Nope. Looks good. Can't even see em!


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## lesmor (Dec 30, 2009)

bcarver0267 said:


> Panels are up, I hope it isn't overkill.
> 
> View attachment 114834


You got that back wall well covered, be good to hear if it does anything for the null
If nothing else it should improve ringing


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