# Room Mode Question~



## red99 (Dec 10, 2014)

First, is a Room Mode different than a Null? In that a null can be changed with speaker (Subs in this case) placement, but a room mode only with MLP change? 

Im trying to find if anything can be done to flatten out a Room Mode at 17Hz besides moving the MLP? The only way I have found to flatten it out is to move the MLP back in the kitchen. (I have a open floor plan family room / kitchen). The room is about 30 feet from front wall to back of kitchen. The MLP is about 11 feet from the front wall ( 19 feet from back wall). If I move the mic back about 4 or 5 feet, then it flattens out. But I would be sitting in the kitchen then. 

I have moved the subs around every inch of the walls in the family room using REW RTA. The only thing as far as movement goes that has any effect on the 17Hz dip, is moving the MLP back. (Sub movement has no effect)

So is there any kind of treatment or tricks I can do to either flatten it out that I am not aware of, or am I just stuck with this? 

And can someone explain what is happening here? Is this a cancellation from a reflecting sound wave off the back wall? (Which is kitchen cabinets mostly).



NOTE: The upper sweep is after tweaking the DSP with a low shelf gain. So I can bring up the 12 Hz, but it has no effect on the 16-17 Hz range.


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## bpape (Sep 14, 2006)

The wavelengths are so long - no real way to do it. And honestly - it really isn't that far out of whack. Additionally, it is at 17hz - lots better things to worry about.


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## Talley (Dec 8, 2010)

I agree. Your talking about something you cannot even hear. You have a good amount in the 14-15hz range and again you cannot even hear what your talking about and the fact that it is impossible to detect a difference between 15hz to 17hz that you'll never know it at all. Little nulls like the one you have at 130hz range your brain would not detect during normal playback because your brain hears the entire range at once... not individual notes.

I've read alot that says the bass hitting impact that you typically feel is in the 40-70hz range anyway and all the sub 25hz frequencies does is "rattles" things with more rumbling but less punch. Under certain situations it helps create realism though as it really moves the air in the room.

Remember not to play test tones at 106db either... you'll cause damage to your equipment if you do. Keep it between 75-85db for best and most consistent results.

To answer your question. Nulls are speaker placement and/or MLP placement together. Modes are room resonance that is more affected by the sound in your exact room dimension. Modes are better to EQ down and/or bass trap tuned to whatever frequency your having issue to flatten out the "boominess" of the mode. Nulls you leave alone as there is only MLP/speaker location that can improve it but again... quick dips are really undetectable but in only certain situations like maybe solo cello music where a perticular frequency note is hit in that exact dip area might cause some dB loss that is detectable.


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## Lumen (May 17, 2014)

Brian gives rock solid advice! If the null nags away at you, try different sub angles 45 degrees at a time (15 , if you have the patience). And you may want to try sweep measurements for each orientation instead. An RTA cannot accurately display narrow, deep nulls (you may unintentionally skip over them).


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## red99 (Dec 10, 2014)

Ok, thanks for the feedback. I'm aware you can't hear that low and it's a pressure wave. Im a bit of a petfectionist I guess and was just wondering if it could be flattened out. I'll just ignore it though.


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## Lumen (May 17, 2014)

red99 said:


> First, is a Room Mode different than a Null? In that a null can be changed with speaker (Subs in this case) placement, but a room mode only with MLP change?


Almost. A room mode has one or more nulls, but a null isn't always a room mode. Nulls can also be caused by speaker/sub location. But room modes cannot be changed. They are determined solely by room dimensions and exist as long as the room exists. Moving the LP and/or speaker only shuffles the modes you hear, and those you do not. 



red99 said:


> The upper sweep is after tweaking the DSP with a low shelf gain. So I can bring up the 12 Hz, but it has no effect on the 16-17 Hz range.


If the dip/null does not respond to EQ, it's a room mode, and you're stuck with it.


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## red99 (Dec 10, 2014)

Lumen said:


> Almost. A room mode has one or more nulls, but a null isn't always a room mode. Nulls can also be caused by speaker/sub location. But room modes cannot be changed. They are determined solely by room dimensions and exist as long as the room exists. Moving the LP and/or speaker only shuffles the modes you hear, and those you do not.
> 
> 
> *If the dip/null does not respond to EQ, it's a room mode, and you're stuck with it*.


I was thinking this was the case. But I wanted to make sure. Thanks


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## dgmartin (Oct 29, 2011)

Lumen said:


> Almost. A room mode has one or more nulls, but a null isn't always a room mode. Nulls can also be caused by speaker/sub location. But room modes cannot be changed. They are determined solely by room dimensions and exist as long as the room exists. Moving the LP and/or speaker only shuffles the modes you hear, and those you do not.
> 
> 
> If the dip/null does not respond to EQ, it's a room mode, and you're stuck with it.


In general, a room mode should respond to EQ. There is no way you can change its frequency and spatial distribution. However you can change the excitation magnitude (with EQ), the excitation location (speaker position) and output location (LP). 

EQ cannot cope with deep nulls like you have slightly above 100Hz. This can be moved in frequency by moving the sub position wrt near walls (or other subs) but not eliminated. About the 17Hz peak, I would try to tame it down anyways since it would mean reduced driver excursion and distortion (i.e. the benefits might be in the upper frequencies of your sub).

Hope that helps. If anything just ask.


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## Lumen (May 17, 2014)

dgmartin said:


> In general, a room mode should respond to EQ. There is no way you can change its frequency and spatial distribution. However you can change the excitation magnitude (with EQ), the excitation location (speaker position) and output location (LP). EQ cannot cope with deep nulls like you have slightly above 100Hz. This can be moved in frequency by moving the sub position wrt near walls (or other subs) but not eliminated...
> Hope that helps. If anything just ask.


Well, no it really doesn't, because you contradict yourself. Your opening statement asserts that a room mode responds to EQ, but later you state EQ cannot cope with deep nulls. If you re-read my post, it states that a null WILL respond to EQ if and only if it's NOT a room mode. IOW, you can have modal nulls (determined by room dimensions), and non-modal nulls (determined by speaker/listener location). Think of a modal null as a drain: it will never fill up with water no matter how much you pour into it. Likewise, you can keep pouring power into a null, but that won't change it's magnitude. An article written by Dick Stroud (an AES patent-holder and acoustics engineering expert) explains nulls and what to do about them appears here. More information on room modes by Acoustic Sciences Corporation  appears here and here

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dgmartin said:


> About the 17Hz peak, I would try to tame it down anyways since it would mean reduced driver excursion and distortion (i.e. the benefits might be in the upper frequencies of your sub).


Agreed! Taming the fundamental should also positively affect its harmonics.


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## dgmartin (Oct 29, 2011)

Lumen said:


> Well, no it really doesn't, because you contradict yourself. Your opening statement asserts that a room mode responds to EQ, but later you state EQ cannot cope with deep nulls. If you re-read my post, it states that a null WILL respond to EQ if and only if it's NOT a room mode. IOW, you can have modal nulls (determined by room dimensions), and non-modal nulls (determined by speaker/listener location). Think of a modal null as a drain: it will never fill up with water no matter how much you pour into it. Likewise, you can keep pouring power into a null, but that won't change it's magnitude. An article written by Dick Stroud (an AES patent-holder and acoustics engineering expert) explains nulls and what to do about them appears here. More information on room modes by Acoustic Sciences Corporation  appears here and here


My reply about modes was really made in the context of the OP who wanted to address a peak caused by a mode (so far I never had issues with modal nulls but only with boundary reflection nulls -my reply might have been biased in that direction), so I agree it was self contradicting without specifying that. I fully agree EQ can only help to tame the peaks caused by the mode(s) and not the nulls. 



Lumen said:


> Agreed! Taming the fundamental should also positively affect its harmonics.


Yes that's another benefit.


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## red99 (Dec 10, 2014)

dgmartin said:


> My reply about modes was really made in the context* of the OP who wanted to address a peak caused by a mode* (so far I never had issues with modal nulls but only with boundary reflection nulls -my reply might have been biased in that direction), so I agree it was self contradicting without specifying that. I fully agree EQ can only help to tame the peaks caused by the mode(s) and not the nulls.
> 
> 
> Yes that's another benefit.


I never said anything about a peak. I was talking about the dip at around 17 Hz that can only be changed by moving the MPL back about 4 feet. Sub placement has no effect (at least in the family room).


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## Lumen (May 17, 2014)

red99 said:


> I was talking about the dip at around 17 Hz that can only be changed by moving the MPL back about 4 feet. Sub placement has no effect (at least in the family room).


I have a silly question, red99: can you move the L/R mains forward toward the LP? Chances are you won't have to go the full 4 ft. If the dip won't respond to EQ, another way to tailor the response is to change the room size. But that's not an option for most people.

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## Lumen (May 17, 2014)

If the dip responds to moving the MLP, it will also respond to moving the mains. Have you tried that? They don't necessarily need to come forward 4 ft. It's hard to tell whether the mode is axial, tangential, or oblique--though judging by it's magnitude, it's most likely axial, since that's the strongest of the three. You may be able to get by with moving the speakers just a few inches in any direction to avoid exciting that particular 17Hz mode.


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## vidiot33 (Dec 12, 2013)

Nulls, if I understand them correctly, exist in rectangular rooms at the 1/4, 1/2 and 3/4 long axis points. Ideally, you don't want your listening position in any of them, but in a multiple of 3rds or fifths the length of the room.

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## red99 (Dec 10, 2014)

How does the mains have any effect at 17hz?


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## Lumen (May 17, 2014)

Ummm, they don't if they're not reproducing it! Sorry about that :R
Let me start over:

If aesthetics/WAF interfere, then the following tip also will also sadly not work. You mentioned you moved the sub along every inch of the walls, but what if it were positioned more out into the room? Close to the room's center is impractical from both a lifestyle and acoustic standpoint, so it can't go there. But there should be a location somewhere other than against the wall where the dip can be smoothed out. Room response is funny that way, though. I wouldn't be surprised to find that the "problem" merely shifts to a different portion of the spectrum.

Your PSA XS15se subwoofer has a high-calibre pedigree, but is rated 23Hz-200Hz +/-3dB (9Hz-16Hz typical in-room extension). The key term here is "typical." Your mileage may vary!


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## vidiot33 (Dec 12, 2013)

red99 said:


> How does the mains have any effect at 17hz?


Good point. Don't know of ANY full range speakers that have any significant response at 17hz.

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## vidiot33 (Dec 12, 2013)

Also, 17 HZ is below the range of human hearing, the vibrations can be felt, but not heard.

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## Talley (Dec 8, 2010)

vidiot33 said:


> Good point. Don't know of ANY full range speakers that have any significant response at 17hz.
> 
> Sent from my iPhone using HTShack


Golden Ear Triton Ones

nouf' said :T

And my uncles Vandersteen 5As have good output to right at 17hz then falls off.


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## red99 (Dec 10, 2014)

Lumen said:


> Ummm, they don't if they're not reproducing it! Sorry about that :R
> Let me start over:
> 
> If aesthetics/WAF interfere, then the following tip also will also sadly not work. You mentioned you moved the sub along every inch of the walls, but what if it were positioned more out into the room? Close to the room's center is impractical from both a lifestyle and acoustic standpoint, so it can't go there. But there should be a location somewhere other than against the wall where the dip can be smoothed out. Room response is funny that way, though. I wouldn't be surprised to find that the "problem" merely shifts to a different portion of the spectrum.
> ...


I dont have the dual PSA subs any longer. I have dual F.A. 18.0's. And I dont see an edit feature on my equipment list. So I just did it on my edit profile. Not sure if it will get changed by someone from there or not.


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## Talley (Dec 8, 2010)

Also moving speakers or sub I don't think will matter when your talking about a audio wavelength of 66 feet.


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## dgmartin (Oct 29, 2011)

red99 said:


> I never said anything about a peak. I was talking about the dip at around 17 Hz that can only be changed by moving the MPL back about 4 feet. Sub placement has no effect (at least in the family room).


Sorry for misreading you. I shall not read posts again past midnight. Re-reading the OP I think this thread should have ended at post #2...


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## Lumen (May 17, 2014)

dgmartin said:


> Re-reading the OP I think this thread should have ended at post #2...


But then we wouldn' t have been able to milk it! :bigsmile:


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## Lumen (May 17, 2014)

Talley said:


> Also moving speakers or sub I don't think will matter when your talking about a audio wavelength of 66 feet.


That's the part of room acoustics that doesn't seem to make common sense. It's not about how long the wavelengths are. It's about room dimensions and how they carve room response in stone. The dimensions establish resonant modes that are excited only when a sound source is placed in the proper location in the room. You can excite all modes by placing the speaker in a corner. Moving that speaker out into the room excites a subset of the room's modes. If the speaker is in the right location, presto! - the room mode gets excited. If not, then it doesn't. 

Many speaker-positioning guides take advantage of room mode science for best bass response from the mains. The Rule of Thirds is one of these. Placing a sound source 1/3 of a room's length and 1/3 of its width supposedly excites the least number of modes (assuming the speaker is not rolled off at a lower frequency). When a subwoofer is involved, mains can be placed for best SS&I largely without consideration for modes. If what you say were true, then how does a sub-crawl help find the spot for smoothest bass response?


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## willis7469 (Jan 31, 2014)

Yep. Kinda like a drum has a resonant frequency of its own. So does a room.


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