# hmm, treatment didn't work



## ToBeFrank (Feb 27, 2010)

I have a null at 149.8Hz at my listening position. According to the dimensions of my room I have an axial mode at 148.7Hz. I made two 4" 2'x4' panels (8lb mineral wool) to try to treat this mode. I played a 150Hz sine wave and placed both panels where I heard the mode the loudest. That ended up being in the middle of the wall near the ceiling and in the middle of the wall near the floor (same wall, no treatment on the opposite wall due to a sliding glass door). Unfortunately, this didn't touch the null and actually made it slightly worse. What should this be telling me? Am I doing something wrong?


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## bpape (Sep 14, 2006)

Which room dimension has the mode at 148? 

Bryan


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## ToBeFrank (Feb 27, 2010)

The room is 22.8x14.7x10.4 ft. One corner is a 100 sq ft opening to an entryway and the kitchen. The mode is along the 22.8 dimension. Note that since this is a combined dining area and living room, the LP is along the width, not the length.


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## ToBeFrank (Feb 27, 2010)

I guess I should mention I have 24" superchunks floor to ceiling in the back corners, three 4" 2'x4' panels behind the LP (the LP is close to the back wall), and a 2" 2'x4' panel behind each front speaker.


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## ToBeFrank (Feb 27, 2010)

BTW, when I play the 150Hz sine wave, I can feel the drywall of the wall I'm treating vibrating. I'm not sure if this has any bearing on the matter.


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## ToBeFrank (Feb 27, 2010)

Any suggestions?


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## bpape (Sep 14, 2006)

How close are you sitting to the wall? Corners will only do so much. Sometimes you need to address specific places - like behind you. 

If the wall is not insulated, it could well be adding to the problem by resonating at that frequency.

Bryan


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## ToBeFrank (Feb 27, 2010)

Thanks Bryan,



bpape said:


> How close are you sitting to the wall? Corners will only do so much. Sometimes you need to address specific places - like behind you.


Here is what I have so far:

1) 24" superchunks in both back corners, floor to ceiling. I don't have front corners... one is the 100 sq ft opening and the other is a fireplace

2) Three 4" 2'x4' 8lb mineral wool panels behind the listening position. The back of the couch is 16" from the back wall which puts my ears in the main LP 24" from the wall. This really is as far forward as I can push the couch. I currently have the panels standing up straddling the floor and wall. This puts the top of them about 16" above my ears. Should I put them in a different position?

3) Two 2" 2'x4' 8lb mineral wool panels on the front wall... one behind each front speaker. I'm sure I'm getting reflections off the fireplace, but obviously not much I can do there.

4) Two 4" 2'x4' 8lb mineral wool panels on the wall to the left of the listening position. This is the dining area and in the lengthwise dimension of the room. These are the panels I tried to set up to tame the 150Hz null. They are placed at the top and bottom of the wall.



> If the wall is not insulated, it could well be adding to the problem by resonating at that frequency.


It is not insulated as it's an interior wall. Based on feeling it vibrate I was wondering if that was the problem. Other than filling it with insulation is there anything I can do?


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## deang (May 18, 2006)

Sorry, this got kind of long and it rambles. Feel free to ask questions. I fix your kind of problems for a living. 

Treating a small area to remove a room mode isn't very effective (I guess you know that). You'd need significantly more wall coverage to modify a large problem. 

Try this: You can also minimize the mode by moving/making sure the speaker's driver (each speaker) is at the 1/4 wavelength of 150Hz ((1130/150)/4=1.88ft or about 23") and you may immediately cancel you're problem. That's about 23" from any wall surface (side or front wall) to the driver. Multiples of 23", like 46", also work. You're trying to create a null at that frequency. Move the speakers within a few inches of those distances only as an experiment. 

The other "trick" is to place a mid-basstrap (small ASC Tube Traps work great for this) at the rear or rear corner of the each front speaker and not on the wall. A simple open basstrap (depends on the wall type) won't work.

I carry a tape measure in 1/4 wavelengths with me to client's houses that have your problem. I also take a variety of basstraps to experiment with. Using both testing software/mic and listening I can fix your type of probem often but not always. 

You might also consider equalization if it can't be controlled with acoustical materials.


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## SierraMikeBravo (Jul 1, 2007)

Ah, hang on a second, did you say you have a NULL? If you have a null, you can't really treat it. It is the absence of energy and the wave is actually at one of it's two slowest speeds at the null. The only way to effectively treat it is by moving your seating. Peaks are what you are trying to treat for amongst other things. In addition, you are going to have boundary gain along any wall regardless of how perfect your room is. This is because air particles have higher pressure along the boundary as they have nowhere to go but jam against each other. Physically, the only way to treat the mode is the use treatment at 1/4 wavelength of the offending mode away from whichever opposing boundaries the harmonic frequency is resonating between. This means, somewhere out in the room, and this is where frictional absorbers technically have their maximum effect since the particles are moving the fastest. Practically speaking, moving the seating out of the null and perhaps using electronic correction is really the only way to deal with this problem. Good luck and best wishes!


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## deang (May 18, 2006)

SierraMikeBravo said:


> Ah, hang on a second, did you say you have a NULL? If you have a null, you can't really treat it. It is the absence of energy and the wave is actually at one of it's two slowest speeds at the null. The only way to effectively treat it is by moving your seating. Peaks are what you are trying to treat for amongst other things. In addition, you are going to have boundary gain along any wall regardless of how perfect your room is. This is because air particles have higher pressure along the boundary as they have nowhere to go but jam against each other. Physically, the only way to treat the mode is the use treatment at 1/4 wavelength of the offending mode away from whichever opposing boundaries the harmonic frequency is resonating between. This means, somewhere out in the room, and this is where frictional absorbers technically have their maximum effect since the particles are moving the fastest. Practically speaking, moving the seating out of the null and perhaps using electronic correction is really the only way to deal with this problem. Good luck and best wishes!


Absolutely correct, you can't treat a null with acoustical treatment. That's what I get for not asking enough questions first. I assumed, and we know what that means, when he said room mode he has a peak and not a hole at that frequency. 

I can't even assume that the problem is actually related to a room dimension at all.

However, if there is a peak at 150Hz then he certainly can place his speakers in the 1/4 wavelength position and create a null. 

On the other hand, if there is a hole and his speakers are at 1/4 wavelength to one or both walls then he can certainly see if moving them out of that position will remove the hole. That really isn't treatment but placement.


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## glaufman (Nov 25, 2007)

Keeping in mind that when dealing with a mode, peaks and nulls are caused by the same phenomenon: interference between waves. The only difference is where in the resulting standing wave you're listening. Any acoustical treatment that would affect a modal peak does so by reducing the wave reflections, and as such in the case of a null should leave more of the direct wave "uncancelled" as it were.

That being said, there are other phenomena that can be causing your trouble.


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## ToBeFrank (Feb 27, 2010)

glaufman said:


> Any acoustical treatment that would affect a modal peak does so by reducing the wave reflections, and as such in the case of a null should leave more of the direct wave "uncancelled" as it were.


That's the theory I was going on.



> That being said, there are other phenomena that can be causing your trouble.


Yep. I've stopped taking measurements until my calibrated mic gets here next week. Then I can be fairly confident I'm getting good readings versus the somewhat unknown I'm getting with my RS meter.


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## bpape (Sep 14, 2006)

Sorry. I'll have to disagree there Mike. You most certainly can treat nulls. I'll agree that sometimes moving things is a good way to handle it though.

A null is caused by 2 waves colliding with each other and cancelling each other out. If we can address the wave from 1 of the 2 directions, we can absolutely address the null.

Bryan


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## SierraMikeBravo (Jul 1, 2007)

bpape said:


> Sorry. I'll have to disagree there Mike. You most certainly can treat nulls. I'll agree that sometimes moving things is a good way to handle it though.
> 
> A null is caused by 2 waves colliding with each other and cancelling each other out. If we can address the wave from 1 of the 2 directions, we can absolutely address the null.
> 
> Bryan


Hi Bryan,

Well, my name isn't Mike, it's Shawn. The handle is military phonetic and stands for my initials SMB. Mike is the phonetic for the letter "M" but that's ok and I digress . Well, I will also agree with you to a point. It just depends on what is creating the null...is it modal or is it SBIR (then it isn't a null). If we go by the strict definition of how a null is created, which is modal, then the only way to change the position of that null is to change the frequency which is dependent on the opposing boundary dimension. You can change the nulls effect by moving speakers/subs to add support in the mode...by reducing the peaks...or by changing the seating position (hence your one dimensional solution unless you count adding another wall ), but the null does not disappear at that position which is frequency dependent. The best, simplest and most economical way to treat it, if it is modal, is to move the seating forward/backward. If it is SBIR, then you attack it by moving speakers.


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## bpape (Sep 14, 2006)

Sorry about the name thing Shawn. 

I'll agree that IF it's SBIR and IF you CAN move the speakers/sub(s), that's the way to go - assuming that you don't cause other issues in doing so. SBIR creates a series of peaks and nulls but I'm sure you understand that. Just putting it out there for those reading along. It's caused by spherical waves hitting close boundaries to speakers and coming back to integrate both constructively and destructively at different frequencies with the direct waves. In smaller spaces, by the time you get speakers out into the room enough to minimize most of the issues, you can create problems with sight lines and several other things.

I'll agree that IF it's purely a room mode and too deep for conventional treatments, that moving the seating would likely be the best solution. ANY time we can get by with movement rather than treatment or EQ, that's a preferable solution. 

Both of the above are still caused by waves meeting and destructively interfering with each other. If we can address one of the wave sources and minimize it's intensity, we can therefore impact the depth of the null since now the interference is not as strong. We do this all the time. 

What if it's neither? What if you're not in a modal area and you are still getting a strong bass cancellation off the rear wall? What if it's related to the height and we can't move the seating in the Z axis? What if it's a combination of several tangential and obilque modes which happen to occur at a position that is otherwise relatively free of stronger axial modes?

Bryan


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## SierraMikeBravo (Jul 1, 2007)

bpape said:


> Sorry about the name thing Shawn.
> 
> I'll agree that IF it's SBIR and IF you CAN move the speakers/sub(s), that's the way to go - assuming that you don't cause other issues in doing so. SBIR creates a series of peaks and nulls but I'm sure you understand that. Just putting it out there for those reading along. It's caused by spherical waves hitting close boundaries to speakers and coming back to integrate both constructively and destructively at different frequencies with the direct waves. In smaller spaces, by the time you get speakers out into the room enough to minimize most of the issues, you can create problems with sight lines and several other things.
> 
> ...


Hi Bryan, 

I'll give you kudos on the tangential . Yes, best way to deal with that is through treatment of some kind. Oblique's usually are not problematic, but there is the possibility. The other possibility is always the riser or some other room "suck out". This of course, begs for the fact of calculating what the modal frequencies are in the space before hand so you know to look elsewhere. 

Yes, you can of course, mitigate a null by minimizing the source's intensity, but as I am sure you know, that is not a really good solution if I am understanding your meaning correctly. By reducing the amplitude of the wave, we are reducing the volume. Reducing the volume causes other problems such as the fact that we are more "deaf" to the loudness of low frequencies, and that causes problems attempting to get a good house curve in the room for low frequencies. This is the very reason why subwoofers have such high amplification because of our perception of loudness at such frequencies. In fact, I often "bump" up the low frequencies just to give the room a little more "excitement". 

Now, we are of course, philosophically debating, but given the frequency of the OP, 150 Hz or so, this is not often that difficult to treat. But you are correct that it warrants additional investigation as to the cause. Best wishes!


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## glaufman (Nov 25, 2007)

SierraMikeBravo said:


> Yes, you can of course, mitigate a null by minimizing the source's intensity, but as I am sure you know, that is not a really good solution if I am understanding your meaning correctly. By reducing the amplitude of the wave, we are reducing the volume. Reducing the volume causes other problems


The idea is really to reduce the amplitude of the reflected wave(s) only, whether it's SBIR or modal, not the amplitude of the direct source. In this way the overall amplitude can actually increase. But as you've correctly pointed out, the treatments needed to do this in some situations and frequency ranges may not be practical.


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