hmm, treatment didn't work - Page 2 - Home Theater Forum and Systems - HomeTheaterShack.com

Old 04-03-10, 09:06 PM
New Member
Deang

Join Date: May 2006
Location: Seattle
Posts: 3
Re: hmm, treatment didn't work

Quote:
SierraMikeBravo wrote: View Post
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.
deang is offline

Old 04-03-10, 09:22 PM
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Greg

Join Date: Nov 2007
Location: NY
Posts: 2,601
Re: hmm, treatment didn't work

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.

-Greg

Don't worry... nothing new here, I've already made that mistake. Trust me.
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Old 04-03-10, 09:50 PM Thread Starter
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Frank

Join Date: Feb 2010
Posts: 74
Re: hmm, treatment didn't work

Quote:
glaufman wrote: View Post
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.

Quote:
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.
ToBeFrank is offline
Old 04-04-10, 04:20 PM
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Bryan Pape

Join Date: Sep 2006
Location: Wildwood, MO (St. Loui
Posts: 5,209
Re: hmm, treatment didn't work

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

I am serious... and don't call me Shirley.

Bryan Pape
GIK Acoustics
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Old 04-04-10, 06:25 PM
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Shawn

Join Date: Jul 2007
Location: Topeka
Posts: 144
Re: hmm, treatment didn't work

Quote:
bpape wrote: View Post
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.

Architectual Acoustics & Cinema Designer
HAA Level III/THX Video Certified Calibrator
Audyssey Professional Installer/Technician
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Old 04-04-10, 06:39 PM
HTS Senior Moderator

Bryan Pape

Join Date: Sep 2006
Location: Wildwood, MO (St. Loui
Posts: 5,209
Re: hmm, treatment didn't work

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

I am serious... and don't call me Shirley.

Bryan Pape
GIK Acoustics
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Old 04-04-10, 07:10 PM
Senior Shackster
Shawn

Join Date: Jul 2007
Location: Topeka
Posts: 144
Re: hmm, treatment didn't work

Quote:
bpape wrote: View Post
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
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!

Architectual Acoustics & Cinema Designer
HAA Level III/THX Video Certified Calibrator
Audyssey Professional Installer/Technician
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Old 04-05-10, 08:25 AM
Elite Shackster

Greg

Join Date: Nov 2007
Location: NY
Posts: 2,601
Re: hmm, treatment didn't work

Quote:
SierraMikeBravo wrote: View Post
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.

-Greg

Don't worry... nothing new here, I've already made that mistake. Trust me.
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