Help with correction of room resonance - AVS Forum
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post #1 of 15 Old 11-18-2013, 06:34 PM - Thread Starter
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Ok so I have recently completed my IB subwoofer build and have completed EQ'uing the with REW and the Behringer FBQ1000.

Here are the results with REW



The higher line is a measurement at close range at the outlet of the manifold and the lower line is the output from the primary listening position.

As you can see from the graph I have a pretty good handle on the room resonances from 20 HZ and above via the FBQ1000. However, my calculations based on room size (19 x 40 x 12) show and the graph above confirms I have a big room resonance around 13 - 14 hz.

Pictures of my room can be found http://www.avsforum.com/t/1484030/new-house-and-new-to-avs

What can I do to help correct this resonance?
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post #2 of 15 Old 11-18-2013, 09:42 PM
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A couch made of jello! wink.gif
JK
I'm interested in answers on this as well.
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post #3 of 15 Old 11-18-2013, 10:25 PM
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Not much if you can't move the listening position, short of herculean efforts at bass trapping.

Unlikely but possibly that you have a wall resonance coincidentally at the same freq as the 1st mode, so it acts as a tuned bass trap.

Fix by filling with concrete smile.gif

Another outlandish idea would be to port it a la Adventures in Closet Sized Subwoofers thread, and use a very large long tuned to 14 Hz that exits part way down the room.

But if you can do that, just move the driver there.

Where along the 40 ft length is the driver?

Noah
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post #4 of 15 Old 11-18-2013, 10:56 PM
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Quote:
Originally Posted by volfan6415 View Post

What can I do to help correct this resonance?
Unbolt your IB sub and re-install it at the midpoint of the 40' dimension (a mode cannot resonate if the source of sound pressure is at its null location).

Sanjay
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post #5 of 15 Old 11-19-2013, 09:02 AM - Thread Starter
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The sub is positioned at approximately 15 feet from the front wall. I am aware that I am dealing with an odd room.

I can't move the subwoofer to the midpoint because that is where the fireplace is that extends into the attic space and there isn't room to install the manifold at that location.

Also placing the subwoofer there would place it behind the primary listening position. As I said the room is oddly shaped. Originally when the house was built the space was two rooms with a common fireplace in the shared wall. At some point the wall on either side of the fireplace was removed to make one large room with a floating fireplace in between, fairly common in New Orleans. What was the "front room" is now being used as the living room and is where the TV is, the "back room" is our dining room.

If this was a dedicated theater room I would not have had to make as many concessions as I have. However, from yalls comments it looks like this might be something that I am going to have to deal with in this space.
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post #6 of 15 Old 11-19-2013, 09:03 AM - Thread Starter
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Quote:
Originally Posted by superedge88 View Post

A couch made of jello! wink.gif
JK
I'm interested in answers on this as well.

Not sure the couch made of jello would pass the WAF test, but would be super comfortable and tasty too. smile.gif
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post #7 of 15 Old 11-19-2013, 09:06 AM - Thread Starter
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Quote:
Originally Posted by noah katz View Post

Not much if you can't move the listening position, short of herculean efforts at bass trapping.

Another outlandish idea would be to port it a la Adventures in Closet Sized Subwoofers thread, and use a very large long tuned to 14 Hz that exits part way down the room.

This would mean that the subwoofer no longer operated as an IB subwoofer correct?
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post #8 of 15 Old 11-19-2013, 01:46 PM
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Quote:
Originally Posted by volfan6415 View Post

This would mean that the subwoofer no longer operated as an IB subwoofer correct?

Correct, but you get more output and lower distortion for the same power

Noah
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post #9 of 15 Old 11-20-2013, 10:45 AM
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Quote:
Originally Posted by volfan6415 View Post

As you can see from the graph I have a pretty good handle on the room resonances from 20 HZ and above via the FBQ1000. However, my calculations based on room size (19 x 40 x 12) show and the graph above confirms I have a big room resonance around 13 - 14 hz.

What can I do to help correct this resonance?

Good question.

In my opinion, your modal calculations, and your observance of the FR dip in the measurement, aren't necessarily cause and effect.

Yes, room modal resonance behavior is absolutely tied to a room's dimensions between boundaries. Although there's three variations, we primarily are most interested in the axial, as they're often the most influential, thus the most important. The Q, or sharpness of said resonant behavior, depends on boundary impedance, ie., how lossy or rigid each boundary is. For example, a fully concrete bunker type room would possess sharply defined modal resonances. And if any boundary was relatively lossy, like a long stretch of typical wood stud/drywall that had no perpendicular walls built off of it, then the previously well defined modal resonance involving that boundary is now less so well defined.

I shared all that because your room appears quite complex, and therefore I'd be less confident in any type of modal calculations, and much more inclined to merely focus on observed measurements. In a best case scenario for accurate modeling, whereas there's a perfectly symmetrical rectangular box, with all boundaries identically built, and no furnishings, it's still difficult to model modal behavior in a perfectly accurate manner. So as easy as it may seem to link two components, in this case your modeled expected results, and the actual measurements taken, in reality acoustics is very complex and often counter-intuitive.

That said, if your room possesses a longest dimension of 40', then the calculated first axial mode is about 14hz. A room's first axial mode, 1, 0, 0, doesn't typically cause a dip in freq response. Modal resonances within the subwoofer range typically cause constructive, supportive acoustic observances. Conversely, dips in FR are typically destructive interference.

Examining your measurement, it shows your response peaking in the low 20s, and rolling off from there. In my experience, the dip in the low teens is likely a ceiling resonance, diaphragmatically subtracting energy from the playback. What's more concerning to me is the area above 50hz, as this is a vital range that's important for both music and movie playback. It's possible we're seeing a robust house curve of 15dB from 100hz downward, in that case the response is relatively smooth with the exception of the small excess of ~50hz energy. That small peak there may very well be a product of the floor to ceiling axial mode, and other modal influences a touch higher.

So bottom line, nothing really appears that unusual. It would appear that given the equipment used, the FR peaking in the low 20s, and rolling off from there is not unexpected at all. Continue to experiment, and remember you can't boost a destructive null. Peaks are mostly caused by modal influences, and nulls and most often destructive 1/4 wave acoustic interference. And most importantly in the context of this thread, the destructive dip in the low teens in your FR, is likely caused by ceiling motion.

That's my take, good luck and welcome to AVS

Quote:
Originally Posted by sdurani View Post

Unbolt your IB sub and re-install it at the midpoint of the 40' dimension (a mode cannot resonate if the source of sound pressure is at its null location).


For what I'm striving for in my playback system, I'd suggest most every room with a largest dimension eliciting reciprocal gains around 20hz and lower, is a good thing. cool.gif
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post #10 of 15 Old 11-20-2013, 11:14 AM
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Place it at the mid point from the attic thru the ceiling


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post #11 of 15 Old 11-20-2013, 11:54 AM
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Quote:
Originally Posted by FOH View Post

A room's first axial mode, 1, 0, 0, doesn't typically cause a dip in freq response. Modal resonances within the subwoofer range typically cause constructive, supportive acoustic observances. Conversely, dips in FR are typically destructive interference.

I believe they just as typically create nulls, though they're sharper in both freq and spatially, and that the first axial mode has a null halfway between the walls.

Noah
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post #12 of 15 Old 11-20-2013, 01:51 PM
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Quote:
Originally Posted by noah katz View Post

I believe they just as typically create nulls, though they're sharper in both freq and spatially, and that the first axial mode has a null halfway between the walls.
Yup, the first-order mode has a null at the midpoint of its dimension. So does every odd-order mode (though they have nulls elsewhere as well).

Sanjay
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post #13 of 15 Old 11-20-2013, 02:26 PM - Thread Starter
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Quote:
Originally Posted by FOH View Post


Examining your measurement, it shows your response peaking in the low 20s, and rolling off from there. In my experience, the dip in the low teens is likely a ceiling resonance, diaphragmatically subtracting energy from the playback. What's more concerning to me is the area above 50hz, as this is a vital range that's important for both music and movie playback. It's possible we're seeing a robust house curve of 15dB from 100hz downward, in that case the response is relatively smooth with the exception of the small excess of ~50hz energy. That small peak there may very well be a product of the floor to ceiling axial mode, and other modal influences a touch higher.

So bottom line, nothing really appears that unusual. It would appear that given the equipment used, the FR peaking in the low 20s, and rolling off from there is not unexpected at all. Continue to experiment, and remember you can't boost a destructive null. Peaks are mostly caused by modal influences, and nulls and most often destructive 1/4 wave acoustic interference. And most importantly in the context of this thread, the destructive dip in the low teens in your FR, is likely caused by ceiling motion.

Thanks for all this information! I am new to the audio side of the home theater and this is my first real dabble into any significant upgrade to the sound. I am new to this so any advice is welcome. I understand that my room is far from ideal but want to do everything I can to optimize the listening experience. And if nothing else at least be aware of where the shortcomings of the system are.

You mention continuing to experiment, what additional measurements would you suggest taking? Also what range should I be showing? I chose a cutoff of 100HZ because my crossover on the AVR is set to 80 hz so I assumed any measurements after this point were not needed and or helpful.
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post #14 of 15 Old 11-20-2013, 02:49 PM
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Quote:
Originally Posted by volfan6415 View Post

You mention continuing to experiment, what additional measurements would you suggest taking?

You could see if there are alternate locations where the null isn't; it might only take a foot or two.

Noah
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post #15 of 15 Old 11-20-2013, 06:08 PM - Thread Starter
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Ok so I have taken some more measurements. I put the Feedback destroyer in Bypass mode and made 10 measurements in the following locations:



Each measurement was taken at my ear height while standing, I am right at 6 feet. All have 1/6 smoothing applied

Position 1

Position 2

Position 3

Position 4

Position 5

Position 6

Position 7

Position 8

Position 9

Position 10



I should also note that I am using an air vent return grill as the "cover" - here is a close up of the subwoofer outlet. The grill is position so that the "openings" are facing the back of the room.

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