The Official Magnepan Owners Thread - Page 158 - AVS Forum | Home Theater Discussions And Reviews
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post #4711 of 4715 Old 09-20-2016, 05:17 PM
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Google "room modes". What are the dimensions of your room? It sounds like a room mode/boundary cancellation. The wavelength of a signal at 91 Hz is about 12' 5" so dimensions around that, twice that, or half that from MLP (main listening position) to a wall will put a node (null) in the response. If that is the case then yes, bass traps can help, but you'd need a goodly number of them. Probably better to move the MLP. Chances are moving the speakers will not help since it is standing waves in the room that are the likely culprit.

Note you will always have a null in the center of the room at the fundamental frequency established by the room's dimensions. That is one reason 1/3 and 2/3 of the long dimension are often suggested as places to put the MLP.

HTH - Don
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post #4712 of 4715 Old 09-20-2016, 10:25 PM
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Quote:
Originally Posted by DonH50 View Post
Google "room modes". What are the dimensions of your room? It sounds like a room mode/boundary cancellation. The wavelength of a signal at 91 Hz is about 12' 5" so dimensions around that, twice that, or half that from MLP (main listening position) to a wall will put a node (null) in the response. If that is the case then yes, bass traps can help, but you'd need a goodly number of them. Probably better to move the MLP. Chances are moving the speakers will not help since it is standing waves in the room that are the likely culprit.

Note you will always have a null in the center of the room at the fundamental frequency established by the room's dimensions. That is one reason 1/3 and 2/3 of the long dimension are often suggested as places to put the MLP.

HTH - Don
Don, my room dimensions are 26' x 20.7' with 11.8' ceilings. My speakers are set up along the long wall. My MLP is about 4' from the back wall and about 8.3' from the nearest side wall.

I think you're right about room modes, because when I play a 91Hz tone and move from the back wall towards the speakers, I can hear the tone getting quieter and louder at regular intervals across the room.

So sounds like I should just move my listening seat forward out of the node. I appreciate your help.

Edit: I just read up on SBIR, so I'm wondering if that might be a factor also. They're about 3.64' from the front wall, which seems to be a no man's land for SBIR. I might try moving them closer to the wall, like 3' or less.

Living room: Magnepan .7, JL Audio D110, ATI AT1202, Oppo HA-1, Sennheiser HD600
Office: Elac B5, Mission 700ASi, NAD D 3020

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post #4713 of 4715 Old 09-21-2016, 07:25 AM
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I lumped SBIR into "boundary reflections" in my post. Fundamentally it's all wave cancellation, ripples in a pool where the waves bounce off a wall and recombine constructively and destructively. Where they add you get a little louder; where they cancel you get a dip (null, much quieter).

The further from the wall behind, the lower in frequency and thus (hopefully) less noticeable cancellations from the back wave hitting the wall, bouncing back, and recombining with the front wave. So you could move them back, but I'd focus on the big nulls by moving the MLP first. Moving them closer to the wall behind, unless it is treated, will probably make comb filter effects more noticeable. You will hear more amplitude variations as you sweep frequency.

FWIWFM I tend to treat the wall behind the panels unless I can get them way out in the room.
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post #4714 of 4715 Old 09-21-2016, 09:20 AM
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I can't really move the speakers further out into the room (according to formulas, I'd have to place them 6.6' away from the wall to push the cancellation notch below 60 Hz). Directly behind the speakers are windows with blinds and bookcases with glass doors and I can't really add any treatments, although what I have there probably provides more scattering than a blank wall.

I'll keep the current speaker positions for now and experiment with moving my listening position and taking some new measurements. That will require moving some furniture around so I might save it for the weekend.

Again, I appreciate all your advice Don.

Living room: Magnepan .7, JL Audio D110, ATI AT1202, Oppo HA-1, Sennheiser HD600
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post #4715 of 4715 Old 09-21-2016, 11:37 AM
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NP. Almost certainly moving the MLP will have the largest impact on that null. I had to do that in my media room despite having a plethora of absorbers.
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