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Discussion Starter #1
Hello everyone
after setting up my AV system in REW i have noticed some nasty dips around the 129 hz and 320hz area i have enclosed a plan of my room and the REW response graph and waterfall -----
What causes these nulls? Can they be treated with panels? where would i place panels?
The room layout and listening position has to stay as it is i'm afraid
thanks for any replies to my post
regards
Darren
 

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Is this purely the mains and no sub? If not, what is the xover frequency and slope. The 120 may be a sub phase issue.

As for the other, hard to say without knowing a little more. With the chairs against the wall, you may not be able to fix everything.

Bryan
 

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Discussion Starter #3 (Edited)
soz my bad should have been more specific:doh:
the graphs are mains and sub combined- 50hz crossover 0 phase,this was the best setting for the low bass extension i required.
Enclosed is another graph of mains only-Target of 75db, no smoothing, no house curve.
 

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The 300ish is most likely a boundary anomaly from the mains. If you really want to see what's happening, don't smooth anything at all and set the horizontal scale to just be 20-400hz. While this won't deal with the 320, if the slope is shallow, some of the 120 may be from mains/sub interaction

Also, it seems as though the sub level is set quite high in relation to the mains and you're getting a pretty severe rolloff starting at about 35hz.

Modal distribution around the problem frequencies

Frequency hz Spacing % Wavelength 1/2 Wavelength 1/4 Wavelength l w h Mode

119.2 10.4 9'6" 4'9" 2'4" 2 1 1 Oblique
119.6 0.3 9'5" 4'9" 2'4" 0 2 0 Axial

122.4 2.2 9'3" 4'7" 2'4" 3 1 0 Tangential
124.8 1.9 9'1" 4'6" 2'3" 1 2 0 Tangential


320.1 0.7 3'6" 1'9" 0'11" 4 3 3 Oblique
320.2 0 3'6" 1'9" 0'11" 9 0 0 Axial
320.6 0.1 3'6" 1'9" 0'11" 6 4 0 Tangential
321.3 0.2 3'6" 1'9" 0'11" 0 2 4 Tangential
321.3 0 3'6" 1'9" 0'11" 8 0 2 Tangential


Note the bolded lines. These are SO close to each other in the case of 120Hz that they're right on top of each other. Wouldn't be so bad if they weren't basically all 1st and 2nd order modes.

On the 320ish - they're higher order but there are so many piled pretty much exactly on top of each other, they're going to build up

Shift the mic about 1' to the left or right and see what happens to the 120hz.

Bryan
 

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Discussion Starter #5
Thanks for your help on this Bryan- i have enclosed a new plot of the main speakers, no sub, no crossover,no smoothing and with the mic position changed to the left (green trace) and to the right (blue trace).
The sub is a little bit hot on the earlier sweeps i know and will be lowered slightly, i was just following a recommended house curve!
 

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And this is still back where your ear is length wise and height wise - correct?

Bryan
 

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Discussion Starter #7
yeah bryan same height and length from original position i just moved the mic about a foot to the left and right i also ran a sweep further forward (2/3 length) from normal listening position and got this-
 

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OK. So, both of the nulls are definitely at least partially related to the length dimension as neither appears when you moved forward in the length dimension. The 70ish peak is still there so that's most likely either height related or something we can fix via movement of the mains to take advantage of boundary cancellations.

If the seating has to stay back pretty much against the wall, I'd pull it forward a bit and experiment with a couple of bales of insulation laid behind the couch.

Bryan
 
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