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Discussion Starter · #1 ·
So I have some pot holes in my response at about 125 Hz, 250 Hz, and 500 Hz. 125 Hz = about 9ft.

My room dimensions are 8 x 11' 4" x 15' 3". The 8' ceiling mode would be more like 140 Hz, but that's close. MLP is 4' 6" from the back wall so that's the 250 Hz hole.

I'm planning to build new L&R speakers using the Cinema 10 kit. This may help with the ceiling mode. Just wondering if there are other options.


 

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If the nulls are due to room modes the practical solutions usually involve changing (moving) the listening position and/or changing the room (adding treatments to walls/floor/ceiling). A null due to signal cancellation is often not easily solved with EQ alone and EQ'ing a null tends to waste a lot of power and headroom. It looks like you have wall treatments but none on the ceiling?
 

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If the graph is the result of a single measurement position, there's not much to be commented on. Need a bunch of positions, averaged.
 

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Discussion Starter · #5 · (Edited)
If the nulls are due to room modes the practical solutions usually involve changing (moving) the listening position...
It's a small room. I can put the MLP 3-5' from the back wall. Everything below 80 looks the absolute best at 4.5' so that's where I've got the MLP.

...and/or changing the room (adding treatments to walls/floor/ceiling). A null due to signal cancellation is often not easily solved with EQ alone and EQ'ing a null tends to waste a lot of power and headroom. It looks like you have wall treatments but none on the ceiling?
That's what I'm thinking, but want some guidance before I go throwing up more bass traps. Is one on the ceiling and one on the back wall really going to fix this? Is it fixable with more super-chunk style traps in the corners? (none in the rear of the room yet).

If the graph is the result of a single measurement position, there's not much to be commented on. Need a bunch of positions, averaged.
It's not a bunch, but 3 averaged across the MLP. This is the three:

The nulls at those 3 frequencies are in each of those graphs.
 

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Take your averaged chart and run a spectrogram or waterfall analysis. Look for two things, resonances, frequencies with very slow decay, and dips, frequencies with no/little resonance and fast decay.

Treat the resonances with room treatment(s) of a type that's effective at the frequencies that resonate. I'm betting you won't have bad resonances, as they normally show up at low frequencies first... it's unusual to have a 4th or 5th harmonic room mode without the 1st, 2nd and 3rd.

Treat the holes with EQ, to bring up the level at a frequency that's not resonating, just missing. I'd also suggest a measurement on your speakers to be sure they're producing the dip frequencies in the first place. I'd do this outside, so there are no room effects, if possible.

Have fun,
Frank
 

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What are the distances from the speakers to the wall behind them, to the celling, to the floor, to the sidewall?

Those dips look like reflection cancellation. My guess is that 4.5ft will be the answer to some of the above.
 

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Yikes--maybe not so easy!

I looked more closely at you pic--you could try it with the center channel.

The 125 Hz suckout looks fairly high Q so could indeed be near boundary effect. The 1/4 wave distance for that is about 2.25 ft.

BTW, if you are getting an effect from back wall that would be around 67-ish Hz. Again, 1/4 wave distance.
 

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Discussion Starter · #15 ·
"The 125 Hz suckout looks fairly high Q"

What tells you it's high Q and what does it mean to be high Q?

Front baffle (which is angled) of the L &R speakers is 23-27" from the front wall. The inner edges closest to the screen of the L & R speakers are 23" from the side walls. There is a lot of fiberglass on both of those surfaces though.

The center is about 2" from the front wall and has 1" of 703 behind it. The center isn't in that sweep though. Just L &R & Subs.
 

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Post 5 has a trace labeled Center--

High Q means, uh, narrow.

When people talk about the distance of a speaker to a boundary for acoustic purposes they mean the actual transducer, the driver(s), not the box.
 

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"The 125 Hz suckout looks fairly high Q"

What tells you it's high Q and what does it mean to be high Q?
Q means "quality", and while the full definition is a bit more complex, it's commonly used in reference to bandwidth. High Q means less bandwidth, narrower peaks/dips. Low Q is the inverse, wider bandwidth, wide peaks/dips. You can tell all most of what you need to know by looking at the bandwidth of the peak on a graph.
 
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Discussion Starter · #18 · (Edited)
Ok, the Q thing makes sense.

BTW, if you are getting an effect from back wall that would be around 67-ish Hz. Again, 1/4 wave distance.

67 hz isn't a real problem. There's a small dip but not too bad.

So the big hole, or low Q, problem spot is at 250 which is a 1/4 wavelength of 1.12'. I can't think of any boundaries at 1'.

Post 5 has a trace labeled Center--
Center = center of the room at 4.5' from the back wall. In other words, the measurement taken from the center of the MLP. Left is the seat to the left of center and right is the seat to the right of center.

When people talk about the distance of a speaker to a boundary for acoustic purposes they mean the actual transducer, the driver(s), not the box.
Yes, thanks for clarifying. I was trying to highlight that the entire enclosure was closer than 2.25' from the side wall. The driver center is going to be 25" from the back wall, just shy of 2.25' but possibly the culprit.
 
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