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Post by atledreier on May 30, 2017 3:57:53 GMT -7
I've been struggling with my room, and somewhere in the process I decided to measure the nearfield output of my manifold, just to make sure I wasn't starting at the wrong end of the problem. Well, as it turns out, the rolloff seems excessive to me. I'd like some input on it. Equipment-wise I have a Denon AVR-4311, Sub1 output to a MiniDSP 8x8, to a Behringer EP2500, bridged mono to parallell/serial pair of drivers. I made sure all processing were off in the receiver, no audyssey, nothing. Only processing in the receiver is a 250Hz lowpass filter, that's as high as it will go. I've checked that the highpass-filter in the EP2500 is disabled, and I've checked with another amplifier altogether, and the FR-plots line up almost to the pixel. I also tried taking the MiniDSP out of the loop, and again, very close to identical FR plot. Now, is this rolloff normal? It seems to end up at around 15dB/oct which seems excessive. I was expecting a 6dB/oct roloff below FS of the driver?  |
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Post by ThomasW on May 30, 2017 7:33:23 GMT -7
Roll-off rate of a sealed box sub is 12dB/octave. Yours appears a bit more that that. I suspect it's a function of the roll-off rate of one or more of the pieces of electronics in the signal path.
What happens when you dial in a fairly wide boost filter with a 15Hz hinge point?
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Post by atledreier on May 30, 2017 8:31:53 GMT -7
+12dB low shelf, Q=1 Fc=15Hz It's up about 10dB @ 12Hz, so it seems to respond as expected. My measurement chain is down 3dB at about 5Hz. I just wasn't expecting such a steep rolloff. Attachments:
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Post by ThomasW on May 31, 2017 6:28:14 GMT -7
Interesting.
It looks like there are some room induced issues in the plot.
Are the measurements taken with the mic less than 1" in front of the dust cap, with the mic capsule at a 90 degree angle to the driver?
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Post by atledreier on May 31, 2017 13:00:40 GMT -7
It's at the mouth of the manifold, centered on the manifold mouth both vertical and horizontal. I could try a measurement as far in as I get the mike. It won't be all the way centered on the dustcaps, my boom isn't long enough for that I think. But I would think the mouth of the manifold is the interesting part?
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Post by ThomasW on Jun 1, 2017 6:17:51 GMT -7
It's possible there's some box induced artifact. Were you to post some pics of your manifold and install it would be helpful.
Is MiniSDP the only EQ in the signal path?
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Post by atledreier on Jun 2, 2017 4:55:14 GMT -7
I've tried taking the MiniDSP out of the equation completely, and also measured the subwoofer output from the processor just to make sure there's nothing funky going on. The rolloff is real.   |
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Post by ThomasW on Jun 2, 2017 7:07:20 GMT -7
There are dozens and dozens of 4 driver manifolds and none have produced a plot like we're seeing here..
I don't think the driver plot is as severe as it looks. I think we're seeing a combination of both driver and electronics roll-off
Let's start with some fixes...
It appears your manifold is a single layer of MDF?
Using 1" X 2" or similar solid wood, not MDF, fabricate a 'X' brace from corner to corner on the outside of the rear surface of the manifold. Use epoxy to attach the brace.
Next make a plate of MDF or ply that can slide through the interior of the manifold and reinforce the rear panel from the inside. That it will be a inch or so 'small' on each side doesn't matter
After these braces have had time to cure run a new plot to see if there are any changes.
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Post by atledreier on Jun 3, 2017 2:44:54 GMT -7
I'll brace the manifold and see what happens. I'll also make a calibration file in REW that compensate for the very slight roloff in the measuring chain, even though that's no really relevant here. The rolloff is way steeper than the eletronics can account for anyway.
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