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Multi-Way Conventional loudspeakers with crossovers

S. Harsch XO
S. Harsch XO
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Old 1st October 2020, 08:23 PM   #801
AntM is offline AntM  United Kingdom
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Hi xrk971
Thanks for you feedback. The dips are 1 or 2 dB - they are higher than in the Harsch paper. I'll need to figure out how to show the plots in the way you are suggesting - since these are post-EQ, I'll have to remeasure them, or find a way of exporting them from the REW filter optimiser. Or possibly use the ACD spreadsheets.

Some older measured responses 1m above floor, 1m from driver, prior to driver EQ:
mid bass
1.png
tweeter
2.png



I had dug out an old 50cm measurements - 1m from floor for the LR4 speaker after individual driver EQ, but without BSC.
Note the dB impulse response still shows significant floor bounce at 4.4ms.
Screen Shot 2020-10-01 at 20.52.21.png

When windowed with 4.4ms Blackman-Nutall window and 10 cycle frequency-dependent window.
Screen Shot 2020-10-01 at 21.08.59.png

And the suggested 6 cycle freq-dependent window:
Screen Shot 2020-10-01 at 21.07.06.png
Curious as to the justification for the 6-cycle window.
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Old 17th October 2020, 07:39 PM   #802
454Casull is online now 454Casull  Canada
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Am I doing the math wrong? A 1.5 ms delay means an acoustic offset, on-axis, of (0.0015 sec) * (343 m / sec) = 0.514 m. Does that means it's practically impossible to achieve the necessary delay by physically offsetting the drivers?
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Old 17th October 2020, 09:48 PM   #803
xrk971 is offline xrk971  United States
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S. Harsch XO
Yes, that is why a Harsch XO is not practical for a passive XO unless the frequency is much higher. With DSP, a 1.5ms delay is trivial.

Quote:
Curious as to the justification for the 6-cycle window.
I got this from private communications with Wesayso and Byrtt, the masters of in-room measurements, REW, and FIR DSP processing. It might be suggested somewher in this massive thread:
The making of: The Two Towers (a 25 driver Full Range line array)

here, Wesayso actually suggests 5 cycles. I think the reason is that within 5 cycles of the sound waves of interest (above some cutoff frequency), the sound is gated so that it represents that part that hasn't interacted with the room yet.
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Last edited by xrk971; 17th October 2020 at 09:56 PM.
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Old 19th October 2020, 03:00 PM   #804
AntM is offline AntM  United Kingdom
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Thanks - gosh that is a long thread.

I noticed this in another thread:
Speakers white paper | Grimm Audio

The designer is discussing using LR4, with phase correction.

In section 2.4 he points out:
"The sum of an ideal LR4 system is a second order all-pass with a Q of 0.7. In order to avoid the problems associated with correcting phase exactly, build an inverse all-pass filter based on the theoretical ideal. This filter will be non-causal so there’s a good reason for using FIR."

This is quite appealing - it implies we can have our cake and eat it - get the better above- and below-axis response from LR4, but with perfect phase response on-axis.

He points out the non-causality, as if it is a minor fly in the ointment. I assume his FIR comment means he solves this by delaying the entire signal through an FIR filter, thus allowing a negative delay relative to the output. In other words he is approximating the phase correction (+delay) with an FIR filter.

Here is a related paper:
AES E-Library >> Correction of Crossover Phase Distortion Using Reversed Time All-Pass IIR Filter
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Old 19th October 2020, 10:15 PM   #805
xrk971 is offline xrk971  United States
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That’s why the link I gave is actually the part of the thread that discusses it. Not just page 1.

In that same thread a lot FIR work is used by Wesayso to make his speakers sound perfect. I have heard them before and they are one of the best sounding speakers I have heard anywhere at any price. Amazing sounding. The FIR convoluting and room treatments are key.
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Old 19th October 2020, 10:19 PM   #806
fluid is offline fluid  Australia
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The 5/6 cycle window in REW is a reasonable approximation of the window lengths wesayso uses in his filters so viewing the measurement through it after correction gives you a good idea if it worked.

There is no special significance to it, change the length to see more or less of the room in the measurement. It is better for overall in room responses to gating at a specific point because that will always affect the lower frequencies in a negative way.

There is a pdf here that seems to describe a use of the reversed IIR filter approach that doesn't require AES membership.

http://www.sga-ssa.ch/docs/events/08...008_05adam.pdf

It is still a look forward system but the latency can be less than what you might need for an FIR.

The block diagram could probably be replicated in sigma studio but I'm not aware of anything off the shelf that does it.

Rephase has a filters linearization option where you can generate the inverse phase shift for LR filters. If you really want the phase shift gone and can live with some latency this is an easy way to do it for an existing crossover.
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