DEQX calibration at low frequencies - best approach?

almikel

New Member
Sep 18, 2010
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Hi guys,
I have a DEQX HDP3, and I'm about to head down a major speaker upgrade, keeping the DEQX at the core and will need to re-do all my DEQX speaker measurements and calibrations. I am going to a 4 way, but will leave that discussion to another thread.

The new setup will be a pair of Synergy 2 way horns, crossing to stereo sealed mid woofers crossing to a tapped horn sub.

I'm looking for some input into how to take meaningful DEQX measurements at low frequencies, interpreting the DEQX measurements and implementing appropriate correction in the low end (ie 300Hz and below, but especially below 100Hz).

I realise the limitations of Windowing and the impact on resolution at low frequencies - hence this thread...

Normally I do my measurements outdoors with the speaker elevated 2m off the ground facing up, with the mic 1m above.
That setup provides the most distance between the direct sound and the first reflection.

My planned mid woofers, for use between 50Hz and 300Hz will be very heavy, so lifting them high off the ground to perform measurements is really not an option (safely), 1m off the ground would be OK, but ground plane easier again.
The planned tapped horn will operate between 25 and 50Hz, and also be heavy and unwieldy, so again either 1 m off the ground or ground plane measurements will be done.

Q1) Given the ground bounce will be the first reflection, and very soon after the direct sound, is there benefit from measuring outdoors?
Is having the ground bounce the only reflection better than lots of reflections soon after the ground bounce for meaningful results?

Q2) is close micing recommended? ie 20mm off the driver for the mid bass and right at the horn mouth for the tapped horn?

Getting back to the DEQX specifically, I have a DEQX Technical Support Advisory (051018b) discussing low frequency measurements with the DEQX.
It describes how windowing and smoothing can be adjusted independently to assist in making DEQX calibrations at low frequencies, but gives no detail on how to interpret the graphs.

I took some screen shots of a close mic'd measurement I did of my current 15" woofer, and my primary goal is to adjust group delay rather than frequency response
The first 2 graphs are with the window truncated right back to the finish of the main energy pulse (ie around 8ms).
100% smoothing
truncate 100ms max smoothing.JPG

no smoothing
truncate 100ms no smoothing.JPG

tuncation out to 100ms
truncate 100ms.JPG

100% smoothing
truncate early 100% smoothing.JPG
no smoothing
truncate early no smoothing.JPG

Can anyone provide some insight to interpreting the results?
Based on the results, what factors should be considered/configured when performing a calbration?

Greatly appreciate all input

cheers
Mike
 

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