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Here are some in room measurments of JBL 4320s and DM's DIY DRD 300B mono-blocks.

Blue line is the lousy loop back characteristic we got from the old PC laptop (Macbook Pro is much flatter).


We started off listening to the JBLs in their manufacturer recommended state (horn driver out of absolute phase with the correctly phased 15" woofer). They use 1st order xover on the horn and a 2nd order xover on the woofer. We noticed a dip at the xover region (800hz) so decided to flip the horn phase (now in absolute phase similar to the woofer). Both drivers in absolute phase removed the xover region dips.

Red line is horn out of absolute phase, Green line is horn in absolute phase.


However, after listening for a while I found that with both horn and woofer in phase, vocals sounded somewhat recessed. I preferred the worse measuring horn out of phase setup.