OK, so here are some pictures. This first set of pictures just shows my Door and Dash speakers - individually and the combined L+R response of each set (L+R Door and L+R Dash).
Here is what the combined responses looked like before applying any allpass filters, with little red arrows showing the phase-related dips (3 in combined midbass response and 2 in combined dash speaker response) - I could probably use another one for the dash speaekers, but...:
Here is what the combined response looks like after I applied allpass filters to those dips - they magically just disappear that easily! Awesome! This is with 1/6th smoothing (with 1/3rd smoothing, things look perfect):
I applied the following allpass filters to fix those dips shows with the arrows (no EQ, just an allpass filter per dip):
L+R Door (all 2nd order):
L+R Dash (all 2nd order):
- 197hz, Q of 4
- 414hz, Q of 9
- 630hz, Q of 9
No idea if this is "normal" (to need to apply so many allpass filters), but they definitely addressed the big dips, so I'm happy.
- 672hz, Q of 6
- 985hz, Q of 5
So I figured - cool! I'm done - everything is lining up just right phase-wise. NOT. I forgot that I haven't deal with the DashDoor crossovers yet! So far I just fixed the phase-related issues when playing speakers in L+R pairs....
I'll be back to show the response of each side (Left Door+Left Dash and Right Door+RIght Dash). I can't seem to get those right - at all, which obviously affects the overall response...
It's becausse first order allpass filters don't affect the frequency you are setting them at. They roll the phase of things 1+ octaves above it. The second order has a much more pronounced impact at the frequency you set it at. You can see clearly where the filters affect phase if you watch this video. Then you can use first order filters as well, the next time!