Interpreting GD on 4way open baffle

My drivers are open baffle 4 way with 4th order LR xovers. I’m using IIR filters in FIR files and the frequency response is nearly flat. Crossover points are 150, 500, 1800. I haven’t done any allpass or linear filters to flatten phase.
Audibly it sounds great but the GD has me baffled (no pun intended). Any thoughts?

Can the GD be reduced using all pass filters? If not, is the only way to reduce xover order and or 3way instead of 4way?

sm52 said:

There are two ways. Move the fastest drivers physically away from the listener. Thus, the low-frequency drivers will be on the same time line with the mid-high frequencies. The second is to pass the hi-mid frequency signal through a sound processor that can delay. Filters introduce delays, but the value of these delays is very small. That is, you need to delay the mid-high by 50 ms, and any filter can give no more than 3 ms.

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Thx - I tried a 50ms delay FIR and all I got was an echoed hi end. I tried moving the midbass speakers closer - no change.
Finally I tried using a REW measurement sweep file stored in Roon just like my music is. The 50ms GD delay disappeared!
Prevously I was routing the measurecent sweep from REW laptop out usb to toslink and into toslink usb on RooExtend RooPlay Pi where it gets played in Roon as a radio station. Thats how I listen to the occasional analog source like LP’s aor SACD’s. But obviously the signal encounters a massive non- linearity or phase delay in mid bass with that approach.

Thanks for your help - it helped me get accurate GD measurements and vindicate my OB 4way speakers!