Pro Audio Subwoofer special DSP filtering

smitske96 wrote: The allignment of subs to tops is imho nothing 'special'. You won't use allpass or FIR that low as Jo Bg commented. i think the ts is making things more special then they are.  If you have the tools you can get really close or beyond the current offerings. As long as you have the knowledge and $$$.

All-pass filters are definitely used for LF and subwoofer sections of PA systems, regardless of enclosure type. Even a generic pair of matched Linkwitz-Riley HPF and LPFs constitutes an all-pass response when they’re summed in the electrical domain.

Here's a poorly formatted phone copy & paste from a PDF copy of 2013 paper by a designer at Nexo on the matter:

6.2. Defining LF CrossOver As we have seen, FIR filters require lot of DSP resources when trying to reach lower frequencies. Straight forward FIR of 1024 taps (@48kHz the impulse response is truncated to 213ms) allows a good control down to about 250Hz (231ms ~= 5 periods @ 250Hz). Under this value it will be difficult to implement a filter without downsampling (with all anti alias and imaging filter required in the down sampling operation). For this reason all LF Crossovers under 250Hz have been made with standard recursive filtering. As we said before, the need to have magnitude compatibility on all sub basses of the range has led us to standardise the cut-off frequency as well as the shape. Four frequencies where chosen with a logarithmic step 60Hz 85Hz 120Hz 180Hz. The shape of the filter was chosen so that the summation of two systems at the same frequency would give a flat response. Acoustical order has been set to 8th@180Hz & 120Hz, 6th@60Hz & 85Hz. Please note we are not referring to electrical orders (such as Linkwitz-Riley 8th order for instance); in our case we are also including the acoustical filter order (different from a reflex enclosure than a band pass for instance). The result is shown on Figure 20, all sub-basses and main cabinet are now sharing the same low end pattern. Figure 20 : LF crossover template It allows us to get the same summation independently of the cabinets (Figure 21). The LF extension and SPL capability depends of course on the cabinet. Figure 21 : Main and sub bass summations The time alignment is done with a mix of pure delay and all pass filters on the subs and all pass filters only on the main cabinet (pure delay is impossible as it would spoil the time alignment up to 20kHz with other systems).

Taken from https://www.aes.org/e-lib/browse.cfm?elib=16976

Production Partner include the electrical filter response when they publish tests for subwoofers too. You may need to actually subscribe to the magazine to see the good stuff.