When we combine the outputs of two loudspeaker drivers with a crossover, we usually want the drivers’ outputs to add in-phase. Otherwise the summed frequency response gets wobbly, as does the polar response, thanks to destructive interference. Even with an even-order crossover designed to have its outputs in-phase, there’s usually some confounding factor to mess things up: physical offsets of the drivers, and/or phase shifts introduced by equalization, the acoustic response of the baffle, and the high-pass characteristic of the drivers themselves.
The remedy here is to delay the signal to one of the drivers, bringing them into alignment at the crossover frequency (and hopefully out to an octave or so on either side). In the analog world this can be done by exploiting the group delay of an all-pass filter. With a digital crossover it’s easier: we just hold back the signal for as many samples as needed.
But how to set the right amount of delay? Here’s the standard trick: Continue reading