Time or phase alignment has a big impact on imaging and it can also affect the frequency response. If you have a significant phase shift at a crossover point, you will see a dip in the response. This can happen with any type of crossover, including the one between mains and a subwoofer.
Spatial perception of sounds is largely based on arrival times and intensity. Each ear picks up a different arrival time and intensity. Our brain uses this information to work out where the sounds are coming from. Normally sources are coherent, originating from an area. We also normally have a visual connection to also make sense of what we hear and tend to rely on sight to a greater degree.
When listening to a sound system, we are attempting to recreate the original source, but at the same time we are trying to contradict what our eyes are telling us. We try to create the impression that the sounds originate from an invisible source, not the speakers. To make it more difficult, the sounds are reproduced by multiple sources. If each of those have a different dispersion pattern and different time arrivals, this only creates confusion. Is it any wonder that accurate imaging can be so elusive?
Our goal with time alignment
Our goal is simple: to create an audio holograph that places sounds where they were recorded. This goal will often be undermined by poor recordings, but when a suitable recording is played, we want to experience the image they have created.
Is time alignment important? By now you should realise my answer to that is an emphatic YES! If you don't care about imaging, then it matters only as much as it affects the frequency response. In that case, it's a phase alignment issue, where misalignment can introduce a dip at the crossover.
Time alignment series
Why you should time align >
Is it really necessary?
Time alignment overview >
A bird's eye view of various ways that are used to time align speakers. Does physical offset really work?
Time alignment with Behringer DCX >
This is about as easy as it gets. You need a mic and a few minutes to run the auto routine.
Digital time alignment >
My preferred method. No need to build bizarre baffles that can introduce their own issues.
Subwoofer phase alignment
Yes, even subwoofers benefit, but not for the same reason