This is an area where more DBT research should be updated as more powerful computers and DSP software have made time aligning drivers a fairly easy task, to a level of accuracy and precision previously unheard of. In my experiments in time aligning drivers, with different speaker systems, and playing with different delay values between drivers, I find it does make an audible difference. Some proponents like Rod Elliot, who set out to debunk time alignment, now think all speakers should be time aligned: "For what it's worth, I originally started this article not to praise, but to debunk the theory that time alignment is the only way a speaker should ever be designed. Having done the research, run tests, and written the article, I confess that I must agree with many (perhaps even most) of the points made by the time alignment proponents. My overall opinion, based on the research for this article (primarily tests and simulations), is that time alignment is a very good thing, and perhaps all speakers
should be designed this way." From:
http://sound.whsites.net/ptd.htm
The step response is a good measurement to not only identify time alignment, but other aspects as well, which are also audible, at least in my experiments. Take this example, where I have time aligned a 2-way system using Audiolense 5 and then only changed one parameter: more low frequency correction at the listening position and tooo another measurement. Nothing else has changed, levels the same, mic position same, etc. This is using a linear phase digital XO FIR filter:
The red and blue have less low frequency correction versus the green and magenta. If one switches to a group delay view of the same chart:
Note the increase in group delay with less low frequency correction for the red and blue traces. And corresponding phase display, again at the listening position:
Is it audible? To my ears it is. I can switch the two level matched FIR filters in real time listening to music and readily pick out the filter with less group delay below 100 Hz. To my ears, the bass has more weight and sounds in phase with the rest of the music, is the subjective evaluation, even through there are currently no DBT's showing one can hear group delay below 100Hz...
Which is why I suggest more testing of time alignment and group delay properties using DBT's would be an interesting area of research to embark on, given the power of computers and sophisticated DSP that did not exist ten years ago.
Part of my point is the comparison of the two step responses and that there is an audible difference between them. Looking at what
@watchnerd posted for step responses, some of them are wildly different, and I would expect an audible difference for sure. I believe part of the issue is that many reviewers may not have heard a legitimately time aligned system and therefore have no reference to compare or have not taken the time to educate ones ears by AB'ing FIR filters with different levels of delay between drivers and tuning into what that sounds like... Just like all things, once one tunes into what dials are being adjusted and what it sounds like, and becomes aware, it is no longer a subtle difference. However, it is indeed complicated, with a lot of moving parts to keep track of to ensure only one parameter under test is actually being varied...