I just want to share another technique for time alignment that comes from another field I have an interest. Let me start by discussing the technique and how I applied it to Car audio. The technique in question is a method for getting two (or more) microphones in phase. This technique is well known by musicians and recording engineers. It is used for miking guitar cabinets to get a fuller sound and it is also good for noise cancellation. Two microphones in phase record a bigger and fuller sound, specially good for metal (distorted) guitars. Two microphones out of phase are good for spoken word in a noisy environment where one mic will capture the voice and noise and the other only the noise with its phase inverted. when you add up the two signals you get "noise cancellation" and the voice remains intact.
How do you get the mics in phase?
The human voice and the guitar are mono signals, but for this example I will pick on the guitar (no pun intended). We first get an idea of where do we like the mics to be by listening to the signal while we move them around in front of the guitar cab. Then we proceed to fix the first mic in place. Now the fun part, we are going to monitor the signal from both mics in "mono" with the second mic set to 180 degrees out of phase. We are going to start moving the second mic around the location that we liked until the monitored mono signal gets to the thinnest and weakest point. Now we fix the second mic in place and switch the out of phase setting back to normal and your mics are now perfectly in phase.
How do I apply this technique to Car audio?
Easy. The only requirement is that you have to be able to feed a mono signal to your speakers and be able to isolate the sound of two speakers at a time. For this example I am going to assume you have a mono sub, so the first Speaker pair to time align would be the sub and your right mid speaker. You are going to have to adjust the crossover a bit to get some overlapping frequencies, say 80Hz to 250Hz on both. Also try to keep the levels to sound even as it will make it easier to listen to both speakers at the same time. We are going to start the time alignment of the mid with respect to the sub. The technique only works if you can set the Speaker that we want to time align out of phase, you may have to do this manually if you don't have a dedicated switch for it. You don't need white noise, your favorite song actually works best. In my opinion, a recording that has a bit of everything as far as frequencies go works very well (Bass for subs and mids, female voice for mids, guitar solos and cymbals for highs, you get the idea). Start delaying the signal of the mid a little at a time and listen for the signal to start cancelling. You are going to look for the point where it sounds its worst (weak and thin). Once you find it that will be your setting and you can now bring the mid back in phase. Now your right mid and subs are in phase. The next step will be to mute the sub and unmute the other mid. Don't forget to revert the settings on your crossover, both mids should have similar crossover settings now. Now you are going to do the same thing with the left mid, you are going to time align the left mid with respect to the right mid. The only time alignment you are doing at this point is to the left mid only as the right mid is already aligned with the subs. You are going to set the left mid out of phase and start increasing the time and listen for the signal to start to cancel and to sound weak, thin and unfocused. Find the worst sound and that is where you want to be. Now the fun part, bring the left mid back in phase and you will immediately hear the sound JUMP in front of you. That is because your two mids are PERFECTLY time aligned.
Now you are going to mute your right mid and unmute your left tweeter. What you want to try to do is to time align speaker that are opposite to each other (one left and one right) and try to have their frequencies overlap a bit but safely. If you have your crossover set to not overlap you will not be able to align a mid with respect to a sub, nor you will be able to align a tweeter with respect to a mid. If there is no frequency overlap there will be little or no cancellation to listen for when out of phase. Just repeat the process until you are done.
I hope this helps and please post your comments with the results you get.