In a car every speaker sits at a different distance from your head, so their sound does not arrive together — the stereo image falls apart and pulls to the left. Delay fixes that: you measure the distances and make the closer speakers wait.
Distances
from your ear to the speaker, cm
Measure from the centre of the cone to ear height at the headrest, not to the seat. Use a tape measure, or a piece of string you measure afterwards. A few centimetres matter: 3.4 cm is already 0.1 ms.
Conditions
the speed of sound depends on temperature
°C
m/s
Sound travels at 343 m/s at 20 °C and 355 m/s at 40 °C. In a car baking in the sun that difference is only a fraction of a millisecond even on the longest path — but set it if you want to be precise. The formula: c = 331.3 + 0.606 · T.
The farthest speaker is the reference. It gets zero, because you cannot make it arrive sooner — everything else is delayed to match it.
Delay to dial in
What it looks like from above
the dashed circle is the common distance
What order to do it in
if you are starting out
Polarity. First check that every speaker is wired the same way round. One mid wired backwards cancels bass, and no amount of time alignment will fix that.
Crossovers. Set the crossover points and slopes. Alignment builds on them, so do not move them afterwards.
Alignment. Enter the values above into the DSP. From here the image sits centred, up on the dashboard instead of in the left door.
Sub phase. For the sub the calculated value is only a starting point — the low-pass filter adds delay of its own. Listen to the handover between sub and mid, and nudge the sub's delay a few tenths of a millisecond either way until it is fullest.
EQ last. Only start shaping tone after all this. EQ first and you will be flattening cancellations caused by bad timing, which does not work.