This article is from the Car Audio FAQ, by Ian D. Bjorhovde (firstname.lastname@example.org) with numerous contributions by others.
There is a lot more to mounting speakers in the kick panels than just
equalizing the path length difference (PLD). Two of which are: on-axis
response, and angling for pattern control. On-axis response refers to
the fact that most speakers sound best when listened to on-axis, or as
close to on-axis as possible. Second, after mounting your speakers in
the kick panels you can then angle the speakers to take advantage of
their off-axis response to use output level to overcome any PLD that is
still present. The pattern control I am mentioning is one of the ways a
horn loaded compression driver works very well, they not only use
amplitude to overcome any PLD that is still present they minimize early
reflections that can destroy imaging staging and spectral balance.
PLD can be improved more than marginally when you consider the stock
locations in a lot of vehicles, or the locations most installers choose.
Measure the PLD between tweeters when mounted high in the dash or at the
front corner at the top of the door and you will notice its probably on
the order of 24". This mounting setup requires a lot of amplitude
adjustment to correct the problems induced by this difference. The
nearer tweeter is out phase from the opposite side and is arriving much
sooner and with much greater amplitude due to the fact is not as far
away. When all these factors are added together, it is very difficult
for even the most flexible DSP unit to correct. On top of that, not
many people or installers have access to the necessary tools to properly
set up time delays using a DSP - TEF, MLSSA or other very expensive
time domain measuring equipment are required to do the job properly.
There will always be trade offs involved and deciding which trade offs
to take can be very hard. A small dropout due to phase cancellation will
probably not be noticed by most people but most people will quickly
notice when a vehicle is not imaging properly, and if you can move the
problem to higher frequency where we determine localization more from
amplitude rather than phase differences, it will be much easier to deal
with. Also, if you minimize the time/phase difference it will be much
easier to correct with amplitude.
Some people complain that kickpanel mounting gives a low sound stage.
However, keep in mind that when any stereo system is imaging properly
the point sources can no longer be localized. When our brains can no
longer localize the point sources it will then hear things at eye level.