Splits are a separate mid-woofer and tweeter, with another little black box to split the frequencies up between the two.
Advantages - high frequency sound (which comes from the tweeters) is very directional and your ears are good at picking up where it comes from. Therefore it is important where they are mounted. This sound is easily knocked out by obstructions such as knees/ steering columns etc when the speakers are mounted low. Also as the tweeter is kinda small mounting them up higher is easier. This has the effect of lifting the sound stage to a more natural height and providing good stereo imaging. They should be mounted in front of you - preferably at around ear height (not usually too many mounting points at this position in a car) but also not too far away from the midwoofer. Generally a good position is the top of the doors at the front, maybe behind the mirror mounts, if you look at higher spec AU's I am sure you will find that there is a windscreen molding to suit mountings in this position.
The mid-woofer being larger can stay in conventional positions in the doors. The sound is not as directional and being larger speakers are only going to fit there anyway.
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