The benefit of equal path lengths is that the time it takes for the signal from each driver to reach your ear is about the same, and it appears to your brain as though you are sitting between the drivers. All of this speaker position fiddling and stuff can categorized under time alignment, and psycho-acoustics.

How to fool your brain (my favourite hobby of all!)

A long time ago (if you believe in evolution), the nature of a creatures existence would have determined it's physical characteristics. What does this mean? Well, take a lizard for example, usually it's eyes are in sockets basically outside of it's cranial cavity (skull perhaps). This allows them to almost look behind themselves to watch for predators. Almost everything was based on being a predator, or the prey. The cheetah can run like the wind to catch it's food, while a kangaroo has enormously strong hind legs, which I am sure a lion or cheetah, or whatever, would not want to be hit by. What is my point? Human's have very few predators of their own, as we where predators ourselves. We also could not fly (if that's news, you need help quick!). So, our hearing is designed to allow us to pinpoint sounds that encircle us. We can not accurately judge whether or not a sound is above or below us, because our ears are on the sides of our heads, not the top and bottom.

The same way we can judge the distance an object is away from us by the difference in angles of our eyes, our ears and brain can perform a similar feat by judging the difference in arrival times of the sound to each ear. But, there are limitations. If you calculate the wavelength of a sound to be more than 4 times the distance between your ears, you can not accurately judge it's location. In common terms, bass is not directional to our hearing system. Unless your head is about 10 feet across.

OK, now we know why the sound of those wonderful midrange and highs can appear to be on windshield. What other considerations are there for speaker placement? I told you that having the midrange and tweeter as close together as possible is the way to go. Here's why. Each speaker that is designed has a specific purpose and application in mind. The little 2-inch speaker in your computer isn't going to have much cone excursion, and likewise, neither are the tweeters you use in your kick-panels. On the other hand, a great big 12-inch woofer is a bit heavy to be moving back and forth accurately enough to clearly reproduce the range from 10kHz-20 kHz. So, we choose the drivers for our system based on their capabilities, and then run them only in that range using some form of crossover. A crossover is a device, or circuit which limits the frequencies that are sent to your amplifier or to the speaker. Obviously, I need a section on crossovers now as well. The range in which you should operate specific drivers depends on their inherent characteristics. And you should ask your salesperson or the manufacturer directly for this frequency range. But note, the operating range is dependent on 2 things. The drivers that will be operating at the neighboring frequency ranges, and the amount of power you want a speaker to handle.