It is of utmost important
that the speakers you use in your car have a very accurate and
neutral response. Like an amplifier or head unit,. their job is
to reproduce only what is given to them in the form of an A/C signal.
The ideal speaker, if it existed, would have perfect frequency
response (20 Hz to 20 kHz), perfect time response (no delay in
getting the signal into the air), no phase changes (no timing
differences at different frequencies), and would employ only one
cone to do this. Well, the laws of physics have pretty much taken
care of anything like that ever being created. However, if they did exist,
and you sat at an equal distance from these speakers, the image
that was created in front of you would be awesome.
Since this will never happen, what do we do in our cars? It's actually quite simple really. Use the best possible drivers in the best possible locations in your vehicle.
How do you choose the best driver for your system. The first thing to do is become familiar with some music. The speakers you want to listen to should be auditioned under identical conditions. This is pretty rare actually, as most stores display their product by manufacturer, rather than application. This makes a true comparison nearly impossible. You must become very familiar with the music you are going to be listening to. Listen for the levels of instruments, background sounds, and of course, frequency response. I have a stack of CD's that are all but worn out from being played on different systems, while auditioning them. They will show off problems very quickly and I can judge an audio system in seconds. I have upset some friends who thought their cars sounded great, until I got there, but it gave them a new level of perfection to strive for!
You need to install these speakers in the best way possible. There have been some neat articles in magazines on the frequency and staging responses derived from various driver mounting locations. I can simplify this pretty fast for you. Get the speakers as far away from you as possible! I'd love to leave it at that, but...
OK, what does this equal path length mean? If you take the average car (ugh, a Ford Tempo!), and you put you wonderful new MB Quart co-axials (that you just paid 450-550 dollars (Canadian) for) in the stock door locations, you are robbing yourself of the best possible performance from those drivers, and you'll probably be dissatisfied The result will be that you tell everyone your Quart speakers are junk. You are only doing yourself an injustice by not installing the drivers in a way that allows them to perform to their highest potential. The idea behind the whole thing is again governed by the laws of Physics. Sound travels at a certain speed. This speed doesn't change noticeably enough with temperature variances to cause problems, and we'll assume it is a constant. The speaker you just mounted in the drivers door of your Tempo (AURGH!) is probably 3 or 4 times closer to you, than the speaker in the passenger door. So you'll hear it sooner and it will be louder, and this is what causes you to hear the image (the point from which you think the sound is coming from) either on the door itself, or if you were slightly luckier, on the far left end of the dash.
There are two ways to fix this problem. Make the speakers an equal distance from your ears, or delay the left speaker a bit, so it fires after the right one. The delay thing is great, and simple, but sounds terrible from the passenger side of the car. I wouldn't bother. So the equal path length route is the way to go. What is in front of you and probably the furthest point away from you in the car, that is of course, unobstructed? Those lovely kick panels! Take some time to think about why the difference in path lengths makes a difference.