Speaker installation is very critical for performance. Whether you spent $50 or $1000 on a set of speakers, if they are not properly installed, the sound will not be up to par.

What makes a good installation? Well, certainly mounting speakers in most factory locations, such as on the bottom of the doors pointing at your legs, are not acceptable. In this cases a new mounting location might need to be improvised.

Distance

The first thing to consider is distance. If the left speaker is only a couple feet away from your ears, while the right speaker is several feet away from you, then the sound will arrive at different times giving you poor sound. The left speaker will sound louder since it is closer.

The best solution is to figure out a location where the difference between the distance of the right speaker to your ears and left speaker (also known as path length difference), are minimal. This is where kick panels shine, making it the preferred location for many audiophiles and competitors alike.

The other solution, which can get expensive, depending on the gear you get, is delays. By adding a delay to the left speaker, the sound can be doctored to arrive from both sides at the same time. This is only a patch, and does not sound as well as equally spaced speakers, but is the second best alternative.

Multiple Speaker Placement

If you have a system with two or more speakers per side, you need to carefully try different locations to obtain the best possible sound in your car.

Let's take a 2-way system with a tweeter and woofer per side as an example. The woofers are mounted in the factory location at the bottom of the door. The tweeters are mounted high up on the front corner of the door panel. Looking at the speakers from the driver's seat, you can see that there are 4 speakers all aimed towards different orientations and all at a different distance to your ears. This interaction of sound waves at different frequencies arriving at your eras at different times seldom sounds good. The best thing to do is mount the woofer and tweeter on each side as close as possible to each other. This is also why kick panels are used so much these days.

Professional installers do use some tricks such as inverting the tweeters' polarity when mounted for example on top of the dash while the woofers are low. Achieving good sound with unconventional mounting schemes is very, very hard and is only achieved after plenty of time has been spent trying different configurations.

Aiming

Our ears can distinguish the direction of sound more easily at higher frequencies.  This means that aiming the mids, and most importantly, tweeters towards your ears play a critical role in sound imaging.  Midbases are not so critical, but should be also aimed towards the listener's ears if possible.

To figure out the best aiming angle involves many hours -even days- of work.  To start, try to aim the speakers towards the center of the car.  Play around with different angles until you obtain the best "sweet spot".

Subwoofers should be mounted up front for best sound. Since this is not possible in most cars, mounting subs in the back is not such a bad thing, since most people can't distinguish where bass comes from. If you have good midbases going down to 60 Hz or less and subs picking up the signal below 60 Hz, then the bass will seem to come from the front.

Enclosures

Everyone is aware that subwoofers need a properly designed enclosure to give top performance. How about midbases and mids? They also do sound much better if they are installed in enclosures. The best sounding and easier to build enclosure type for midbases and mids is sealed.

Mounting Speakers

If you are using speakers that fit into a factory location, make sure there are no gaps or holes.  Sometimes building a wood or fiberglass baffle helps reduce holes and gives you much better sound.  Always be careful when using power tools around speakers.  Warranties usually don't cover holes in speakers.

For unconventional speaker locations, sometimes metal has to be cut.   If you have the resources, plasma cutters and pneumatics tools work great.   For most mortals that do not have these tools, a pair of metal snips (left and right cut) will do the job.