The purpose of a loudspeaker (also known as a driver) is to convert the audio signal in its electrical form (from the amplifier) to a pressure wave that we perceive as sound. The sound starts out as a low level signal in the head unit and is transmitted to the amplifier by RCA cables. The amplifier then amplifies (makes greater) the level of the signal and passes it to the speaker through speaker wire. The speaker then converts this electrical energy into mechanical energy and we hear the result as sound. See the diagram below.
Like any component in a car stereo system the ultimate goal of the speaker is to do its job without adding any coloration (changing of the original sound) to the audio signal. Ideally only one loudspeaker per side would be needed to reproduce the entire audio spectrum. Unfortunately a speaker that can produce low frequencies has too much mass to be able to produce the higher frequencies. This is why we have coaxial and component speakers. Subwoofers are also used in order to produce the extreme lows of music with impact that can be felt as well as heard. Speakers have an extreme range in quality and price. They also have the greatest impact on the overall sound of a car stereo system.
When shopping for car speakers be sure and listen to the speakers before you buy them. There is no other way to know how the speakers will sound to you. Be sure to bring music that you normally listen to when you audition the loudspeakers. This will give you a better idea of how the speakers are reproducing sounds you are familiar with. Listen closely for low level details and how natural different instruments sound. When you have narrowed down your choices to a couple of different speakers, audition them at length. A poor quality car speaker will become tiresome to listen to, usually because of an exaggerated low and high end. Be sure to adjust the bass and treble as well as the volume to levels that are comfortable to you. Beware of salespeople that do an A/B comparison of loudspeakers for you. When switching between speakers, unscrupulous salespeople will turn up the volume slightly on the more expensive speakers to persuade you to buy them. It is an established fact that people tend to prefer the loudspeakers that they determine to be louder.
When choosing loudspeakers, be realistic. If you don't want to modify your existing speaker holes, shop for speakers that will fit in the current space available. Keep in mind depth considerations, especially in doors where speakers may interfere with windows rolling down, dashes where speakers may be limited by ducts for heat and air, and rear decks where depth is limited by torsion bars that run underneath the rear deck. When shopping for subwoofers don't get crazy trying to stuff your car with subwoofers. Consider how much room you have and how much of it you are willing to sacrifice to achieve the low end. This will determine size as well as enclosure type for the woofers you choose. This is all part of system planning.
All speakers will have a power handling specification. Usually two specifications, RMS or continuous power handling and peak or maximum power handling. As in amplifier ratings you can ignore the peak or maximum specification and concentrate on the continuous rating. Let's say a speaker has a rating of 150 watts RMS. This does not mean you have "150 watt speakers" or that your speakers "put out 150 watts". It means your speakers can handle 150 watts of power without being damaged (assuming clean power, see next paragraph). Does this mean you need a 150 watt amplifier? No, you will still choose the amplifier based on your listening tastes and your budget. It simply means you should not use an amplifier that is significantly greater than 150 watts to power this speaker.
When choosing an amplifier for your speakers it's actually better to have a little too much power than not enough. A speaker with plenty of power will be able to play louder and cleaner than a speaker driven by an undersized amplifier. Much of this depends on your listening habits. If you like loud music and find yourself turning the volume up to the maximum then you are probably going to be driving your speakers with "dirty power". This is when you push an amplifier past its limits and drive the speaker output into clipping. Clipping is the term given when a normal music wave is cut short because the amplifier runs out of power. This is the biggest killer of speakers, not overpowering. See the image for an example of a clipped waveform. Notice how it compares to a unclipped waveform.
To give an example of acceptable power you can estimate that a speaker could receive 25% more power than its RMS rating and still have a long life. So a speaker that is rated for 100 watts RMS could safely handle 125 watts RMS. Of course if you push the 125 watt amplifier to its limits you're back in the same boat and your speakers will eventually die a hard death. Be kind to your speakers and to your ears. If your ears or the music sounds bad then turn it down. Your speakers and your ears are being damaged and your speakers are the only one that can be replaced. Use common sense.
You may also be interested in How to Install Your Own Car Stereo System . It covers many topics including factory speaker removal and new speaker installation. Click here.