Speakers Overview:
The car speakers you use will have the absolute say in how your system will sound. There are many different types of speakers available. A single speaker can be used to reproduce the full range of sounds but it is not ideal. If the speaker is too great it will have problems reproducing high frequencies which require rapid movement of the speaker. If it is too diminutive it will have problems reproducing low frequencies which require large amounts of air to be moved. Because a single speaker cannot reproduce all sounds accurately multiple speakers are used each of which imitates sound in the frequency range it was designed for. A speaker called a tweeter reproduces high frequencies generally above 2 kHz.
Tweeters are small and lightweight so they can respond fast. Very little power is required for powering tweeters because they are very efficient. Woofers are the exact opposite because they usually require considerable amounts of power to really move air. Woofers are meant to produce sound at frequencies below 250 Hz and often just below 100 Hz (in the case of subwoofers). Because a woofer must move considerable amounts of air they are usually large with typical sizes of 10″, 12″, 15″ and even 18″! On the other hand tweeters are usually very small ranging in size from 1/2″ to 2″ in size. Typically, tweeters larger than 1″ in size cannot respond fast enough to sound good and are too directional. In between are midrange speakers which handle the frequencies between the woofers and tweeters. Further separation can be done but is usually unrequired and just complicates the crossover which must separate the full audio signal into multiple parts for each speaker.
Things to look for:
Power Handling: Just as with amplifiers, RMS or continuous power is pivotal here. Some manufacturers will assert very high power handling figures but they are usually for very short peaks only. Granted music is not continuous but the continuous power handling gives you a much better impression of how much power a speaker can really handle. For tweeters and midranges, power handling is not as critical since it does not take much power for them to play loudly. For woofers though a rough match should be made between the woofer and the amp driving it.
Sensitivity: This is a very important specification for a speaker. It gives you an idea of how loud a speaker will play given a certain input power. If a speaker is insensitive then it will require greater power to play at the same volume level than a speaker that is more sensitive. Figures between 85 dB and 95 dB at 1 watt RMS at 1 meter are regular. If you use anything outside of this range you may have trouble matching the output levels of the speakers relative to each other. .