AmplifierThis is the workhorse of audio performance. An amplifier gives your speakers a stronger, cleaner signal to work with, allowing your speakers to push harder with less sound distortion. Many head units will have an internal amplifier that moderately boosts the signal, which might be good enough, depending on your needs. But, for those who want better overall sound quality, and perhaps more importantly, want enough power for thumping bass, get a stand-alone amp or two. You won't be sorry. Amplifiers are marketed by output in watts, number of channels, and impedance. Watts measure the power output of the amplifier. Since the strength of the signal will fluctuate, RMS (Root Mean Square) is used as a measure of how much continuous wattage an amplifier will deliver, while the peak rating tells you the absolute maximum it can crank out. The number of channels tells you how many speakers or subwoofers the amplifier can drive, and impedance (in ohms) measures the resistance of the amplifier or any circuit to the flow of electric current. For example, the 800-watt Rockford Fosgate Punch 801S has a rating of 200 watts x 2 at 4 ohms. This means it has two channels and delivers 200 watts per channel when there is an impedance of 4 ohms. Lowering the impedance to 2 ohms allows that same amplifier to provide 400 watts per channel (400Wx2), or it can be bridged to power a subwoofer at 800 watts over 1 channel at 4 ohms (800Wx1). Beware flea market specials that try to lure you with a ridiculous rating like 1,500 watts per channel for only $5. If you're not sure if an amplifier will live up to such an uber-rating, check the fuse. An amplifier needs a 30-ampere fuse for every 100 watts of performance. Amperes, also known as amps, measure the amount of electricity flowing through a circuit. A 200-watt amplifier that works on a 30-ampere fuse is really wired like a 100-watt amplifier. Another 200-watt amplifier that pops a 30-ampere fuse will never live up to its full potential. Either way, both of these amplifiers are substandard.
SpeakersMost come in two styles. First, you have coaxials, which consist of a single speaker body with a tweeter placed over a midrange or woofer cone. Their cousins, triaxials, have a tweeter and midrange mounted over a woofer. Secondly, component speakers separate the midrange and tweeter, allowing an installer to place each component in different locations in your vehicle to better fit your vehicle's acoustic environment.
A tweeter operates in the higher frequencies of 20 kilohertz (kHz) to 6kHz. A midrange speaker will generate sounds in the 6kHz to 500Hz range, and a woofer works from 500Hz to 100Hz. Crossovers separate the signal coming from the head unit or amplifier, and send the high-frequency signals to the tweeter, mid-frequency signals to the midrange, and low frequency signals to the woofer. This prevents distortion caused by a midrange speaker trying to play sounds best handled by a tweeter. Another variety of speaker has niche applications in a vehicle: horn-loaded compression drivers. They look like long, skinny megaphones, and sound like brassy tweeters.
Speakers and subwoofers are often given a rating in watts. This tells you how strong a signal the speaker can take from an amplifier. Don't drive a 100-watt speaker with an amplifier that pumps 400 watts per channel or you'll get sound distortion and probably damage the speaker's cone. Some manufacturers bolster their product ratings by advertising the peak wattage, but don't be fooled. A speaker might be able to take 100-watt spikes from time to time, but will be better suited for continuous use at a lower wattage.