Car stereos have been standard accessories in automobiles since the late 1970s, and as audio technology has improved, car stereo systems have improved with them. However, even with these advances, car stereos still work based on the same basic procedures pioneered in the 1930s: playing sound so that different parts of the audio come from distinct right and left channels.
Basic Stereo Sound
The most basic kind of stereo in a car -- or in any environment -- is the simple right and left channel stereo. This takes a piece of music recorded with two tracks -- recorded at different distances through different microphones -- and plays the two sounds through two different speaker systems, positioned to the listener's right and left. Each of those audio signals is called an "audio channel." In a car, the left and right channels usually refer to speakers planted in the doors of the passenger compartment. Even in systems with more than two speakers, the channels are split left and right.
Advanced Speaker Controls
A benefit of stereo sound using distinct right and left channel audio is better control over the sound that comes out in your car. Even the most basic car stereo systems give you independent volume controls over the left and right channels. More advanced controls may give you a graphical mixer, allowing you to fine-tune the audio using specific frequency ranges.
Most car audio systems have a pair of general speakers that try to replicate the full range of human hearing from 20 Hertz to 20,000 Hertz. According to Crutchfield Stereo Systems, most factory speakers are lower in quality than aftermarket speakers. While replacing all of your speakers at once saves on labor costs, if you're comfortable doing the speaker upgrades yourself, you can do it in stages. Replace the worst sounding speakers first; this gets you the greatest sound quality improvement for the least amount of money.
Quadraphonic and Surround Sound
Quadraphonic and Dolby Surround Sound systems add separate speakers for the higher-pitched sounds in music, often positioning them a bit forward of the "baseline" speakers to enhance the illusion of different instruments being in different locations relative to the listener. Subwoofers are used to drive lower frequency sounds -- because these sounds aren't as directional as higher frequency ones, the subwoofer is usually a separate channel from the traditional or enhanced left and right speaker channels. While Quadraphonic sound systems usually have four speakers and an optional subwoofer, Dolby sound systems for cars have a subwoofer, and position four or six speakers in different locations through the passenger compartment.
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