Better car audio, Tip #4: Add a signal processor or an equalizer
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Welcome to this article from the Crutchfield archives. Have fun reading it, but be aware that the information may be outdated and links may be broken. 
A car interior presents some serious problems when it comes to sound quality. Glass and plastic surfaces reflect sound like crazy, while carpet, seat covers, and other absorbent materials soak it up. Add poorly-placed speakers to the mix, and you'll find significant peaks in frequency response in most car interiors. These peaks make your music boomy in the bass or shrill in the upper frequencies, causing "ear fatigue."
Most car receivers give you treble, midrange, and bass controls - useful for global fixes but not for zeroing in on problem areas. You'll need a sophisticated equalizer to kill these peaks, whether it's built into your receiver or in a processor that you mount in your dash or near your amplifiers.
An outboard equalizer gives you multiple points for adjusting frequency response, so you can iron out the peaks in your system. A parametric equalizer allows you to vary the centerpoint and width of each EQ band, so you can really zero in on a problem area. Sound processors can help you eliminate frequency response peaks and increase bass response, and some even include a microphone for analyzing your car's acoustics.
This post is excerpted from a recent article in our Learning Center, Jeff's Tips for Getting Maximum Sound Quality in Your Car.
