Choosing the Right Speakers (and the Left Ones Too!)
So, you're thinking about a sound system for your car. The first thing you should consider is what you expect to get from such a system. Let's look at how you are planning to use sound equipment, and I'll note a few recommendations along the way.
But first, a rule of thumb: Remember that the woofer(s) determine all parts of the bass system. Always base your decision about amplifiers and enclosures around the woofer or speaker you select. See the 5 levels below:
ROCK! LOUD AND HARD.
You want the biggest baddest bass system you can afford. Of course, you risk a certain lack of appreciation from your neighbors, or maybe the whole town ... I urge you however, to consider the rights of others to a quiet environment. And remember that there are new noise abatement laws in many locales, so ... Boom Responsibly!
For most cars, 10, 12 or 15" subwoofers mounted in a heavy dual bandpass or sealed box and connected to compatible 500 to 2400 watt (RMS) amplifiers may just do the trick.
Look for woofers that have dual voice coils with 4 ohm impedance, and between 300 to 2500 watts RMS total. The best performing and most efficient systems will often have dual 4 ohm woofers with all coils connected in parallel to form a 1 ohm total mono load. This will require a 1 ohm compatible mono amplifier with a capacity equal to the total RMS power rating of the woofers, plus 10 to 30 percent more as an overhead to allow high transients with minimal distortion.
The amount of power (in RMS watts) determines how much clean loudness the system can provide. Power is exponential in relation to loudness, which is linear. This means that for a 10 watt system to produce twice its clean power, output must increase to 100 watts. Twice the loudness again requires 1000 watts, and so forth.
Systems of more than 500 watts RMS are fairly typical of many of the "heavy hitters" heard from way down the street. But do keep in mind that along with being hard on neighborly relations, they can also be damaging to one's hearing. So we recommend using this system at only 'moderate' strength most of the time. But rev it up when you're in an appropriate place and time, and ready to make an impression.
Less expensive setups, with smaller speakers and less powerful amps can provide good volume and bass as well, provided the components of the system are compatible. Essentially, you will have to decide your purchases on a combination of wallet size and bass loudness desired.
I JUST WANT A FEW TUNES AND A LITTLE TALK RADIO TO ACCOMPANY ME TO WORK EVERYDAY
Almost anything the car manufacturer puts in the car will serve this purpose, and some of them go well beyond. If your car is an older model, and you have noticed a marked deterioration in the sound, it may be attributable to the original speakers showing the effects of age and their usually cheap construction. A new (and technologically improved) set of same size speakers will usually perk it back up to better than new performance.
JAZZ, LOUD AND BRASSY
Go with the recommendations above, and in addition you may want to add small front or dash mounted mid range units to help bring out the full timbres of the middle toned instruments and the human voice that lies at the heart of nearly all traditional jazz.
BIG BAND MUSIC FROM THE 30S TO THE 50'S
So you want a bit more "oomph" to bring out clarinets and trumpets. As above, try an upgrade on those original speakers. If you want a bit more bass response, add a small (75 to 100 RMS watt-per-channel) amplifier and add a small (8" to 10") single subwoofer. Equipment of this type can often be hidden under or behind the back seat.
Use the considerations as above, but you may also want to consider adding a small pair of tweeters to the front, mounted on the dashboard. These will allow you to hear the full range of high frequency expressions embedded in many CD's.