Start at the Source
When you are trying to get the best sound out of your system, you want to start at the source and work your way from there. The best speakers in the world will sound terrible if you are trying to stream Pandora through them.
CDs or lossless music files are the way to go these days, although most head units still don’t support lossless files. I would say the lowest bit rate you should go on mp3s is 320kbps AAC. Most head units will support this format, and iPods support this format. Most head units have great iPod support. Win-win.Also, it will be very hard if not impossible for most normal people to distinguish between this format and lossless. Especially in a car environment (terrible acoustics, road noise, etc).
The most popular first upgrade is the radio itself, or head unit. This is for many different reasons: features, looks, customization, better processing, etc. Car companies use cheap parts to keep the cost of their cars down. It’s amazing how much difference a good head unit can make, especially when you can add features your car might not have come with in the process.
Units like this new Pioneer AVIC-8000NEX ($859 with free shipping on Amazon) fit most vehicles with the right parts and greatly improves the looks and functionality of your dash. DVD playback, smartphone compatibility, navigation, are all features you can have when adding an aftermarket head unit.
You can see how this is a popular first upgrade for your sound system. You can even add external monitors and sources such as headrest TVs or a backup camera. Adding an aftermarket radio also makes it easier to add external amplifiers.
Not only do aftermarket radios offer better features, they usually offer better sound quality, too. Most stock head units have cheap DACs (digital to analog converters). When you upgrade your head unit, most of the time you are upgrading the DAC as well. Units like the Pioneer DEH-80PRS ($248 on Amazon with Prime shipping) offer three separate Burr-Brown DACS and 3 volt RCA outputs for the ultimate in sound quality.
Aftermarket units also offer more power than stock head units (not stock amplified systems, though). Most factory radios have around 5-10 watts RMS per channel on their built in amplifiers. Most aftermarket radios offer around 15-18 watts RMS per channel (although they advertise 50W; this is a max power rating, though).
Subs are probably the next most popular upgrade whether it be for sound quality or SPL (sound pressure level, aka loud).
Subwoofers are not just to make loud boomy noises. They also greatly increase the sound quality in a few different ways.
One way they help with sound quality is they introduce low frequency sounds that most factory speakers or even aftermarket mid range drivers can’t reproduce clearly. This lets you hear low sounds that you couldn’t hear before that add depth and richness to your music.
Another way they help with SQ is take bass out of your mid range speakers. Most factory systems have full range speakers. This means the signal going to them consists of the very low frequencies and the very high frequencies that these speakers can’t produce. This makes them sound distorted when you try to play bass through them.
When you add a sub, you can set it to play the low frequencies only and set your other speakers to play only the frequencies higher than that. This allows each speaker to be within its sound reproduction range and makes everything much less distorted.
Sound Pressure Level
Another popular use of subwoofers is to be loud. This is usually for people trying to show off or compete in SPL competitions. In these setups you usually have the biggest or most subs you can fit/afford with a lot of power going to them. These are the systems that go bump and wake your neighbors late at night.
Check out the Infinity Basslink 2×10″ powered sub on Amazon
In order to add a sub, you have to have an amp to power it. Since head units usually only put out 50W max, you need to add a more powerful external amp to put out the higher power levels subs need since they are a much bigger driver than your typical 6.5″ speaker. Subs usually require at least 150W RMS to perform (depending on the driver).
Some people want to add a little bit of bass in a small package. Sub and amp combos like this Infinity Basslink ($165 on Amazon with Prime shipping) powered subwoofer have the amp built in and take up less room without sacrificing too much bass performance
The next upgrade step is usually the speakers, specifically the front. This can be done with or without adding another amp, but you will get much better performance in terms of quality and loudness with one. The front speakers come first because that is where the sound is being projected towards you, and setting up a good front stage can make it sound like a concert in your car. Rear speakers are usually just used for fill or not used at all when quality is your concern. For loudness, you’ll add all the speakers you can.
Check out the db651 on Amazon
Coax speakers are the ones with the tweeter built into the center of the speaker like these Polk Audio db651‘s ($57 on Amazon with Prime shipping). This design allows for more simple installation than component speakers but still adds much better quality than factory speakers. These are a very popular choice due to being more inexpensive and easier/cheaper to install than component setups.
The tweeters are usually crossed over with the use of a capacitor which gives it a -6dB/octave slope at a fixed crossover point to block low frequencies from playing through, and damaging, the tweeter.
Component speakers, or separates, are speaker setups that separate the tweeter from the mid range driver so the tweeter can be located to better improve sound quality. Since higher frequency sounds are easier to localize than low frequency sounds (hence why the sub is all the way in the trunk usually, the speakers are way down in the door panels, and the tweeters are up high), placing the tweeters closer to ear level and facing them at the driver raises the sound stage and greatly improves the sound quality.
Components usually come with an external crossover with a more advanced network than a single capacitor. These usually have a second or third order crossover set at a certain frequency offering steeper crossover slopes and usually consist of an L pad to dim the tweeters down a few dB to make them not as harsh.
Speakers can be amplified with the head unit, but adding an external amp greatly increases output and sound quality. This is especially true when running components since the passive crossovers aren’t too efficient and require the recommended RMS power to sound best.
Basically, if you can afford it, do it.
These are the basics of sound system upgrades. Depending on your budget you can do one or all of them to improve your sound.