There are a lot of misconceptions about what capacitors are, what they do, and what to look for in one. A capacitor will not make your system suddenly sound much louder (although it can improve it slightly). It will not increase the voltage in your cars system (but it will help maintain it). A capacitor will not provide extra power for devices (but it will maintain it). A capacitor, however, will provide a benefit for most audiophiles in overall sound quality, and can benefit your cars electrical system.

For starters, a capacitor has a specific construction. The most basic form of a capacitor is two metal plates, which are very close to each other, but do not touch. The plates are separated by some type of insulating material. Air, which does not conduct, can be used as an insulator, and on older AM/FM radios, the tuning capacitor uses air. Other materials can be used, like plastics, or electrolyte pastes. When power is applied to the capacitor, the charges flow from positive, to the positive plate of the capacitor. The charges stop at the one plate, because of the insulating material, but the electrostatic charge pushes against the charge of the other plate, and causes the current to flow on that side. This flow will happen as long as the charges are able to build up on the plates. The size of the plate, and the distance between the two plates will determine how much charge the plates can hold. When the power source is removed, the charges are still there, and they have no place to go. The charges would like to meet each other, to equalize the charges between the two plates, but the insulator between the plates prevent this. If the plates were allowed to touch, the charges would all flow to the other side at once (which could be very dangerous). If some device, such as a light bulb, speaker, or motor were used to bridge the two sides of the capacitor, the charges would power that device until both plates evened out their charges. In this manner, a capacitor stores electrical charge.

When a capacitor charges, it does so at a certain rate. Normally, a capacitor will try to take as much charge as it possibly can, as quickly as it can. Like the shorting of a capacitor, this can be very dangerous. When installing a power supply capacitor (often called a stiffening cap) in a car, you should always charge it through a resistor. A resistor will slow down the rate at which the capacitor charges or discharges. This is known as an RC time constant. This is useful when a capacitor is used for audio. As I said earlier, current will flow through a capacitor until it is saturated. The speed at which a capacitor can take on a charge, and let it out is the frequency at which that capacitor prefers to work. Capacitors, to a certain degree, will resist lower frequencies than it can handle, but will present very little resistance at higher frequencies. This makes small capacitors ideal for blocking low frequency bass from speakers which cannot handle them (you are probably familiar with these as "bass blockers"). The property where a capacitor has more resistance to lower frequencies is called Capacitive Reactance. There is a neat formula for it too, its like this:

X_c=1/(2 PI FC)

Where X_c is Capacitive reactance. PI is 22/7, or more commonly used 3.1415..... F is frequency, and C is the capacitance in farads, . Now since a farad of charge is highly impractical for audio uses, you will have to convert the rating on your capacitor (which is usually in microfarads, or "µf") to a decimal equivalent of a farad. The easiest way to do this, of course, is to move the decimal point to the left.

Anyway....

When a resistor is placed in series with a capacitor, it will change the characteristics of the capacitor, with respect to its reactance. For most car applications, that resistance is the speaker it is used on.

Knowing this, we want to look at the two applications of capacitors for car audio:

1. Power Supply Stabilization
2. Crossovers

Very large capacitors are used for stabilizing the electrical system. As we have seen, a capacitor stores energy. In a system where several electrical devices are competing for power, when an amp starts using power at an inconstant rate (a little here, a little there), head and dash lights begin to dim. This can also affect other amps in the system. Amps have internal capacitors on the other side of their power supply which hold power for those times when it is really needed. However, the power to charge those capacitors has to come from somewhere, and during peak usage, the power is not always available. The result is that the distortion in the music increases slightly: Sound suffers. A capacitor will store the energy, and will give the energy up when it is needed by other components.

Another benefit of the large capacitor, is that as we mentioned, it passes high frequencies. Any electrical noise will be filtered by a capacitor since the noise is AC. In addition, since the capacitor is a large one, then the "high" frequency can actually be pretty low. In this manner, a capacitor helps reduce power supply noise in your audio system. Unfortunately, the larger capacitors are usually polar, meaning that they have a + and - side, and do not work properly when working backwards. The saving grace here is that, even in reverse, there is some capacitance, just not as much as when polarity is respected.

The second use of a capacitor in car audio, is as a crossover. As we've seen earlier, the capacitor (smaller, non polar ones) will pass AC of higher frequencies, and resist lower ones. Bass blockers are one example of this. When capacitors are used along with coils, they form crossover networks, which improve the sound going to a driver, and can make a speaker system sound a lot better. Many component sets have crossovers, which use a capacitor to carry the + signal* to the tweeter, and a coil would carry the + signal to the woofer. The negative signals do not need the components, because as long as the capacitor (or coil) are in the circuit somewhere, they will do their job. Without the coil and capacitors, the tweeter would distort badly, the woofer would usually do a really bad job of reproducing the highs, and there would be a nasty peak where the woofer and tweeter overlap.

Finally, capacitors can serve several other functions. Getting back to the RC time constant, you could use a capacitor to do things such as allow lights to "fade out" when you turn them off, or to charge a relay for keeping power to your system until you arm the alarm/open the door. You can even make a buzzer from a small capacitor and a relay. The applications are endless if you use your imagination.

* - While sound is AC, + and - are still used for polarity. In truth, the sound is only - or + for half of the signal, but using the + and - help us keep speakers in phase with one another.