Voltage is defined as "electrical pressure" measured in Volts.
To clearly define this word, I'll define a couple of other relavent words as well: Volt: One volt will, when applied across a resistance of one ohm, force one amp of current to flow through the circuit. Current: A measure of electron flow, measured in Amperes (amps). Ampere: One amp of current will flow through a resistance of one ohm if one volt of electrical force is applied to the circuit . Resistance: Resistance is the opposition to the flow of electrical current. Impedance: The AC (alternating current) equivelant of Resistance. Ohm: One ohm of resistance will allow only one amp of current to flow through a circuit when one volt of electrical force is applied to the circuit. Coulomb: The S1 unit of electric charge , equal to the quantity of electricity conveyed in one second by a current of one ampere.
Ok, there are two places we usually see voltage brought up in car audio. 1: Charging systems (voltage to amplifiers, or from alternators/voltage regulators, car batteries, etc) and 2: head unit pre-out "voltages."
Voltage is generally a fixed valeu in a car, being 12.5 voltd (DC or direct current) when the car is turned off, or at idle, and 13.8 to 14.4 volts when the car is running, and above idle. This is because the car's BATTERY supplies voltage when the engine is off, and the ALTERNATOR puts out the same voltage as teh car's BATTERY at idle. 12.5VDC. When the engine is above idle, the ALTERNATOR puts out a higher <b?voltage</b> in order to recharge the car's BATTERY, and as a result, your amplifers usually see that higher voltage. If the amplifier has a regulated power supply, it always sees 12VDC regardless of input voltage, but with an unregulated amplifier power supply, the power output of the amplifier varies directly in accordance with the input voltage. We'll get into that more another time though.
The head unit's pre-out voltage is a measure of the signal strength being sent from the head unit to the amplifier, and this voltage is an AC (alternating current) voltage the magnitude of which varies with the volume level of the head unit. a "4 volt" pre-out will provide 4 volts at full volume. The higher the pre-out voltage is, generally the cleaner the signal will be, and the higher the resolution of that signal, so the better detail you'll get from the amplifiers being driven by that input signal. This does not make your amplifiers any "louder" but it will make the SQ better, and the signal being amplified much cleaner and more accurate and detailed.
If a car's voltage drops to below that of about 12 volts, you can start to have problems with any electronics in a car, including ammplifiers and stereo systems. When the voltage to an amplifier is reduced to below the rated operating voltage of that device (12 volts) the amplifier's output voltage (the signal sent to the speakers) will no longer be a clean sine wave AC voltage you want. When voltage going into teh amp is reduced, the power supply of the amp can no longer produce a clean output signal, and "clipping" occurs. This is when the peaks of the AC sine wave output are clipped off due to sagging voltage rails, and the signal sent to your speakers becomes a DC voltage. This DC voltage causes the cooling effect produced by the movement of the speakers to be reduced or completely cancelled, resulting in overheating of the voice coils, and distortion as well as physical, thermal damage to teh voice coils of the speakers. This is why a suitable charging system is a necessity in a car with a high current audio system to avoid damage to both the car and the audio system.
This is really just a primer on voltage. You can easily spend an entire semester in college learning about the basics of voltage, current, and resistance alone but I've tried to give you the need to know parts in relation to car audio. I hope it helps some of you gain a better understanding of electricity and it's relation to our hobby.