There seems to be a number of questions regarding how to wire up decks, speakers and how to use combinations of speakers to create a certain load to have an Amplifier make as much power as it can.
I will try to explain everything you need to know to talk knowledgeably about, and safely install car audio products. But you'll have to bear with me, there's a great deal to cover.
Let's start with some basic physics. The flow of electrons is what causes work to get done. Now, work doesn't mean taking out the trash, or flipping burgers. Work is simply the process of having something happen. From something as minute as a pin falling on the floor, to running a top fuel dragster down the quarter mile, work has to be done, and something has to move or change.
When we talk about work being done in a car audio system, consider what we
are trying to do. We wish to take a source of music, be it a CD, Minidisk, Cassette or radio station,
convert the signal from that source to an analog low level waveform, then amplify
that signal and send it to a transducer (any device that transforms one form of energy into another (speaker, light bulb, motor, engine))
which will in-turn move air and produce sound..
This is actually a very simple task in terms of overall system complexity, and many of the concepts used in car and home audio have been around for decades.
We have two sources of energy in our vehicles to power our audio systems. The first is the simple car battery. I won't get into how a battery works here, suffice it to say that it is able to store a certain number of electrons. The second source is the car's charging system. This consists of an alternator, diodes and a voltage regulator, in simplest terms. This transforms motion (the turning of the engine) into electricity. We must use these sources of energy to power our cars and our audio systems. Again, there are certain laws of physics which will limit how many electronics we can flow at once, and thus, how much work actually gets done. Let's take a peek at those.
The primary law around which all electrical theory is based, is Ohms law. This law states that, when one
Volt is being applied to a load, which has a resistance of one Ohm, one Ampere of current will flow. It's a simple law, really. If you have two
Volts, you get two Amperes; if you have one Volt, and 2 Ohms, you get half an Amp. Follow me so far?
The second law is actually a group of laws: they are known as the power equations. There are three equations to know. Here's the units we will use: I = current in Amperes (or Amps), V = pressure in Volts, R = resistance in Ohms and P = power in Watts. All of these terms are capitalized, because they are named after real people.
P = V x I
P = V^2 / R
P = I^2 x R
OK, what does this all mean? The first equation states that power (in Watts) is the product of Volts times Amps. If you have 12 Volts, and 3 Amps, you have 36 Watts. Simple enough. The second equation states that power can also be calculated from the product of voltage squared, divided by resistance. If you have 10 Volts and 4 Ohms, you get 100 / 4 = 25 Watts. Still with me? OK, the last one: Power is the square of the current, times the resistance. Example, 2 Amps, and 4 Ohms = (2 x 2) x 4 = 16 Watts.