So what are they?

I hope you didn't want a simple explanation :P

To explain exactly what it is, I'll have to explain physically HOW a speaker(any speaker, subwoofer or non) works. There are several parts to a speaker, but I'll only cover the ones necessary to explain dual vs. single voice coils.

The cone of a speaker is just that... the cone area. It's usually made out of plastic(polypropolene), paper, or some type of metal compound. It's the part of a speaker that moves the air for sound.

It's attached to a "pole piece", that runs from the bottom of the cone to the magnet at the bottom of the speaker.

Around the pole piece is a coil of copper, called the "voice coil". When you send current through a copper coil, it becomes an electromagnet(rememer wrapping copper wire around a nail, then hooking it to a 9-volt batter in science class?).

The magnet made by the copper wire is the same charge as the magnet at the bottom of the speaker, so they repel each other. Since the magnet at the bottom of the speaker is attached to the basket, which is screwed down, the other magnet(the voice coil) is pushed upward(or outward), which moves the cone, and the cone moves air. That's all sound is, is moving air.

So a dual-voice coil sub is just that... a sub with two voice coils rather than one. The normal advantage of a DVC sub is that it can handle more power without frying the coil(s). Having DVC subs vs. SVC can also allow you to have a lower(or higher) impedance on the low-end of your system, but that's an entirely different lesson ;)

-Matt

whats impedence?

go get a book about our cars at an auto shop... that wont help you out with this but you will learn alot from it... just study it like you have an exam the next day

Impedance is the amount of electrical resistance, and it's measured in Ohms(symbolized by the Greek omega symbol).

That's what's meant when an amp says it produces 300W at 2 Ohms. It's a VERY complicated and confusing topic, especially when you're brand new to audio.

-Matt