well i hope this helps you out.
Impedance is the amount of resistance speakers provide to the current flowing from the amplifier. Every time you halve the speakers' impedance — switching from a 4-ohm to a 2-ohm sub, for instance — you effectively ask the amplifier to double its output. Some amps can handle this lower impedance, some cannot. The key thing to know about impedance, then, is how to match your speakers to the capabilities of your amp.
The typical car stereo amplifier is stable down to 4 ohms in mono (or bridged) mode or 2 ohms stereo. Some high performance amplifiers can operate safely with loads as low as 1/2-ohm in stereo (1-ohm mono). If you're designing a subwoofer system, ask yourself the following questions:
1. How many subs do I want? More subs = more cone surface area = more air moved = big bass! Balance this equation against the amount of room in your vehicle.
2. How much power do I want? If you're powering your vehicle's speakers with your receiver, you'll need less power for your subs. If you use a multi-channel outboard amp for your speakers, you'll need more power for your subs.
Check out suitable amps in your price range. Once you've found the right amplifier, look for subwoofers that complement your amp in terms of power rating and impedance. Most subs are rated at 4 ohms, but 2-ohm, 8-ohm, and dual voice coil subwoofers have become commonplace, so it's easy to find speakers that will bring out the best in the amp you've chosen.
