8 Ohm Sub

I have a 200 Watt 18" 8 Ohm subwoofer speaker and I was wondering how much power in 4 Ohms I would need to drive it to the full 200 Watts? I know most amps don't say how much their bridged power is in 8 Omhs.

In theory when you double impedance, power is cut by half, so if an amplifier produces 400 watts @ 4 ohms, it should produce 200 watts @ 8 ohms.~~wirewise~~

Or you can find a device called an Accumatch by Veritas http://www.veritasaudio.com/. It is a complex device, but simple to hook up. It will effectively halve or double the ohmage (called impediance) the amp "sees" depending on how you hook it up. It will then allow you to use a smaller amp to drive the same sub. It works great, I used one on a Pheonix Gold Cyclone to drop the impediance for my amp. I was able to make my amp run at 2 ohms using an 8 ohm sub with the Accumatch! I could actually HEAR the difference. They are awesome devices.Computer Guy & Electronics Engineer.