Originally Posted by Bill Fitzmaurice
4 ohm speakers require twice the current capacity as 8 ohm. Most high quality amps and receivers can handle 4 ohm loads, but not all can. There's no advantage to using 4 ohm speakers, so all things considered IMO you should avoid them.
It's not twice the current, it's about 40% more current for a given amount of power dissapation by the speakers. Power is proportional to the current squared.
The impedance of a speaker isn't like the resistance value of a resistor. It varies with frequency. The actual impedance will fluctuate a lot over the course of a song. The impedance values are just a ballpark figure to give you a general idea of what kind of resistive load a speaker might give you.That's why the rating are always a big round number like 4,6, or 8 and not 6.71. Many speakers, paricularly cheap speakers made in China, just blindly stick a 8 ohm rating on the speaker. I've found, in general, 4 ohm speakers to be of better quality than 8 ohm in that a manufacturer probably took the time to measure an accurate impedance and hence probably took more time to build a better speaker.
The other thing to keep in mind is that while a 4 ohm speaker requires more current for a given volume level, at the same time, the receiver or amp is capable of delivering more current. The lower the resistive load, the higher the power rating for the receiver or amp.
My personal experience is the ohm rating on a speaker is something I generally ignore. I've ownded many 4 ohm speakers and have never experienced that an amp or reciever has any more trouble driving them then similar 8 ohm speakers. Speaker sensitivity is more important than impedance when it comes to how loud you can play a speaker beofre an amp blows.