why low impedance speakers?

To move the speaker cone and create sound, an electro-magnetic field is created to push and pull against the fixed magnet. This is done by forcing current through the voice coil with the voltage developed by the power amp. The lower the impedance of the coil, the more current will flow for a given voltage and the louder the speaker will play. Basically Bob's point above. Power amps are voltage amplifiers that adjust their output current within reason to meet the demands of the load. So if the impedance of the speaker is too high, it will take an extremely powerful amplifier to get any sound out of it. If the impedance is too low, the amp will run out of current and cause distortion and/or blow a fuse. 4 to 8 ohms is a good compromise.

The point about low impedance speakers being "more in control" is also well founded. As the magentic field expands and contracts, a counter voltage is created which also must fight against the impedance of the voice coil. A lower impedance coil will let this energy to dissipate more quickly, as will an amp with a lower output impedance. This is one reason solid state amps typically have tighter bass than tube amps, since the SS amp has a lower output impedance.

I dissagree with abstract7. a typical 8 ohm or 4 ohm speaker has nothing to do with the loudness of the speaker. Only two things affect the loudness of a speaker. 1. Efficiency, (how loud the speaker will be with a given power input.) 2. power input. It is true that most amps will put out far more power into a 4 ohm load than with an 8 ohm load. however, almost all speaker's sensitivity rating are at one watt at one meter and one watt is one watt, regardless of the speaker load on the amp. So, a speaker with a sensitivity of 90 db would play at 90 db at one watt weather it was a 16 ohm speaker or a one ohm speaker. That's a scientific fact. I think what abstract7 was getting at was the fact that a 4 ohm speaker will increase a typical amps output by an average of 70, 80 and in rare cases, 100%. It's that extra power output that makes the speaker louder, not the ohms.

Mborner is correct that a 90dB speaker is just that 90dB at 1 watt 1 meter. But that's not really what I was getting at. It is a combination of his last statement, but more importantly the speaker manufacturer/designer can implement a design on a 4 ohm load that will result in higher output for 1 watt versus a similar design at 8 ohms. So what I am saying is that if the requirement is to design a speaker that has 90db sensitivity--it's easier to do with a 4 ohm design. It is in part a specmanship game. What is important is that if it is a really low impedance--dips into the 1 or 2 ohms--you have an amp to handle it. Most amps have no problems with 4 ohm loads (but again, that's only a nominal load).