What's with 4 ohm speakers?

I was incorrect on the RAAL ribbon's. They are all 8ohm impedance. ScanSpeak has 36 different tweeter model's in production. 23 of those models have 4ohm impedance. Vifa currently has 18 tweeter model's that have 4ohm impedance.

Ribbons are often 8 ohms as the ribbon itself is the equivalent of a voice coil.

Speaking of tweeters, Morel offers nearly identical MDT29's in both 4 and 8 ohm versions. As rated, the 4 ohm is 92 dB sensitivity and the 8 ohm is 89 dB. For a MTM, the 4 ohm would be more likely.

So, just by halving impedance and not doubling driver area and motors, you get +3 dB.

Speaking of tweeters, Morel offers nearly identical MDT29's in both 4 and 8 ohm versions. As rated, the 4 ohm is 92 dB sensitivity and the 8 ohm is 89 dB. For a MTM, the 4 ohm would be more likely.

So, just by halving impedance and not doubling driver area and motors, you get +3 dB.

I may be wrong but FWIU halving the impedance will not increase efficiency by 3db. If you look at the spec's more closely you will notice that Morel spec's the 8 ohm at 89 db at 1W/1M which is 2.83 volts into 8 ohms. If you look at the 4 ohm spec sheet they stayed with a 2.83v input which is actually 2 watts into 4 ohms. So the doubling of the wattage is where the 3db comes from.

It seems to be fairly common practice among driver manufacturers to spec a driver that is available in both 4 and 8 ohm versions in this manner. Can't say as I blame them since they believe that the majority of there drivers will be used on a "constant voltage paradigm" type of amp. Thus the amp is going to try to keep the voltage output the same regardless of impedance.

Jjrenman, you are absolutely correct. People commonly assume that Efficiency and Sensitivity are the same but that is only true into exactly 8 ohms.