When you see speakers that are 4 ohms in the bass and 8 ohms in the mids and highs, quite often the speaker designer is using woofers that are 3 db less efficient, and expecting that the amplifier will double power with the 4 ohm load. This brings the woofer output up to the level that the mids and highs operate at, but you need transistors to do that properly.
Ralph,
This may seem like a matter of semantics, but I want to raise an issue in connection with this statement because I think it has been a point of confusion in some other recent threads.
A speaker that has say a 4 ohm impedance in the lows, and an 8 ohm impedance in the mids and highs, and that may have woofers that are 3db less efficient than the other elements, will, as you say, generally be best suited for a solid state amplifier that can double power into 4 ohms.
BUT, as long as the amplifier has an output impedance which is negligibly small in relation to 4 ohms, and as long as the volume levels it is called upon to deliver are within the limitations of both its voltage swing capability and its ability to deliver current, then a flat frequency response will result. It is not the ability to double power into 4 ohms per se which results in flat frequency response into such a speaker, but rather it is the ability of the amp to act essentially as a voltage source, up to some maximum volume level.
In other words, to cite an example, a solid state amp having low output impedance, which is capable of 200W into 8 ohms but only 300W into 4 ohms, will deliver just as flat a frequency response into such a speaker as an amp which is capable of 200W into 8 ohms and 400W into 4 ohms, IF it is not called upon to generate more volume than its relatively limited current capability can support.
Do you agree?
Regards,
-- Al