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Efficiency and Sensitivity are not the same thing!

Efficiency is 1 watt/1 meter.

Sensitivity is 2.83 volts at 1 meter.

If the speaker is 8 ohms, the two are the same.
(8 ohms = 2.83volts/0.35 amps; 1 watt= 2.83Volts x 0.35Amps)

If the speaker is 4 ohms, 2.83 volts works out to 2 watts, not one. IOW, there is the expectation that the amp is doubling its power!

So by cutting the impedance in half, the speaker can **appear** to be more efficient, which does not happen. If you want to know how efficient the speaker is, the only way to do that so it makes sense it to limit the input power to 1 watt.

Now to play with this a little, if you have a speaker that is 4 ohms and 90 db sensitivity, its efficiency is actually 87 db. This is because going from 2 watts to 1 watt is a 3 db change.

Conversely, if you have a speaker that is 90 db sensitivity and it is a 16 ohm speaker, its efficiency is 93 db.

This can have a profound effect depending on the type of amplifier you want to use. FWIW. One should also keep in mind that all amps regardless of technology have greater distortion into 4 ohms, and so will sound harsher and less detailed.

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