Some ambiguity in the comments here! Let's see if we can straighten them out.
First- Sensitivity is indeed 2.83V at 1 meter. It is not the same as efficiency! Efficiency is 1 watt at 1 meter and there is a difference.
If the speaker is 8 ohms the two numbers are the same, as 2.83 volts driving 8 ohms means that there is 1 watt in the load. However if the impedance is reduced to 4 ohms, 2.83 volts is 2 watts.
What this means is that a 4 ohm speaker of the same sensitivity as an 8 ohm speaker is actually 3 db less efficient. Because the two measurements are often confused, the myth has arisen that if you put two speakers in parallel (for example two woofers) that their efficiency is doubled (3db) but this is not true. The sensitivity is doubled as the impedance is cut in half, but the efficiency remains the same.
Conversely if the same woofers were put in series, the efficiency is still the same as a single woofer, but now the sensitivity has been reduced by 3 db.
What is poorly understood is that efficiency means that there will be 1 watt in the system. So if 2 woofers are involved per our example, each woofer will dissipate 1/2 watt regardless of whether the speakers are in series or parallel.
Where this becomes important has to do with your amplifier. If tubes, the sensitivity rating can be misleading as the impedance plays a big role. Tube generally are less effective into lower impedances and are not known for doubling power as the impedance of the load is cut in half.
It might help to read this article- it explains why this is going on and why there are two specs:
http://www.atma-sphere.com/Resources/Paradigms_in_Amplifier_Design.php
As a further note something not discussed in high end audio but certainly should be is the fact that there really is no good justification for four ohms as a speaker load if sound quality is your goal. It does not matter what kind of amplifier you have! All transistor amps (while maybe quite comfortable driving 4 ohms) make more distortion into lower impedances.
You might think that the change in distortion is insignificant but its not. Our ears use odd ordered harmonics to sort out how loud a sound is; in essence any amplifier driving a lower impedance will sound brighter and harsher. Now if *sound pressure* is your goal then there is a weak argument for four ohms, as you might get 3 db more output out of the amp if its solid state. However you will find that the speaker cables are much more critical for four ohms.
First- Sensitivity is indeed 2.83V at 1 meter. It is not the same as efficiency! Efficiency is 1 watt at 1 meter and there is a difference.
If the speaker is 8 ohms the two numbers are the same, as 2.83 volts driving 8 ohms means that there is 1 watt in the load. However if the impedance is reduced to 4 ohms, 2.83 volts is 2 watts.
What this means is that a 4 ohm speaker of the same sensitivity as an 8 ohm speaker is actually 3 db less efficient. Because the two measurements are often confused, the myth has arisen that if you put two speakers in parallel (for example two woofers) that their efficiency is doubled (3db) but this is not true. The sensitivity is doubled as the impedance is cut in half, but the efficiency remains the same.
Conversely if the same woofers were put in series, the efficiency is still the same as a single woofer, but now the sensitivity has been reduced by 3 db.
What is poorly understood is that efficiency means that there will be 1 watt in the system. So if 2 woofers are involved per our example, each woofer will dissipate 1/2 watt regardless of whether the speakers are in series or parallel.
Where this becomes important has to do with your amplifier. If tubes, the sensitivity rating can be misleading as the impedance plays a big role. Tube generally are less effective into lower impedances and are not known for doubling power as the impedance of the load is cut in half.
It might help to read this article- it explains why this is going on and why there are two specs:
http://www.atma-sphere.com/Resources/Paradigms_in_Amplifier_Design.php
As a further note something not discussed in high end audio but certainly should be is the fact that there really is no good justification for four ohms as a speaker load if sound quality is your goal. It does not matter what kind of amplifier you have! All transistor amps (while maybe quite comfortable driving 4 ohms) make more distortion into lower impedances.
You might think that the change in distortion is insignificant but its not. Our ears use odd ordered harmonics to sort out how loud a sound is; in essence any amplifier driving a lower impedance will sound brighter and harsher. Now if *sound pressure* is your goal then there is a weak argument for four ohms, as you might get 3 db more output out of the amp if its solid state. However you will find that the speaker cables are much more critical for four ohms.