Center channel Impedance.

[E-Stat]

But where did extra 3 db came from?

3 db from acoustic gain. 3 db from a doubling of power. If you had a transformer driven amplifier whose output was consistent, then you would get 3 db from merely doubling the speakers. Additionally, you are now running twice the power to double the speakers.

If we halv the impedance (add another speaker in parallel), wouldn't double power be needed to keep same volume as when there was only one speaker.

Nope. Re-read you own source!

"If we have two sources of the same volume, the loudness acoustic gain is 3 db."

Before the Sound Labs, I used Acoustat 2+2s which had four panels per speaker. There was also a 1+1 which used the exact same transformer for driving half the number of panels. While there were different taps to the bass transformer to optimize the frequency response, the 2+2 was twice as efficient having double the panel area. 400 watts into a double pair of Advents has 3db more output than 400 watts into a single pair.

Also remember that relation between power output and acoustic power out is not linear as for every 3 db increase in loudness require doubling the power.

Or having two sources with the same power! You still don't get it?

I went to their web site to look at First Watt amplifiers specifications. If you look at their amp's THD, you will see that %THD are only for 1 watt output.

You didn't look very far. Try again.

I would like to know what is THD for full power output. I bet it does not look pretty

And you would be wrong. Again.

The extra power is not for loudness,

At least you are consistently wrong. I assure you that the measurable levels I get at 400 watts is much higher than for 4 watts. You can teach me nothing in this regard. I know exactly the difference between what the Stasis output is at 0 db and at -20 db.

rw