Really, why do more speakers mean more volume?

[alexclaber]

So if my amp is (power) rated the same at 8 and 4ohms, then adding an additional 8 ohm cab in parrallel to the first 8 ohm cab will result in being just noticeably louder (3db). IF i had an amp that was rated at twice the power at 4 than 8 ohms, the setup would be 6db louder.

Yes but only if the single cab could handle the full output of your amp without power compression, otherwise going to two cabs would add a more than 3dB. And I think the directionality thing makes a difference too but I'll wait for another brain to contribute on that topic.

Alex

 

[billfitzmaurice]

You've mentioned that the choice of drivers in your line array cabs in less important due to the line array flattening the frequency response - how does this happen? I've googled it to death and can't find an answer - at least until you hit the maximum sensitivity limit!

That's what happens, you hit the sensitivity limit. But if the response isn't perfectly flat to begin with then you hit the sensitivity limit with a different number of drivers at different frequencies; with enough drivers eventually all frequencies hit the sensitivity limit and response for the system flattens out.

What happens with HF units? The mid (crossed over at 1kHz) and tweeters in the two cabs are about 20" apart - do they exhibit any sensitivity gain or is there purely a gain in output due to increased power handling?

20" is a quarter-wavelength at 170 Hz, so perfect coupling cannot happen. Depending on the phase angles of the summed responses at any particular frequency they'll either add or subtract from each other. In other words, comb-filtering. That's why it's critical those sources be vertically aligned, so that the combing occurs on the vertical plane where it's not heard, as opposed to the horizontal plane where it would be.

I also improve the on-axis sensitivity through the midrange due to the four vertically stacked 10"s acting like a short line-array.

Not really, it's way too short for that, but you do reduce vertical dispersion which significantly reduces room reflections.