Originally Posted by
pos
The situation is quite simple:
when doubling the amplitude of a signal you get a 6dB gain.
To achieve this acoustically you need to double the volume of air that is moved.
For one speaker that means doubling the excursion, which requires to double the tension, which implies multiplying the power by four (P = U^2/R, ).
You can also achieve the same result by keeping the excursion constant and doubling the cone area (still twice the volume of air moved) OR doubling the number of cones of same area (and feeding each cone with the same power).
of course running one amplifier with half the impedance load is a way to acheive a (near) doubling of the power, but lets say for the sake of simplicity that we use two amps instead of one, with the same power for each.
So with two speakers you double your power (same power for each speaker) but you get a 6dB boost. BUT there is a catch: you only get that boost where the signal from the two cones sum in phase. That means on a straight line between the two cones for all frequencies, and also around that line for the lower frequencies (the lower you go, the further around the line you get the inphase summation). This is a basic directivity phenomenone, similar to what you get with a single one, but of course with two cones it strikes lower in frequency...
Where the summation is not in phase (farther from the line and higher in frequency) you only get a 3dB boost (quadratic summation).
Of course in a real room the situation is more complicated, but the reasoning is the same. This is only the places where you get that in phase summation (not a straigth line anymore...) that are more complicated to predict.