Sound power radiation of two monopole sources For a fixed given electrical power to two monopole sources, producing the same frequency. Which mode, either in-phase or out of phase, will radiate the most sound power into the far-field? 
 A: Short answer: in phase radiates more in the far-field. But read the long answer.
Long answer: this actually depends on the frequency and the distance between the sources.
For a wavelength significantly larger than the distance between the sources, the pressures emitted by each source simply add up when in-phase; and they cancel when out-of-phase.
But what happens when the wavelength is _not_ larger than the distance between the sources?
In this case, the resulting pressure depends on the position of the listener. At the listening point, the interferences could be constructive or destructive. And this is true for the in-phase and out-of-phase case...
EDIT: a short note on power
When the wavelength is much larger than the distance between the loudspeakers, the power radiated by the 2 sources is:


*

*when in-phase: approximately the same as as one source which would receive the added powers received by 2 sources.

*when out-of-phase: approximately zero.


You may ask: where does the energy goes when the sources are out-of-phase?
When the sources are out-of-phase, each source is changing the acoustical impedance seen by the other one: the mechanical motion of the loudspeaker diaphragm is not transformed into acoustical energy.
Thus, the efficiency of that system is approximately zero: none of the electrical power transmitted to the speakers is transformed into acoustical power. All the electrical power is transformed into heat!
