A nearby wall doubles a source’s radiated power via reflected amplitude

Evidence

The constructive addition of amplitude from the source and its very nearby (compared to a wavelength) image means that the amplitude is almost twice what it would have been for a single source alone. Since power is the square of the amplitude, the total power is everywhere about four times what it would have been for a single source. Remembering that only the physical region counts (or that there is only one real source), we must divide by two, but the wall has caused the source to put out a factor of 2 more power than it would have. — Eric J. Heller, Why You Hear What You Hear- An…, 516