Nearby walls amplify monopole but not dipole sound sources

Evidence

How does an object a short distance away from a sound source induce it to put out more total power? We have seen that confinement or reflection (echo) leads to amplification of a monopole source, even with something so simple as a nearby wall (see, for example, section 7.1). By the method of images (section 7.2), a nearby wall turns a dipole source into a quadrupole source for the perpendicular orientation of the dipole relative to the wall, and into a pair of in-phase dipoles for the parallel orientation. A “naked” dipole source is shown in figure 20.4a. The perpendicular orientation of the dipole, “dressed” by a nearby wall, is shown in b. The image dipole has the opposite orientation of the real one, which worsens the situation and leads to even more short-circuiting. The result is a quadrupole source, which can be considered as a dipole short-circuiting another dipole, reducing its already meager output. The dipole dressed by a parallel wall (case c) doubles its amplitude (for the source very near the wall) and induces twice the total power out of it, as with a monopole source. But it remains a self-short-circuiting dipole source. There is a dramatically better strategy available: turn the dipole source into a monopole source! — Eric J. Heller, Why You Hear What You Hear- An…, 1881