Reflecting surfaces within an eighth wavelength amplify a source via proximity resonance

Evidence

Almost all sound sources expend only a tiny percentage of their energy making sound, “wasting” the rest on friction of various sorts. As we just noted, only a tiny fraction of the force exerted is owing to sound radiation. The truth is that the source hardly notices the extra load. This is self-amplification—an example of a resonance phenomenon, as we shall see in chapter 9. We also call the effect proximity resonance, as it is caused by the proximity of nearby reflecting objects. Engineers call it “loading the source.” It is effective until the reflected amplitude no longer comes back sufficiently in phase, which happens if the round-trip distance is greater than a quarter wavelength—that is, a distance to the reflecting wall over an eighth of a wavelength. After this distance, proximity resonance is no longer operative. For fixed distance d from a wall, proximity resonance will hold as long as the wavelength λ is greater than 8d, or equivalently the frequency is less than f = c/8d, since fλ = c. — Eric J. Heller, Why You Hear What You Hear- An…, 513

Categorization

• DEFINE: Defines proximity resonance/loading the source and its quarter-wavelength threshold.