Refraction, unlike wall diffraction, can create near-total acoustic silent zones.

Evidence

One explanation for the dramatic silence experienced by Kean and Randolph is refraction if the sound entering a cooler layer of air aloft. As we will explain in the next section, refraction of sound can leave some zones almost completely silent. These are places where ray tracing reveals no rays penetrate starting at the source; they gently bend away. True, no rays penetrate into the shadow behind a wall either, but in that case considerable diffraction from the abrupt edge at the top of the wall means it is far from silent behind the wall. Not so with the gentler refractive bending of rays: there is little if any diffraction, and regions not visited by rays are very nearly silent. — Eric J. Heller, Why You Hear What You Hear- An…, 2595

Categorization

• DEFINE: Distinguishes diffraction (abrupt edges, weak shadow) from refraction (gradual bends, near-silent shadow zones).
• XREF: Connects to the reader’s broader question of why we hear what we hear — outdoor acoustic shadowing as atmospheric phenomenon.