Concave curved walls confine whispers to travel tens of meters

Evidence

A whispering gallery is a structure where sounds are transported large distances. These are usually one of two types: The first type has gently curving concave walls; a ray-tracing analysis (see figure 27.10, left) shows they keep sound launched near and parallel to the wall confined to the wall region. Although the sound can still spread out in the vertical direction, the confinement of the sound to a layer near the wall may allow a person to whisper to another and be heard plainly 30 or 50 meters away or more. Even more dramatic are whispering galleries shaped so that the ray paths, which stay near the wall but initially spread out along the surface of the wall, come back to a focus at another place near the wall. The inside of a sphere will do the trick nicely, since all circumferential circles radiating from a point on the surface converge again across from the center of the sphere. — Eric J. Heller, Why You Hear What You Hear- An…, 2516

Categorization

• SHARE: A specifically nerdy fact about whispering gallery geometry — great for an acoustics-minded friend.
• XREF: Explains a classic audibility phenomenon; relates to why St Paul’s dome and other whispering galleries work.