Physics · Ch 11 — Waves
Reflection of Sound Through the Curved Surface
Reflection of Sound Through the Curved Surface
Sound reflected from a curved surface behaves quite differently depending on whether the surface curves outward (convex) or inward (concave). Sound reflected from a convex surface is spread out further than the incoming wavefronts, so it is quickly attenuated (weakened) as it disperses over an ever-widening area; sound reflected from a concave surface, by contrast, converges the reflected wavefronts toward a single focal point, where it can be strongly amplified. This focusing property is deliberately exploited in the parabolic reflector (a specific type of curved reflector) used to build highly directional "parabolic microphones", which are able to pick out and amplify a faint, distant sound arriving from one precise direction while rejecting sound from other directions. The same focusing principle explains why any surface -- smooth or rough -- inside a large hall, auditorium, or theatre absorbs some of the sound produced there (via the walls, ceiling, floor, and seats), which can leave some parts of the audience hearing the performer poorly; placing a curved, concave sound board directly behind the speaker or performer reflects their soun …
What this figure shows. Two side-by-side panels compare reflection off a convex reflector and a concave reflector. In the convex-reflector panel, a sound source emits wavefronts that strike the outward-curving (convex) surface and are shown reflecting so that the outgoing wavefronts spread apart more than the incoming ones, diverging away from any common point. In the concave-reflector panel, a sound source's wavefronts strike the inward-curving (concave) surface and are shown reflecting so that the outgoing wavefronts bend inward and converge together toward a common focal region. The figure visually contrasts the two curved-surface behaviours that the surrounding text describes in words: a convex surface spreads and weakens (attenuates) reflected sound, while a con …
What this figure shows. A stage area (labelled "Dais") is drawn with a large curved reflecting board, labelled "Concave Surface", positioned behind and above a speaker standing on the stage, angled so that sound leaving the speaker's mouth strikes the concave board and is reflected forward and spread out evenly across the seating area of the auditorium in front of the stage. The figure illustrates the practical, large-scale application of concave-surface focusing described in this subsection: without such a reflecting board, sound produced on stage would radiate in many directions and be absorbed unevenly by walls, ceiling, floor and seats, giving uneven and weak coverage; the concave board instead redirects and evenly redistributes t …