Physics · Ch 11 — Waves
Reflection of Sound Through the Plane Surface
Reflection of Sound Through the Plane Surface
Sound waves reflect off a plane (flat) wall in a manner that exactly parallels how light reflects off a plane mirror. If a loudspeaker is positioned at some angle relative to a plane wall, treated as a point source, the sound radiating from it travels outward as spherical wavefronts (visualised as expanding concentric circles of compression). When these wavefronts strike the plane wall and reflect, the reflected wavefronts are themselves also spherical, and their centre of curvature -- lying on the far side of the wall from the real source -- behaves exactly as if it were the position of a second, virtual (imaginary) sound source, positioned as the mirror image of the real source in the plane of the wall. A listener on the same side as the real source therefore perceives the reflected sound as though it were arriving from this virtual image source located behind the wall, exactly as a person looking into a plane mirror perceives their own reflection as an image located behind the mirror's surface. This is precisely why a listener in an ordinary room typically hears …
What this figure shows. A loudspeaker (sound source) is drawn at an angle in front of a vertical wall, emitting curved wavefronts labelled "Incident spherical wave" that expand outward and strike the wall. On the far side of the wall, a matching set of curved wavefronts is drawn labelled "Reflected spherical wave", positioned as though radiating outward from a "Virtual source" located behind the wall at the mirror-image point of the real speaker, with a listener drawn on the same side as the real source receiving the reflected wave as if it truly came from that virtual position. The figure establishes, by direct analogy with a plane mirror in optics, that a listener hearing sound reflected off a flat wall perceives it as coming from an imaginary …
What this figure shows. A small illustrative scene shows a person near a wall, with dashed arrows labelled "Direct sound" drawn travelling straight from a sound source to the listener's ear, alongside separate dashed arrows labelled "Reflected sound" drawn bouncing off the nearby wall before also reaching the same listener's ear. The figure gives a concrete, everyday illustration of the plane-surface reflection principle just introduced in Figure 11.18: in ordinary rooms and outdoor spaces, a listener routinely receives both the direct sound travelling straight from a source and one or more reflected copies of the same sound bouncing off nearby walls, arriving sligh …