Physics
Sound Wave Reflection
Quick fact
The law of reflection for sound is the same as for light: the angle of incidence equals the angle of reflection.
Why this is interesting
You shout into a canyon and hear your voice after a delay. What causes that familiar echo?
Read the full explanation
Understanding Sound Wave Reflection
Imagine throwing a rubber ball at a wall: it bounces off at the same angle it came in. Sound waves do something similar. A sound wave travels through air as a series of compressions and rarefactions. When this wave hits a solid surface, like a cliff or a wall, it cannot continue through that material easily. Instead, part of the wave's energy is sent back into the air. The wave that arrives at the surface is called the incident wave, and the one that bounces back is the reflected wave. If the surface is large and hard, you hear the reflected sound as a distinct echo after the original sound.
A deeper explanation
Reflection happens because sound is a pressure wave. When it reaches a boundary where the material's acoustic impedance—a measure of how much pressure a wave needs to cause particle motion—changes sharply, the wave cannot transmit perfectly. The mismatch forces some energy to reflect backward. This is described by the reflection coefficient, which depends on the impedance difference. The angle of reflection equals the angle of incidence measured from a line perpendicular to the surface (the normal). This principle is crucial: sonar uses reflected sound pulses to detect objects underwater; bats use it to navigate; architects use it to control echoes in concert halls. Without reflection, sounds would simply fade away—no echoes, no reverberation, no ability to sense distant obstacles with sound.