Reflection and Refraction of Sound

Explained

Echoes, and sound that bends

Sound is a longitudinal wave, so it reflects and refracts like any other wave. What makes it feel different from light is the speed. Sound travels at about 340 m/s in air, roughly a million times slower than light, and that slowness is why we can hear reflections as separate events.

Echoes

An echo is a reflection of sound from a large hard surface such as a cliff or a wall. Soft, uneven surfaces absorb sound instead, which is why a furnished room has no echo and an empty hall does.

You only hear a distinct echo if the reflected sound arrives sufficiently later than the original. Too close to the wall and the reflection overlaps the original sound, so it is heard as one slightly prolonged noise rather than two.

To calculate with an echo, remember the sound travels to the surface and back, so the distance in the equation is twice the distance to the wall. Halving is the step most often forgotten, and it makes the answer exactly twice what it should be.

Uses of reflected sound

Echo sounding measures the depth of water by timing a pulse to the seabed and back. Ultrasound scanning builds an image from reflections at boundaries inside the body, which works because some of the wave reflects wherever the material changes. Both use the same calculation with the same halving step.

Refraction of sound

Sound refracts when it moves into a region where its speed is different, and in air the speed depends on temperature. Sound travels faster in warmer air.

On a normal day the air near the ground is warmer than the air above. Sound travelling upwards moves into cooler, slower air, so it bends away from the ground and is lost.

On a clear night the ground cools quickly, so the air near it is colder than the air above. Now sound travelling upwards moves into warmer, faster air and bends back down towards the ground. That is why distant sounds carry so much further at night, and it is a favourite context for this topic.

Sound needs a medium

A longitudinal wave travels as compressions and rarefactions of the material it passes through, so there must be particles to compress. Sound cannot travel through a vacuum, which is the one property that clearly separates it from light.

It also travels faster in liquids than in gases and faster still in solids, because the particles are closer together and pass the disturbance on more quickly. That is the opposite of what happens to light, which slows down in denser materials, and questions sometimes rely on the contrast.