Echoes, Reverberation and Sound Reflection

Sound waves reflect from hard, flat surfaces. The reflected sound is called an echo.

Conditions for hearing an echo

An echo is heard as a separate sound when the reflected sound arrives at least 0.1 s after the original sound. The minimum distance to the reflecting surface is:

$d = \frac{v \times t}{2} = \frac{340 \times 0.1}{2} = 17 \text{ m}$

The factor of 2 accounts for the sound travelling to the surface and back.

Using echoes to measure distance

$d = \frac{v \times t}{2}$

Example: A ship sends a sonar pulse downward. The echo returns after 0.4 s. Speed of sound in water = 1500 m/s.

$d = \frac{1500 \times 0.4}{2} = 300 \text{ m}$

Using echoes to measure the speed of sound

1. Stand a measured distance $d$ from a large wall.
2. Clap and listen for the echo.
3. Adjust the clapping rate so each clap coincides with the echo of the previous clap.
4. Time $N$ clap intervals and find the period between claps ($t$).
5. $v = 2d/t$ (the sound travels $2d$ in time $t$).

Reverberation

Reverberation is the persistence of sound in a room due to multiple reflections. A long reverberation time makes speech unclear. Soft, porous materials (carpet, curtains, acoustic tiles) absorb sound and reduce reverberation.

Common errors and how to correct them

• Forgetting to divide by 2 in echo calculations (the sound travels to the surface AND back).
• Confusing echoes with reverberation. An echo is a single distinct reflection; reverberation is the overlapping of many reflections.

How to apply this in an exam

Write $d = vt/2$ and explain the factor of 2. Substitute values carefully. For reverberation questions, explain how absorbing materials reduce it.