Describe an echo as the reflection of sound waves and explain the conditions required for an echo to be heard.
What is an Echo?
An echo is a distinct, audible repetition of a sound caused by the reflection of sound waves from a surface back to the source.
How an Echo is Formed
A sound source produces a wave that travels through a medium (usually air).
The wave encounters a reflecting surface such as a wall, cliff, or building.
Part of the wave is reflected back toward the source.
If the reflected wave reaches the listener after a sufficient delay, it is heard as a separate sound – an echo.
Suggested diagram: Ray diagram showing a sound source, incident wave, reflecting surface, and reflected wave returning to the source.
Conditions Required for an Echo
Condition
Explanation
Reflecting surface
Must be large and smooth enough to reflect a significant portion of the sound energy.
Minimum distance
The surface must be far enough away that the reflected sound arrives at least 0.1 s after the original sound.
Medium continuity
The medium (air) must be continuous; large temperature gradients or wind can affect the path.
Why 0.1 s?
The human ear can distinguish two separate sounds if they are separated by at least 0.1 seconds. This sets the minimum round‑trip distance for an echo.
Given the speed of sound in air \$v \approx 340\ \text{m s}^{-1}\$, the minimum distance \$d_{\text{min}}\$ to a reflecting surface is:
If the time interval between the original sound and the echo is measured as \$t\$, the distance \$d\$ to the reflecting surface is:
\$d = \frac{v \, t}{2}\$
where:
\$v\$ = speed of sound in the medium (≈ 340 m s⁻¹ in air at 15 °C).
\$t\$ = time between the emission of the sound and the reception of the echo.
The factor \$\frac{1}{2}\$ accounts for the round‑trip travel of the wave.
Factors Affecting Echo Intensity
Surface material: Hard, smooth surfaces (e.g., concrete, stone) reflect more sound than soft, porous ones (e.g., curtains, foliage).
Surface size: Larger surfaces intercept more of the incident wave, producing a stronger echo.
Angle of incidence: The reflected wave follows the law of reflection (angle of incidence = angle of reflection). If the angle is too shallow, the reflected wave may miss the source.
Atmospheric conditions: Temperature, humidity, and wind can alter the speed of sound and the direction of the wave.
Examples of Echoes in Everyday Life
Shouting in a canyon and hearing the sound return.
Clapping in a large empty hall or gymnasium.
Sonar systems on ships and submarines use echoes to locate objects underwater.
Common Misconceptions
Echo vs. reverberation: Reverberation is a rapid series of reflections that blend with the original sound, whereas an echo is a single, distinct reflection.
All reflections are echoes: Only reflections that arrive after the 0.1 s threshold are perceived as echoes.
Summary
An echo is the audible reflection of a sound wave from a distant surface, heard as a separate sound when the round‑trip travel time exceeds 0.1 seconds. The distance to the reflecting surface can be calculated using \$d = \frac{v t}{2}\$, and the strength of an echo depends on surface properties, size, angle, and atmospheric conditions.
Practice Questions
A person shouts toward a cliff and hears an echo 0.6 s later. Calculate the distance to the cliff. (Assume \$v = 340\ \text{m s}^{-1}\$.)
Explain why an echo is not heard when shouting at a nearby wall 5 m away.
Describe how temperature affects the speed of sound and consequently the time delay of an echo.