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Reflection of Sound — Echo and Reverberation

Reflection of sound is the bouncing back of sound from a surface; echo is a distinct reflected sound, while reverberation is repeated reflection that prolongs sound.

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Student-friendly explanation

An echo is heard separately only if the reflected sound reaches the ear after at least about 0.1 s. Since sound must travel to the reflecting surface and back, the minimum distance of a reflecting surface in air is about 17 m when speed is taken as 340 m/s. Reverberation occurs in halls when many reflections overlap and make sound persist.

How to write this in exams

  1. 1

    Start with the exact idea

    Reflection of sound is the bouncing back of sound from a surface; echo is a distinct reflected sound, while reverberation is repeated reflection that prolongs sound.

  2. 2

    Then show how to use it

    Identify echo time. Multiply speed by time to get total distance. Divide by 2 to get distance of reflecting surface. State unit clearly.

  3. 3

    Add one concrete example

    A shout near a distant wall may return as an echo, but in a large empty hall repeated reflections may cause reverberation.

  4. 4

    Avoid this incomplete answer

    Wrong answer: distance = 340 × 2 = 680 m for a 2 s echo. This is wrong if asked distance to wall, because sound travels both ways.

Definition

Reflection of sound is the bouncing back of sound from a surface; echo is a distinct reflected sound, while reverberation is repeated reflection that prolongs sound.

Example

A shout near a distant wall may return as an echo, but in a large empty hall repeated reflections may cause reverberation.

Rule to remember

For echo: total distance = speed × time = 2d, so d = vt/2. Minimum time gap for distinct echo is about 0.1 s.

Memory hook

Echo goes and comes back, so divide by two.

Examples and method

Worked example

If sound speed is 340 m/s and echo is heard after 2 s, total distance travelled = 340 × 2 = 680 m. Distance of wall = 680/2 = 340 m.

Method to apply

Identify echo time. Multiply speed by time to get total distance. Divide by 2 to get distance of reflecting surface. State unit clearly.

Diagram support

A ray diagram can show sound travelling from source to wall and reflected sound returning to listener, with distance d each way.

How CBSE asks it

Questions may ask minimum distance for echo, distance of a cliff from echo time, or difference between echo and reverberation.

Avoid common mistakes

Common confusion

Students often use one-way distance instead of round-trip distance in echo numericals.

Common wrong answer

Wrong answer: distance = 340 × 2 = 680 m for a 2 s echo. This is wrong if asked distance to wall, because sound travels both ways.

Exam tip

For echo numericals, always use total distance travelled by sound = 2 × distance from wall.

Quick check

Why is a wall about 17 m away needed to hear a clear echo in air?

A wall about 17 m away is needed because sound must travel to the wall and back, taking about 0.1 s for the echo to be heard separately.

Study the reflection of sound — echo and reverberation diagram carefully

Use the labelled diagram to keep reflection of sound — echo and reverberation clear in short answers and revision.

What this diagram makes clear

This diagram keeps the labels and direction of reflection of sound — echo and reverberation in the right order.

Where this helps in exams

Use this for labelled diagram work and short exam answers on reflection of sound — echo and reverberation.

Revision cue

Revise reflection of sound — echo and reverberation through the labels before writing the answer.

Answer writing and exam use

1-mark answer

Reflection of sound is the bouncing back of sound from a surface; echo is a distinct reflected sound, while reverberation is repeated reflection that prolongs sound.

2-mark answer

Reflection of sound is the bouncing back of sound from a surface; echo is a distinct reflected sound, while reverberation is repeated reflection that prolongs sound. For echo: total distance = speed × time = 2d, so d = vt/2. Minimum time gap for distinct echo is about 0.1 s. A shout near a distant wall may return as an echo, but in a large empty hall repeated reflections may cause reverberation.

3-mark answer

An echo is heard separately only if the reflected sound reaches the ear after at least about 0.1 s. Since sound must travel to the reflecting surface and back, the minimum distance of a reflecting surface in air is about 17 m when speed is taken as 340 m/s. Reverberation occurs in halls when many reflections overlap and make sound persist. For echo: total distance = speed × time = 2d, so d = vt/2. Minimum time gap for distinct echo is about 0.1 s. If sound speed is 340 m/s and echo is heard after 2 s, total distance travelled = 340 × 2 = 680 m. Distance of wall = 680/2 = 340 m. Questions may ask minimum distance for echo, distance of a cliff from echo time, or difference between echo and reverberation. Wrong answer: distance = 340 × 2 = 680 m for a 2 s echo. This is wrong if asked distance to wall, because sound travels both ways.
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