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CraftExam
medium difficulty6 min

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.

In one line: Echo goes and comes back, so divide by two.

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Understand it

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.

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.

See it worked out

Follow the example line by line, then use the same method on new questions.

Worked example

  1. 1If sound speed is 340 m/s and echo is heard after 2 s, total distance travelled = 340 × 2 = 680 m.
  2. 2Distance of wall = 680/2 = 340 m.

Method you can reuse

  1. 1Identify echo time.
  2. 2Multiply speed by time to get total distance.
  3. 3Divide by 2 to get distance of reflecting surface.
  4. 4State unit clearly.

Visual tip

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

Check yourself

Answer the question in your head before opening the answer.

Try it yourself

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

Think first, then show answer
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.

Common wrong answer

Distance = 340 × 2 = 680 m for a 2 s echo.

Why it loses marks: This is wrong if asked distance to wall, because sound travels both ways.

Don't mix this up

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

Exam tip

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

Reflection of Sound — Echo and Reverberation diagram

Open full screen to read every label clearly.

Write it in the exam

Each point is worth about one mark. Write point 1 for a 1-mark question and keep adding points as the marks go up.

How CBSE asks it

Questions may ask minimum distance for echo, distance of a cliff from echo time, or difference between echo and reverberation.
  1. 1

    Enough for 1 mark

    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

    Add for 2 marks

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

  3. 3

    Add for 3 marks

    An echo is heard separately only if the reflected sound reaches the ear after at least about 0.1 s.

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