Sound — Part 1: Echoes and their uses: visual lesson, ICSE Class 10 Physics (Science Paper 1)

Lesson

Hello… hello… hello!

Part 1 — shout towards a hill or a tall building and the sound comes back. Bats, dolphins, ships and doctors all use this returning sound.

In this lesson you will

Explain an echo and state the conditions for hearing a distinct echo
Justify the minimum distance of about 17 m for an echo in air
Calculate the speed of sound or the distance of a reflector from echo times
Explain how bats, dolphins, fishing boats, doctors and SONAR use echoes

Table of four situations in which sound is reflected
Where	What happens
Valley among hills	Your shout comes back a moment later
Empty hall	Sounds seem to linger
Bat at night	Finds insects by returning squeaks
Ship at sea	Measures the depth below it

Diagram

Reflection of sound and the echo

Sound is reflected from hard, large surfaces such as walls, cliffs and buildings, obeying the same laws of reflection as light. An echo is the sound heard after reflection from a distant obstacle, separately from the original sound. The sound travels to the reflector and back, a distance $2d$ , so $d = \frac{v t}{2}, \qquad v = \frac{2d}{t}$ where $t$ is the time between producing the sound and hearing the echo.

Concept

Conditions for hearing a distinct echo

Time gap: the sensation of a sound lasts in our ear for about 0.1 s (persistence of hearing). The echo must arrive at least 0.1 s after the original sound to be heard separately.
Minimum distance: in 0.1 s sound in air (about 340 m/s) travels $340 \times 0.1 = 34$ m, there and back, so the reflector must be at least 17 m away.
Size of reflector: the reflecting surface must be large compared with the wavelength of the sound.
Loudness: the sound must be loud enough for the echo to be heard.

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