Concepts

Class 9
Chapter 12 Class 9 - Sound

## What is Echo?

An echo is sound which is repeated

This is because sound is reflected back

Example-

If we stand on top of mountain and shout our name

We hear the repetition of our sound

This is called echo

## Does all type of surface cause echo?

No

Only Hard Surface cause echo because they reflect sound

Soft surface do not cause echo because they absorb sound

## Why do we hear echo in large hall but not small room

To hear a distinct echo the time interval between original and reflected sound must be atleast 0.1 seconds.

In a small room

Sound reflects back very fast, so we are not able to hear our voice being repeated

But in large room

Sound takes time to strike and reflect back, so we are clearly able to hear our voice echo

Note:

Echoes may be heard more than once due to successive or multiple reflections.

Example- Rolling of thunder is due to successive of sound from multiple surfaces like clouds and land.

Note - The sensation of sound persists in our brain for about 0.1 seconds. That is why we need atleast 0.1 seconds to hear a distinct echo

## What is Minimum Distance Required to hear an echo?

17.2 Meters

The surface which reflects sound should be atleast 17.2 meters from us, only then we are able to hear an echo

Calculation

We know that Speed of Sound is 344 m/s

Also human ear can hear 2 voices clearly if 2 sound have a gap of at least 0.1 seconds

We know that

Speed = Distance/Time

Speed × Time = Distance

Distance = Speed × Time

Distance = 344 × 0.1

Distance = 34.4 meters

So Distance travelled by Sound wave to reach surface and come back = 34.4 meters

Distance one side (from us to surface) =34.4/2 = 17.2 meters

## Questions

Question 1 Page 168 - An echo is heard in 3 s. What is the distance of the reflecting surface from the source, given that the speed of sound is 342 m s –1 ?

Example 12.2 - A person clapped his hands near a cliff and heard the echo after 2 s. What is the distance of the cliff from the person if the speed of the sound, v is taken as 346 m s –1 ?

NCERT Question 11 - When a sound is reflected from a distant object, an echo is produced. Let the distance between the reflecting surface and the source of sound production remains the same. Do you hear echo sound on a hotter day?