Ian Anderson

Topic: Sound

Why do we hear echoes?

Hey. Hold On.

Dont go into that room.

You dont want to listen to me.

Ok. Then go ahead.

See, I warned you.

Will you listen to me now?

Dont worry.

The sound you heard was just an echo of your voice.

Let me explain what an echo is.

When we speak or laugh in a big empty hall, we hear our own sound repeatedly.

This is because our sound waves get reflected from the walls of the hall back to us.

The reflected sound that we hear is called an echo.

Hence, an echo is defined as the repetition of sound caused by the reflection of sound waves from a hard surface back to the listener.

Hey. What are you doing?

Are you trying to hear an echo?

It is not that easy.

There are 2 ideal conditions for an echo to be heard.

When we speak or laugh, we hear our original sound at that moment.

The sensation of this original sound remains in our brain till 0.1 seconds.

This time is called the persistence of hearing.

When we utter sounds, some of our sound waves get reflected while some get absorbed.

If the reflected sound waves reach our ears before the completion of 0.1 seconds, then our brain does not perceive the original and reflected sounds as separate sounds.

They are interpreted as one sound

Therefore, in order to hear two distinct sounds or an echo, the time gap between the original sound and the reflected sound or echo should be at least 0.1 seconds.

This is the first condition for an echo to be heard.

Do you know when the time gap will be more than 0.1 seconds?

This gets us to the second condition for an echo.

The minimum distance between the speaker and the reflecting surface should be at least 17.2 meters.

When the distance is 17.2 meters and we start speaking, assume that the original sound reaches our ears at this time and the reflected sound reaches our ears at this time, then the time interval between the reflected sound and the original sound will be equal to 0.1 seconds.

As a result, we will be able to hear an echo.

Alright.

Now, let us try and hear an echo in this room?

It is even bigger than the previous room.

Now, why cant we hear an echo?

This is because there are many soft furnishings like sofas, carpets, curtains, etc., in this room.

They absorb most of the sound waves.

As the sound waves do not get reflected, we are not able to hear an echo in this room.

Hence, one more thing to consider, if we want to hear an echo, is that there should be minimal use of such sound absorbing materials.