Echo

A Special Feature Of The Human Ear- Persistence Of Hearing

The human ear has a special feature. This is its property of persistence of hearing. When a given sound reaches the human ear and affects the ear drum, its effect ‘stays on’ in the ear drum for nearly 1/10th, i.e. 0.1 of a second. This helps us to get a ‘feeling of continuity’ while hearing one sound after another.

Echo

Let be a common experience that when we shout into a well or inside an empty hall, or inside a dome, we hear our own sound after a short time. It happens because our sound is reflected from the walls. When sound waves strikes a surface, they return back into the same medium. This phenomenon is called reflection, this is known as echo.

Relation between speed of sound, time of hearing echo and distance of reflecting body :

Let t be the time at which an echo is heard, d is the distance between the source of sound and the reflecting body and v is the speed of sound. The total distance travelled by the sound is 2d. Speed of sound, $large v=frac{2d}{t}$ or $large d=frac{vt}{2}$

The sensation of sound last or presists in our brain for 0.1 s (1 /10 sec) even after the source of sound has stopped vibrating. Taking sound speed (v) to be 344 m/s.

$large d=frac{344times 0.1}{2}=frac{34.4}{2}=17.2 m$

Thus, for hearing distinct echoes, the minimum distance of the obstacle from the source of sound must be 17.2 m from the source.

Conditions for the formation of echoes :

The minimum distance between the source of sound and the reflecting body should be 17.2 meters.

The wavelength of sound should be les than the height of the reflecting body.

The intensity of sound should be sufficient so that it can be heard after reflection.

Types of Echoes:

(i) Instantaneous echo: The echoes of sound of short duration is called instantaneous echo.

(ii) Syllabic echo: The echo of syllables of spoken words is called syllabi echo.

(iii) Successive echo : This echo is heard when sound is produced between two distance parallel rows of tall buildings or hills.

Sample Questions

(More Questions for each concept available in Login)

Question : 1

An echo is heared from a distant cliff after an interval of 2 secs. If the speed of sound in air is 340m/s, calculate the distance of the cliff.
A. 430m	B. 340m	C. 440m	D. None oif these
Right Option : B
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Question : 2

To hear a distinct echo, the minimum distance of a reflecting surface should be:
A. 17 metres	B. 34 meters	C. 68 meters	D. 340 meters
Right Option : A
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Question : 3

A boy standing 70 m from a high wall claps his hands. After how much time is an echo heard ? ( The speed of sound in air is $large 350m/sec$ )
A. 0.2 sec	B. 0.4 sec	C. 4.2 sec	D. 4 sec
Right Option : B
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