Case Study Questions for Class 9 Science Chapter 12 Sound

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Case Study/Passage Based Questions:

Question 1:

Read the following and answer any four questions from (i) to (iv) given below :

Sound bounces off a solid or a liquid like a rubber ball bounce off a wall. Like the light, sound gets reflected at the surface of a solid or liquid and follows the same laws of reflection.

The directions in which the sound is incident and is reflected make equal angles with the normal to the reflecting surface at the point of incidence, and the three are in the same plane. If we clap near a suitable reflecting object such as a tall building or a mountain, we will hear the same sound again a little later. This sound that we hear is called an echo.

The sensation of sound persists in our brain for about 0.1 s. To hear a distinct echo the time interval between the original sound and the reflected one must be at least 0.1s. Hence, for hearing distinct echoes, the minimum distance of the obstacle from the source of sound must be 17.2 m. This distance will change with the temperature of air. Another phenomenon of reflection of sound is reverberation.

A sound created in a big hall will persist by repeated reflection from the walls until it is reduced to a value where it is no longer audible. The repeated reflection that results in this persistence of sound is called reverberation. Excessive reverberation is highly undesirable.

(i) Which of the following is true related to the reflection of sound?
(a) Directions in which the sound is incident and is reflected make equal angles with the normal to the reflecting surface at the point of incidence.
(b) Incident wave reflected wave and normal lies in the same plane
(c) Both a and b are true.
(d) None of these

(ii) For hearing distinct echoes, the minimum distance of the obstacle from the source of sound must be
(a) 10m
(b) 20m
(c) 17.2m
(d) None of these

(iii) Excessive reverberation is
(a) Desirable phenomenon
(b) Undesirable phenomenon
(c) Does not exist
(d) None of these

(iv) A person makes a sound near an obstacle and heard the echo after 1 s. What is the distance of the obstacle from the person if the speed of the sound, v is taken as 346 m/s?

Time taken for hearing the echo, t = 1 s
Distance travelled by the sound= v × t = 346 m/s × 1 s = 346 m
In 1 s sound has to travel twice the distance between the obstacle and the person. Hence, the distance between the obstacle and the person= 346/2 = 173 m.

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