NCERT Class 9 Science Solutions: Chapter 12 – Sound Part 3

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Q 1 Image Showing Sound Travels.

Q 1 Image Showing Sound Travels

Question 1:

Q. In which of the three media, air water or iron, does sound travel the fastest at a particular temperature?

Answer:

The speed of sound depends on the nature of the medium. Sound travels the fastest in solids. Its speed decreases in liquids and it is the slowest in gases.

Therefore, for a given temperature, sound travels fastest in iron.

Question 1:

Q. A echo returned in 3 s. What is the distance of the reflecting surface from the source, given that the speed of sound is 342?

Answer:

Speed of sound,

Echo returns in time,

Distance travelled by sound

In the given time interval, sound has to travel a distance that is twice the distance of the reflecting surface and the source.

Hence, the distance of the reflecting surface from the source

Question 1:

Q. Why are the ceilings of concert halls curved?

Answer:

Ceilings of concert halls are curved so that sound after reflection (from the walls) spreads uniformly in all directions.

Question 1:

Q. What is the audible range of the average human ear?

Answer:

The audible range of an average human ear lies between 20 Hz to 20,000 Hz. Humans cannot hear sounds having frequency less than 20 Hz and greater than 20,000 Hz.

Question 2:

Q. What is the range of frequencies associated with (a) Infrasound?

Answer:

(a) Infrasound has frequencies less than 20 Hz.

Q. What is the range of frequencies associated with (b) Ultrasound?

Answer:

(b) Ultrasound has frequencies more than 20,000 Hz.

Question 1:

Q. A submarine emits a sonar, which returns from an underwater cliff in 1.02 s. If the speed of sound in salt water is 1531 m/s, how far away is the cliff?

Answer:

Time taken by the sonar pulse to return,

Speed of sound in salt water,

Distance of the cliff from the submarine Speed of sound Time taken

Distance of the cliff from the submarine

Distance travelled by the sonar pulse during its transmission and reception in water Actual distance

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