# NCERT Solutions for Class 9 Science Unit 12

## Sound Class 9

### Unit 12 Sound Exercise Solutions

Sound is a type of energy. Sound travels in the form of wave from one place to another.
Sound is produced because of the vibration of any object. In other words, when an object is vibrated it produces sound.
When a stone is dropped in pond water, it produces many circular wrinkles and water appears to be travelling outwards from centre. These circular wrinkles are called ripples. This happens because of formation of waves in water.
When a stone is dropped in water, it creates a disturbance in water particles. The water particles pass the disturbance to the adjacent particles and the adjacent particles pass the disturbance to the next adjacent particles and so on. This phenomenon continues and the disturbance moves outward from the centre of disturbance. This makes water appear to be moving. In fact, water molecules do not travel in this case, rather only the disturbance is passed through particles of water. This creates wave in the form of ripples in water.

### Question : Solutions of Questions on Page Number : 162

Q1 :

How does the sound produced by a vibrating object in a medium reach your ear?

When an object vibrates, it forces the neighbouring particles of the medium to vibrate. These vibrating particles then force the particles adjacent to them to vibrate. In this way, vibrations produced by an object are transferred from one particle to another till it reaches the ear.

Q2 :

Explain how sound is produced by your school bell.

When the school bell vibrates, it forces the adjacent particles in air to vibrate. This disturbance gives rise to a wave and when the bell moves forward, it pushes the air in front of it. This creates a region of high pressures known as compression. When the bell moves backwards, it creates a region of low pressure know as rarefaction. As the bell continues to move forward and backward, it produces a series of compressions and rarefactions. This makes the sound of a bell propagate through air.

Q3 :

Why are sound waves called mechanical waves?

Q4 :

Suppose you and your friend are on the moon. Will you be able to hear any sound produced by your friend?

Q5 :

Which wave property determines (a) loudness, (b) pitch?

Q6 :

What are wavelength, frequency, time period and amplitude of a sound wave?

Q7 :

Distinguish between loudness and intensity of sound.

Q8 :

Guess which sound has a higher pitch: guitar or car horn?

Q9 :

How are the wavelength and frequency of a sound wave related to its speed?

Q10 :

Calculate the wavelength of a sound wave whose frequency is 220 Hz and speed is 440 m/s in a given medium.

Q11 :

A person is listening to a tone of 500 Hz sitting at a distance of 450 m from the source of the sound. What is the time interval between successive compressions from the source?

Q12 :

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

Q13 :

An echo returned 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?

Q14 :

Why are the ceilings of concert halls curved?

Q15 :

What is the audible range of the average human ear?

Q16 :

What is the range of frequencies associated with

(a) Infrasound?

(b) Ultrasound?

Q17 :

A submarine emits a sonar pulse, 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?

### Exercise : Solutions of Questions on Page Number : 174

Q1 :

What is sound and how is it produced?

Q2 :

Describe with the help of a diagram, how compressions and rarefactions are produced in air near a source of sound.

Q3 :

Cite an experiment to show that sound needs a material medium for its propagation.

Q4 :

Why is sound wave called a longitudinal wave?

Q5 :

Which characteristics of the sound helps you to identify your friend by his voice while sitting with others in a dark room?

Q6 :

Flash and thunder are produced simultaneously. But thunder is heard a few seconds after the flash is seen, why?

Q7 :

A person has a hearing range from 20 Hz to 20 kHz. What are the typical wavelengths of sound waves in air corresponding to these two frequencies? Take the speed of sound in air as 344 m s - '1.

Q8 :

Two children are at opposite ends of an aluminium rod. One strikes the end of the rod with a stone. Find the ratio of times taken by the sound wave in air and in aluminium to reach the second child.

Q9 :

The frequency of a source of sound is 100 Hz. How many times does it vibrate in a minute?

Q10 :

Does sound follow the same laws of reflection as light does? Explain.

Q11 :

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?

Q12 :

Give two practical applications of reflection of sound waves.

Q13 :

A stone is dropped from the top of a tower 500 m high into a pond of water at the base of the tower. When is the splash heard at the top? Given, g = 10 m s - '2and speed of sound = 340 m s - '1.

Q14 :

A sound wave travels at a speed of 339 m s - '1. If its wavelength is 1.5 cm, what is the frequency of the wave? Will it be audible?

Q15 :

What is reverberation? How can it be reduced?

Q16 :

What is loudness of sound? What factors does it depend on?

Q17 :

Explain how bats use ultrasound to catch a prey.

Q18 :

How is ultrasound used for cleaning?

Q19 :

Explain the working and application of a sonar.

Q20 :

A sonar device on a submarine sends out a signal and receives an echo 5 s later. Calculate the speed of sound in water if the distance of the object from the submarine is 3625 m.

Q21 :

Explain how defects in a metal block can be detected using ultrasound.

Q22 :

Explain how the human ear works.