## 04 December 2011

### IX Physics Sound NCERT Solutions

9th Physics for term-II Sound NCERT Solutions

Index Questions Solution and Exercise

Page 162:  How does the sound produced by a vibrating object in a medium reach your ear?

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

Page 163:  Explain how sound is produced by your school bell.

Ans: 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 creates a series of compressions and rarefactions.
This produce the sound of a bell

Why sound waves are called mechanical waves?

Sound waves need the material medium to vibrate and propagate. Hence, these waves are known as
mechanical waves. Sound waves propagate through a medium because of the interaction of the
particles present in that medium.

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

Sound needs a medium to propagate. Since there is no material medium on the moon due to absence
of atmosphere, you cannot hear any sound on the moon.

Page 166 : Which wave property determines (a) loudness, (b) pitch?

(a) Amplitude of sound wave determines the loudness of a sound. The amplitude of a sound directly
proportional to amplitude of wave. If the amplitude of a sound is large, then the sound produced will
also be loud.

(b) Frequency of sound wave determines the pitch of a sound . The pitch of a sound is proportional to  its frequency. High pitched sounds have high frequency

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

The frequency of the vibration of a sound produced by a guitar is greater than that produced by a car
horn. As we know that the pitch of a sound is directly proportional to its frequency, the guitar has a
higher pitch than a car horn.

Page 166 : What are wavelength, frequency, time period and amplitude of a sound wave?

Wavelength: The distance between two consecutive compressions or two consecutive rarefactions is
known as the wavelength. Its SI unit is metre (m).

Frequency: The number of oscillations produced by vibrating body in one second is known as the
frequency of a sound wave. It is measured in hertz (Hz).

Amplitude: The maximum height reached by the crest or trough of a sound wave is called its
amplitude.

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

Speed of sound wave is product of wavelength and frequency of a sound wave

Speed ( v) = Wavelength × Frequency = ν λ

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

Frequency of the sound wave, v = 220 Hz Speed of the sound wave, v = 440 m/ s

Speed = Wavelength × Frequency

Wavelength of a sound wave = Speed of the sound wave/ Frequency of the sound wave

= 440 m/s ÷ 220 Hz = 2m

Hence, the wavelength of the sound wave is 2 m.

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?

You know that the time taken to complete two successive compressions or two consecutive
rarefactions is called the time period of the wave.

Time period of the wave = 1/Frequency Hence, time period is reciprocal of the frequency of the wave

Now, Time period of the wave=1/500=0.002 second

Page 166 : Distinguish between loudness and intensity of sound.

a) Intensity of sound waves is defined as the average energy transported per second per unit area
perpendicular to the direction of propagation.

b) It is measured in Js-1m-2 or Wm-2.

c) The intensity of sound in air depends on the square of the frequency and the square of the
amplitude.

a) The loudness of sound is defined as the degree of sensation of sound produced in the ear

b) The loudness of a sound depends on its amplitude.

c) The amplitude of a sound decides its intensity, which in turn is perceived by the ear as loudness.

Page 167 : In which of the three media, air, water or iron, does sound travel the fastest at a particular
temperature?

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.

Page 168 : 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 ?

Speed of sound, v = 342 m/ s Echo returns in time, t = 3 s

Distance travelled by sound = v × t = 342 × 3 = 1026 m

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 actual distance of the reflecting surface from the source =1026/2=513m

Page 169 : Why are the ceilings of concert halls curved?

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

Class 09 Sound solved Numerical Test Paper-1
Class 09 Sound solved CBSE Test Paper-2
Class 09 Sound solved CBSE Test Paper-3
Class 09 Sound solved CBSE Test Paper-4