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## Sunday, December 4, 2011

### CBSE PHYSICS IX Physics Sound NCERT Solutions

Explain how sound is produced by your school bell.
When the bell continues to move forward and backward, it creates a series of compressions and rarefactions. In this  way a bell produce the sound .
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.