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#### Class Nine Science NCERT Solutions Chap 12 Sound

Chapter 12: Sound

Q. 1. How does the sound produced by a vibrating object in a medium reach your ear?

Ans. 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.

INTEXT QUESTIONS (Page No. 163)

Q. 1. Explain how sound is produced by your school bell?

Ans. When the school peon strikes at the middle of the bell with the help of a wooden or metallic rod solid particle of school bell start oscillating to and fro and as a result, the sound is produced.

Q. 2. Why sound waves are called mechanical waves?

Ans. Sound waves require a medium, solid or liquid or gas for their propagation and cannot travel without a medium. The velocity of sound in a medium depends on its mechanical properties. Hence, sound waves are called mechanical waves.

Q. 3. Suppose you and your friends are on the moon. Can you be able to hear any sound produced by your friend?

Ans. No. We shall not be able to hear the sound because sound requires a medium for its propagation. In the moon, there is no atmosphere, i.e. there is a vacuum. Sound cannot travel through a vacuum, it needs a medium for its propagation.

INTEXT QUESTIONS (Page No. 166 [i])

Q. 1. Which does wave property determine (a) loudness, (b) pitch?

Ans. The amplitude of the wave determines the loudness, i.e., more the amplitude of a wave, more is the loudness produced.

The frequency of the wave determines the pitch, i.e., higher the frequency of a wave, more is its pitch and shrill is the sound.

Q. 2. Guess which sound has a higher pitch: guitar or car horn?

Ans. Car horn has a higher pitch than guitar because the sound produced by the former is shriller than later.

INTEXT QUESTIONS (Page No. 166 [ii])

Q. 1. What are the wavelength, frequency, time period and amplitude of a sound wave?

Ans. Wavelength: It is defined as the distance between two consecutive compressions or rarefactions of a sound wave.

Frequency: The number of complete oscillations per unit time is called frequency.

Time period: The time taken by two consecutive compressions or rarefactions to cross a fixed point is called the time period of the sound wave.

Amplitude: The magnitude of the maximum disturbance in the medium on either side of the mean value is called the amplitude of the wave.

Q. 2. How are the wavelength and frequency of a sound wave related to its speed?

Ans. Wave speed = Frequency × wavelength

Q. 3. Calculate the wavelength of a sound wave whose frequency is 220 Hz and speed is 440 ms–1 in a given medium.

Ans.

Q. 3. Calculate the wavelength of a sound wave whose frequency is 220 Hz and speed is 440 ms–1 in a given medium.

Ans

Q. 4. 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?

Ans. The time interval between two successive compressions

INTEXT QUESTIONS (Page No. 166[iii])

Q. 1. Distinguish between loudness and intensity of sound.

Ans. Loudness Intensity

1. Loudness or softness is determined by

the amplitude of the sound.

2. It refers to the degree of sensation produced in the ear.

INTEXT QUESTIONS (Page No. 167)

Q. 1. In which of three media, air, water or iron, does sound travel the fastest at a particular temperature?

Ans.

INTEXT QUESTIONS (Page No. 167)

Q. 1. In which of three media, air, water or iron, does sound travel the fastest at a particular temperature?

Ans. Sound travels fastest through iron with a speed of 5950 ms–1

INTEXT QUESTIONS (Page No. 168)

Q. 1. 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 ms–1?

Ans. Speed sound, v = 342 ms–1

Time is taken for hearing the echo, t = 3 s

Distance travelled by the sound = vt = 342 × 3 = 1026 m

In 3 s, the sound has to travel twice the distance between the reflecting surface and the source.

So Distance of the reflecting surface from the source

INTEXT QUESTIONS (Page No. 169)

Q. 1. Why are the ceilings of concert halls curved?

Ans. The ceilings of concert halls are curved because sound after reflection from it reaches all the corners of the hall and is audible to each person in a hall.

INTEXT QUESTIONS (Page No. 170)

Q. 1. What is the audible range of the average human ear?

Ans. An average human ear can hear sound waves between frequencies 20 Hz to 20,000 Hz.

Q. 2. What is the range of frequencies associated with

(a) Infrasound (b) Ultrasound?

Ans. (a) Infrasound: Sound waves between the frequencies 1 to 20 Hz.

(b) Ultrasound: Sound waves of the frequencies above 20,000 Hz.

INTEXT QUESTIONS (Page No. 172)

Q. 1. 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?

Ans. The time between transmission and detection of sonar pulse.

t = 1.02 s

Speed of sound in saltwater, v = 1531 ms–1

The distance of the cliff = d (say)

Then the distance travelled by sound = 2d

But 2d = speed of sound × time = vt = 1531 × 1.02 m