Std. 7.Sound

Question 1:

Fill in the blanks .

(a) Sound is generated by the rhythmic ............. of an object.

(b) The frequency of sound is measured in ............... .

(c) If ............. of sound is decreased , its loudness also decreases.

(d) A medium is necessary for .................... of sound.

ANSWER:

(a) Sound is generated by the rhythmic vibration of an object.

(b) The frequency of sound is measured in Hertz (Hz).

(c) If amplitude of sound is decreased, its loudness also decreases.

(d) A medium is necessary for propagation of sound.

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Question 2:

Match the pairs

Group 'A'                       Group 'B'

(a) Flute                         (1) Frequency less than 20 Hz

(b) Frequency                (2) Frequency more than 20000 Hz

(c) Sound level              (3) Vibrations in the air 

(d) Ultrasonic sound     (4) Measured in Hz

(e) Infrasonic sound      (5) Decibel

ANSWER:

Group 'A'	Group 'B'
(a) Flute	(3) Vibrations in the air
(b) Frequency	(4) Measured in Hz
(c) Sound level	(5) Decibel
(d) Ultrasonic sound	(2) Frequency more than 20000 Hz
(e) Infrasonic sound	(1) Frequency less than 20 Hz

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Question 3:

Give scientific reasons.

(a)  In earlier times, people used to listen for the arrival of a distant train by putting their ear to the rail.

(b) The sounds generated by a tabla and a sitar are different.

(c) If you were both on the moon , your friend will not be able to hear you call.

(d) We can hear the movement of a mosquito's wings but we cannot hear the movement of our hands.

ANSWER:

(a) In earlier times, people used to listen for the arrival of a distant train by putting their ear to the rail because they knew that sound travels faster through solids than gases. Thus, by putting their ear to the rails used to give them the idea of the arrival time of a train in advance.

(b) Due to difference in the pitch and timbre, the sound produced by a tabla and a sitar are different.

(c) We know that sound cannot travel through vacuum. Since there is no atmosphere on the moon atmosphere, so we cannot hear sound on the Moon. 

(d) The frequency of sound produced by the movement of mosquito's wings is in the audible range of humans whereas that produced by the movement of our hands falls in the inaudible range. Due to this reason, we can hear the movement of a mosquito's wings but cannot hear the movement of our hands.

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Question 4:

Write answers to the following questions .

(a) How is sound produced ?

(b) What does the intensity of sound depend upon ?

(c) Explain how the frequency of oscillation is related to the length of a pendulum and the amplitude of its oscillation.

(d) Explain the two ways by which the pitch of the sound generated by a stretched string can be changed.

ANSWER:

(a) The back and forth movement of an object produces sound. An object moving back and forth is said to be in vibration. Hence, sound is produced by vibrating objects.

(b) Intensity of sound depends on the amplitude of vibration of the sound. It is proportional to the square of amplitude of vibration of the sound.

(c) Frequency of oscillation of a pendulum decreases with increase in the length of the pendulum and vice-versa. It is independent of the amplitude of oscillation of pendulum provided the amplitude is small.

(d) Frequency or pitch of vibration of a stretched string can be changed by varying the

• tension in the string: If the string of the instrument is under high tension, then the pitch of the note produced on plucking the string will be high i.e. the note will be shriller because frequency of vibration of the string will be high.
• thickness of the string: If the string of the instrument is thin, then the pitch of the note produced on plucking the string will be high as frequency of vibration of the string will be high.