18. Sound : Production of Sound | Science | Class 7

The blog discusses the Questions and Answers from chapter 18, Sound : Production of Sound, from Science of class 7. Students can use these answers for reference in their studies.

Q1. Fill in the blanks.

1. Sound is generated by the rhythmic vibrations of any object.

2. The frequency of sound is measured in Hertz.

3. If amplitude of sound is decreased, its loudness also decreases.

4. A medium is necessary for propogation of sound.

Q2. Match the pairs.

Ans.

Q3. Give scientific reasons.

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

Ans. People used to listen for the arrival of a distant train by putting their ear to
the rail because :
(a) When a train runs on a railway track its wheels produce vibrations due to friction.
(b) These vibrations produce a sound that can propagate at distant places.
(c) Since a rail is a denser medium the sound can travel to longer distances through
it as compared to through air which is a rarer medium.
(d) Hence sound can be clearly herd if a person puts on his ear to the rail.If you were both on the moon, your friend will not be able to hear your
call.
Ans. If we were both on the moon, my friend will not be able to hear my call, because
:
(a) A medium is necessary for the propogation of sound.
(b) Unlike earth the moon does not have an atmosphere to propogate a sound.
(c) Hence it will not be possible for the sound to propogate on moon, and my friend
will not be able to hear my call.

2. The sounds generated by a tabla and a sitar are different.

Ans. The sounds generated by a tabla and a sitar are different because :
(a) In a table the sound is produced due to the vibration of a stretched diaphram.
(b) In a sitar the sound is produced due to the vibration of stretched string.
(c) Due to this difference in the vibration of the two objects their frequency of vibrations are different and also the pitch of sound are different.
(d) Hence the resulting nature of sound is also different in a table and sitar.

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

Ans. If we were both on the moon, my friend will not be able to hear my call, because :
(a) A medium is necessary for the propogation of sound.
(b) Unlike earth the moon does not have an atmosphere to propagate a sound.
(c) Hence it will not be possible for the sound to propagate on moon, and my friend
will not be able to hear my call.

4. We can hear the movement of a mosquito wings but we cannot hear the movement of our hands.

Ans. We can hear the movement of a mosquito wings but we cannot hear the movement of our hands, because :
(a) A mosquito flaps it wings about 300 to 600 times in a second, i.e. its frequency is 300 Hz to 600 Hz. This falls in the audible range of sound and hence can be heard.
(b) While we humans do not move our hands so fast as the mosquito. Hence the frequency is about less than 20 Hz which is a infrasonic sound and hence not heard.

Q4. Write answers to the following questions.

1. How is sound produced ?

Ans. Sound is generated whenever an object is subjected to vibration. These vibra-
tions travel through some medium and reach our ears and we can hear the sound.
Thus vibration of an object is necessary for generation of sound.
For example: the bell, or the strings or diaphragm of a musical instrument.

2. What does the intensity of sound depend upon ?
Ans. The intensity of sound depends upon a number of factors like :
(a) Amplitude of vibrations : The intensity of vibration is proportional to the square of amplitude of vibration.
(b) Frequency of vibrations : The frequency is directly proportional to the intensity of sound, i.e. if the frequency is higher the intensity of sound is also higher.
(c) Distance from the source of sound : The distance from the source of sound is inversely proportional to the intensity of sound. If the distance between the source of sound and the listener is less, the intensity of sound that is felt is high.

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

Ans. The frequency of oscillation is related to the length of a pendulum and the
amplitude of its oscillation in the following ways :
(a) As the length of the pendulum increases the number of oscillations in one second
or the frequency decreases.
(b) The change in amplitude does not bring about a change in the frequency of the
oscillations i.e. frequency of the oscillations remains unchanged.

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

Ans. The two ways by which the pitch of the sound generated by a stretched string
can be changed are :
(a) By changing the tension in the string: As the tension in the wire is increased, as
the frequency increases a shriller sound is produced. Similarly when the tension
is decreased a less shriller sound is produced.
(b) By changing the length of the wire: When the length of the wire is decreased its frequency increases thereby producing a shriller sound. Similarly when the length is increased a less shriller sound is produced.

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