18 – Sound – Production of Sound
Q. 1. Fill in the blanks with appropriate words and complete the sentence:
(1) Sound is generated by the rhythmic .....................
of any object.
(2) The frequency of sound is measured in
.................
(3) If ................. of sound is decreased, its
loudness also decreases.
(4) A medium is necessary for .....................
of sound
Ans.
(1) vibrations (2) Hertz (3) amplitude (4) propagation
Q. 2.
Match the columns :
Answer
(1)
Flute - Vibrations in the air
(2)
Frequency - Measured in Hz
(3)
Sound level - Decibel
(4)
Ultrasonic sound - Frequency more than
20000 Hz
(5)
Infrasonic sound - Frequency less than 20 Hz
Q. 3.
Answer the following questions :
(1)
How is sound produced?
Ans.
Vibrations are produced in an object when it is struck. Vibrating object is a
source of sound. The rhythmic vibrations of an object produces sound.
(2)
What does the intensity of sound depend upon?
Ans.
The intensity of sound depends upon :
(1)
Amplitude of vibrations : The intensity of sound is proportional to the square
of the amplitude of vibration.
(2)
Frequency of fibrations : If frequency is higher then intensity of sound is
also higher.
(3)
Distance from the source of sound : If the distance between the listener and
the source of sound is less than intensity of sound can be felt greater.
(3)
Explain how the frequence of oscillation is related to the length of a pendulum
and the amplitude of its oscillation.
Ans.
(1) As the length of a pendulum increases, the frequency of oscillations
decreases That means, as the length of a pendulum increases, the number of
oscillations in one second decreases. (2) The frequency of oscillations is not
affected much by the amplitude of oscillations. That means, even if the
amplitude is increased, the frequency does not change much, it remains nearly
the same.
(4)
Explain the two ways by which the pitch of the sound generated by a stretched
string or wire can be changed.
Ans.
(1) When the tension in the wire is increased, the frequency increases and thus
the sound produced is shriller. Also, when the tension in the wire is
decreased, the frequency decreases and the sound produced is less shrill. (2)
When the length of the wire is decreased, the frequency increases. Thus, the
sound produced is shriller. When the length of the wire is increased, the
frequency decreases and the sound produced is less shrill. Using these two
ways, the pitch of the sound generated by a stretched wire can be changed.
Q. 4.
Give scientific reasons :
(1) In earlier times, people used to listen for the
arrival of a distant train putting their ear to the rail.
Ans. (1)
While the train runs on the rail, its wheels bang on the rail and friction
occurs (2) These banging and friction produce vibrations and sound is produced.
(3) These vibrarations propagate to a large distance through a solid medium
such as rail. That means, the sound travels over a large distance through rail.
(4) If ear is put to the rail this sound can be clearly heard and the arrival
of a train can be guessed. Thus, in earlier times, people used to listen for
the arrival of a distant train by putting their ear to the rail.
(2)
The sounds generated by a tabla and sitar are different.
Ans.
(1) The stretched diaphram of a tabla when strucked produces vibrations which generates
the sound. (2) The stretched string of a sitar when plucked produces vibrations
which generates the sound. (3) In this way, due to the difference in the
vibrating object, the frequency of vibrations from both the objects is
different, and the pitch of sound is also different. Thus, the sounds generated
by a tabla and a sitar is different.
(3) Your friend
will not be able to hear your call, if you were both on the moon.
Ans.
(1) Medium is necessary for the propagation of sound. (2) There is no
atmosphere on the moon as that of the earth and thus due to absence of medium
sound does not propagate while talking on the moon. This is the reason, why
your friend will not be able to hear your call.
(4) We
can hear the movement of a mosquito's wings but we cannot hear the movement of
our hands.
Ans.
(1) The up-down movement of a mosquito's wings occur very fast. In one second,
it's wings move about 300 to 600 times. That means, the frequency of movement
of the wings is 300 Hz to 600 Hz. Thus, audible sound is produced and the
movement of the wings can be heard. (2) On the other hand, we cannot move our
hands so rapidly. The frequency of sound produced by movement of hands is less
than 20 Hz. We cannot hear this infrasonic sound Thus, the movement of our
hands cannot be heard.
