Solutions-Class 7-Science-Chapter-18-Sound: Production of Sound-Maharashtra Board

(a) Sound is generated by the rhythmic vibrations 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.

• While the train runs on the rail, its wheels bang on the rail and friction occurs.
• Banging and friction produce vibrations in the rail and sound is produced.
• These vibrations propagate to a large distance through a solid medium such as rail than air. That means, the sound travels faster over a large distance through rail (solid medium) than air
• 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.

• The stretched diaphram of a tabla when strucked produces vibrations which generates the sound.
• The stretched string of a sitar when plucked produces vibrations which generates the sound.
• 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.
 

We know that

• Medium is necessary for the propagation of sound. Sound cannot travel through vacuum.
• There is no atmosphere on the moon as that of the earth.
• Due to absence of medium on moon, sound does not propagate while talking on the moon.

This is the reason, why your friend will not be able to hear your call.

• 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.
• 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.

Vibrations are produced in an object when it is struck. 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.

The intensity of sound depends upon :

• Amplitude of vibrations : The intensity of sound is proportional to the square of the amplitude of vibration.
• Frequency of vibrations : If frequency is higher, intensity of sound is also higher.
• 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.

• Frequency of oscillation of a pendulum decreases with increase in the length of the pendulum and vice-versa. That means, as the length of a pendulum increases, the number of oscillations in one second decreases.
• It is independent of the amplitude of oscillation of pendulum provided the amplitude is small. That means, even if the amplitude is increased, the frequency does not change much, it remains nearly the same.

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.
• Length of string: 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 ways, the pitch of the sound generated by a stretched wire can be changed.