CBSE Class 9 Science Sound Notes

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Sound

Sound is a mechanical wave and needs a material medium like air, water, steel etc. for its propagation.

A medium is the substance or material through which a pulse or a wave moves.

INTEXT QUESTIONS 

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.

LONGITUDINAL AND TRANSVERSE WAVE

A longitudinal wave is a wave where the particles in the medium move parallel to the direction of propagation of the wave.

“If the particles of a medium vibrate in a direction, parallel to or along the direction of propagation of wave, it is called longitudinal wave”

Sound waves are pressure waves caused by objects which are vibrating. Sound waves need a medium through which to travel.

A compression is a region in a longitudinal wave where the particles are closest together. A rarefaction is a region in a longitudinal wave where the particles are furthest apart.

TRANSVERSE WAVE
“If the particles of the medium vibrate in a direction, perpendicular to the direction of propagation, the wave is called transverse wave.” Example: water waves, vibrations of stretched string.

INTEXT QUESTIONS PAGE NO. 163

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

Ans: When the school bell vibrates, it forces the adjacent particles in air to vibrate. This disturbance gives rise to a wave and when the bell moves forward, it pushes the air in front
of it. This creates a region of high pressures known as compression. When the bell moves backwards, it creates a region of low pressure know as rarefaction. As the bell continues to
move forward and backward, it produces a series of compressions and rarefactions. This makes the sound of a bell propagate through air.

2. Why are sound waves called mechanical waves?

Ans: Sound waves force the medium particles to vibrate. Hence, these waves are known as mechanical waves. Sound waves propagate through a medium because of the interaction of
the particles present in that medium.

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

Ans: Sound needs a medium to propagate. Since the moon is devoid of any atmosphere, you cannot hear any sound on the moon.

CHARACTERISTICS OF SOUND
Sound can be distinguished from each other by the following three characteristics:
1. Loudness or Intensity
2. Pitch or Frequency
3. Quality or Timbre
1. Loudness
The sensation produced in the ear, which enables us to distinguish between a loud, and a faint sound is called loudness.
The amplitude of the wave determines the loudness or softness of a sound basically.

Intensity
The intensity of sound is defined as the amount of energy passing per unit time per unit area in a direction perpendicular to the area.

First figure is regular and has a definite amplitude. It represents a musical sound.
A musical sound can be defined as a pleasant continuous and uniform sound produced by regular and periodic vibrations. Example: The pleasant sound produced by a guitar, piano,
tuning fork etc.
The second curve represents a noise. The curve is irregular and has no definite amplitude.
Musical sound is pleasant to hear and is of rich quality. Noise on the other hand is unpleasant to hear.
Noise can be defined as an irregular succession of disturbances, which are discordant and unpleasant to the ear.

TERMS RELATED TO SOUND WAVES
The amplitude A is the maximum excursion of a particle of the medium from the particles undisturbed position.
The wavelength is the horizontal length of one cycle of the wave.
The period is the time required for one complete cycle.

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