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Science Class 9 Notes – Sound
1. Sound : Sound is a form of energy which produces a sensation of hearing in our ears.
2. Source of sound and its propagation : A source of vibration motion of an object is
normally a source of sound.
3. Characteristics of the medium required for the propagation of sound:
(i) Medium must be elastic so that the medium particles have the tendency to return back to
their original positions after the displacement.
(ii) Medium must have the inertia so that its particles have the capacity to store the energy.
The frictional resistance of the medium should be negligible to minimise the loss of energy in
propagation.
4. Types of waves
(i) Mechanical waves : A mechanical wave is a periodic disturbance which requires a material
medium for its propagation. On the basis of motion of particles the mechanical waves are
classified into two parts.
(a) Transverse wave
(b) Longitudinal wave
(a) Transverse wave : When the particles of the medium vibrate in a direction perpendicular
to the direction of propagation of the wave, the wave is known as the transverse wave. For
example, waves produced in a stretched string.
(b) Longitudinal wave : When the particles of the medium vibrate along the direction of
propagation of the wave then the wave is known as the longitudinal wave. For example sound
wave in air.
(ii) Electromagnetic waves : The waves which do not require medium for propagation are
called electromagnetic waves these waves can travel through vacuum also. For example, light
waves, X-rays.
5. Characteristics of a sound wave
Frequency : The number of vibrations per second is called frequency.
The unit of frequency is hertz
(ii) Amplitude: The maximum displacement of each particle from its mean position is called
amplitude.
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Science Class 9 Notes – Sound
1. Sound : Sound is a form of energy which produces a sensation of hearing in our ears.
2. Source of sound and its propagation : A source of vibration motion of an object is
normally a source of sound.
3. Characteristics of the medium required for the propagation of sound:
(i) Medium must be elastic so that the medium particles have the tendency to return back to
their original positions after the displacement.
(ii) Medium must have the inertia so that its particles have the capacity to store the energy.
The frictional resistance of the medium should be negligible to minimise the loss of energy in
propagation.
4. Types of waves
(i) Mechanical waves : A mechanical wave is a periodic disturbance which requires a material
medium for its propagation. On the basis of motion of particles the mechanical waves are
classified into two parts.
(a) Transverse wave
(b) Longitudinal wave
(a) Transverse wave : When the particles of the medium vibrate in a direction perpendicular
to the direction of propagation of the wave, the wave is known as the transverse wave. For
example, waves produced in a stretched string.
(b) Longitudinal wave : When the particles of the medium vibrate along the direction of
propagation of the wave then the wave is known as the longitudinal wave. For example sound
wave in air.
(ii) Electromagnetic waves : The waves which do not require medium for propagation are
called electromagnetic waves these waves can travel through vacuum also. For example, light
waves, X-rays.
5. Characteristics of a sound wave
Frequency : The number of vibrations per second is called frequency.
The unit of frequency is hertz
(ii) Amplitude: The maximum displacement of each particle from its mean position is called
amplitude.
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