NCERT Solutions for Class 9th Science
Chapter 12. Sound
- Sound is produced by Vibrating objects.
- The matter or substance through which sound is transmitted is called a medium.
- In Solid Medium Sound transmits fast.
- School bell is made of metal. When a wooden hammer is stroked on it the metal produces vibrations, which causes sound.
- A sound passes through a medium, the medium shows its physical motion and vibrates. So the sound waves are called the mechanical waves.
- Frequency – The number of oscillations completed by sound wave in per unit time is known as frequency of the sound wave. S.I Unit of frequency is Hertz (Hz).
- Amplitude – The magnitude of the maximum disturbance in the medium on either side of the mean value is called the amplitude of the wave. It is usually denoted by the letter A.
- Speed – The speed of sound is defined as the distance which a point on a wave, such as a compression or rarefaction, travels per unit time.
- When an object vibrates, it sets the particles of the medium around it vibrating. The particles do not travel all the way from the vibrating object to the ear. The particles in the medium in contact with the vibrating object displace from its equilibrium position. It then exerts force on the adjacent particles. After displacing the adjacent particle the first particle of medium comes back in its original position. This process continues in the medium till the sound reaches your ear.
- When a vibrating object moves forward, it pushes and compresses the air in front of it creating a region of high pressure , this region is called a compression.
- When a vibrating object moves backwards. It creates a region of low pressure is called rarefaction.
- He region of high pressure is called compression and the region of low pressure is called rarefaction.
- Sound propagates in the medium as a series of compressions and rarefactions.
- The individual particles of the medium moves in a direction parallel to the direction of propagation of the disturbance. The particles do not move from one place to another but they simply oscillate back and forth their position of rest. This is exactly how a sound wave propagates; hence sound waves are longitudinal waves.
- The wave particles do not oscillate along the line of wave propagation but oscillate up and down about their mean position as the wave travels, such the wave is called transverse wave.
- The loudness or softness of a sound is determined basically by its amplitude. The amplitude of the sound wave depends upon the force with which an object is made to vibrate.
- most common medium through which sound travels is Air.
- Soft sound has small amplitude and louder sound has large amplitude. Soft sound is produced by less energy while louder sound is produced by more energy.
- Tone is known as sound of single frequency.
- Note is known as sound of a mixture of several frequencies.
- The time taken by a pendulum to complete one oscillation is called its time period.
- The distance between two consecutive compressions or two consecutive rarefaction is called the wavelength.
- How the brain interprets the frequency of an emitted sound is called the pitch.
- The different between transverse wave and longitudinal wave.
1.Particles of the medium moves in a direction parallel to the direction of propagation of the disturbance.
2.It forms compression and rarefaction.
3.Example – light wave.
1.Particles do not oscillate along the line of wave propagation but oscillate up and down about their mean position as the wave travels
2.It forms crest and trough.
3.Example – sound wave.