Chapter 13 Sound Class 8 Notes | EduRev

Science - Short Notes for Class 8th

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Class 8 : Chapter 13 Sound Class 8 Notes | EduRev

 Page 1


Chapter 13 Sound 
Sound – A form of energy produced by vibration of an object, which are transmitted through a medium such as air and 
heard by ear. 
Vibration – Rapid to and fro or back and forth movement of an object or medium. 
? Sound in humans is produced by the larynx or voice box present just above the windpipe. Larynx contain two 
cords with a space for air in between. When the air is forced in the slit between the two cords, they vibrate to 
produce sound. 
? Sound travel through a medium and cannot pass through vacuum. The sound that reaches our ear travels through 
the air. If there is vacuum in our surrounding, we will not be able to hear anything. Sound can pass through 
liquids or wires (as in case of a toy telephone made by attaching two cups or glasses with a string). 
 
? When we hit an object, it starts vibrating and produces sound. The air near the surface of the object starts 
vibrating and these vibrations are passed in all the directions. When these vibrations reach our ear, they make the 
eardrum vibrate. Eardrum is a thin membrane present in our ear that receives sounds and passes the vibrations to 
other parts of the ear. 
Some important terms 
1. Oscillations – One complete to and pro movement of an object to reach the point from where it started is called 
one oscillation. 
2. Frequency – Number of oscillations completed per second. Its unit is sec
-1
 or Hertz (Hz). Humans can hear sound 
waves of 20Hz to 20kHz 
3. Amplitude – the highest distance of the oscillation from its mean position is called amplitude. 
4. Crust – the highest point of a wave or vibration. 
5. Trough – the lowest point of a wave or vibration 
6. Wavelength – the distance between two consecutive crust or trough. 
 
 
 
 
 
 
? The amplitude decides the loudness of the sound. Louder sounds have higher amplitudes. 
? The shrillness of pitch of the sound is decided by its frequency. Shriller voices have higher frequency than heavy 
voices. 
Noise – the unpleasant sounds that cause irritation are called noise. 
Music – sounds that are soothing and can be enjoyed, forms the music. 
Noise pollution 
Presence of excessive amount of noises in the surrounding is known as noise pollution 
Causes of noise pollution: 
1. Honking of vehicles 
2. Television, radio and transmitters run at very high volume. 
3. Sounds produced by motors, coolers, or home appliances. 
Page 2


Chapter 13 Sound 
Sound – A form of energy produced by vibration of an object, which are transmitted through a medium such as air and 
heard by ear. 
Vibration – Rapid to and fro or back and forth movement of an object or medium. 
? Sound in humans is produced by the larynx or voice box present just above the windpipe. Larynx contain two 
cords with a space for air in between. When the air is forced in the slit between the two cords, they vibrate to 
produce sound. 
? Sound travel through a medium and cannot pass through vacuum. The sound that reaches our ear travels through 
the air. If there is vacuum in our surrounding, we will not be able to hear anything. Sound can pass through 
liquids or wires (as in case of a toy telephone made by attaching two cups or glasses with a string). 
 
? When we hit an object, it starts vibrating and produces sound. The air near the surface of the object starts 
vibrating and these vibrations are passed in all the directions. When these vibrations reach our ear, they make the 
eardrum vibrate. Eardrum is a thin membrane present in our ear that receives sounds and passes the vibrations to 
other parts of the ear. 
Some important terms 
1. Oscillations – One complete to and pro movement of an object to reach the point from where it started is called 
one oscillation. 
2. Frequency – Number of oscillations completed per second. Its unit is sec
-1
 or Hertz (Hz). Humans can hear sound 
waves of 20Hz to 20kHz 
3. Amplitude – the highest distance of the oscillation from its mean position is called amplitude. 
4. Crust – the highest point of a wave or vibration. 
5. Trough – the lowest point of a wave or vibration 
6. Wavelength – the distance between two consecutive crust or trough. 
 
 
 
 
 
 
? The amplitude decides the loudness of the sound. Louder sounds have higher amplitudes. 
? The shrillness of pitch of the sound is decided by its frequency. Shriller voices have higher frequency than heavy 
voices. 
Noise – the unpleasant sounds that cause irritation are called noise. 
Music – sounds that are soothing and can be enjoyed, forms the music. 
Noise pollution 
Presence of excessive amount of noises in the surrounding is known as noise pollution 
Causes of noise pollution: 
1. Honking of vehicles 
2. Television, radio and transmitters run at very high volume. 
3. Sounds produced by motors, coolers, or home appliances. 
Effects of Noise pollution 
1. Cause irritation and anxiety. 
2. Cause hypertension (increase in blood pressure) 
3.  Deprives sleep or Insomnia 
4. Hearing impairment 
Prevention 
1. Use of silencers in motor vehicles, industries and aircrafts. 
2. Noisy functions should be conducted away from residential area. 
3. Avoid unnecessary honking. 
4. Run television and radio at low volume. 
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