NCERT Class 8 Science Chapter 13 - Sound Notes

Chapter 13 - Sound Notes

1. What is Sound?
→ Sound is a form of energy that enables us to hear and it travels in waves.
→ When matter vibrates or moves back and forth very quickly, a sound is made.
→ Sound waves can travel through solids, liquids or gases.
→ Example: When a school bell rings, parts of bell vibrate creating sound.

2. Sound Waves
→ Sound is a kind of energy that can be heard.
→ A sound is made when things vibrate.
→ Sound travels in waves.
→ Sound must travel through matter to be heard.
→ The vibrating object makes the air around it vibrate.
→ Sound vibrations move through the air into the ears and make the eardrums vibrate.
→ Volume is how loud or soft a sound is.
→ Pitch is how high or low a sound is.

3. Sound travels through matter
→ Solids
• Some sounds that we hear travel through solids.
• Example: When you hit a drum, it vibrates, then the sound travels through the air to your ears.
• Sound travels fastest in solids. This is because the molecules in a solid medium are much closer together than in gases or liquids allowing sound waves to travel more quickly through it.
→ Liquid
• Some sounds that we hear travel through water.
• SONAR is the way to use sound to locate objects under water.
• Few animals such as dolphins, whales etc use SONAR to locate underwater.
→ Gases
•  Most of the sound we hear travels through gases such as air.
• Example: A sound from a bell, a horn from vehicles or alarm clock travels through the air.
• Sound mostly travels slowly in gases because the molecules of a gas are farthest apart.

4. Loudness or Volume
→ Volume is the loudness or the softness of a sound.
→ Loud sound use a lot of energy.
→ Soft sound use a little energy.
→ Example: The harder a drum is hit, more the drum will vibrate. The more an object vibrates, the louder sound it makes.

5. Pitch
→ Pitch is the highest or lowest sound an object makes.
→ Objects that vibrate slowly, make a low pitch. Example: Drum
→ Objects that vibrate quickly, make a high pitch. Example: recorder

6. How does sound travel?
→ Sound passes through medium as longitudinal waves.
→ When the vibrations are fast, you hear a high pitch. When they are slow, you hear a low pitch.

7. How else can we change sound?
→ We can make it louder or softer by changing the amplitude of the height of the wave.
→ The higher the amplitude, the louder the sound. The lower the amplitude, the softer the sound.

8. How do our ears work?
→ Sound waves are sent.
→ The outer ear catches the sound waves.
→ The middle ear takes the sound waves and vibrate the eardrum.
→ The inner ear sends the message to the brain.
→ The brain puts in together and we understand the sound.

9. RADAR and SONAR
→ The full form of RADAR is Radio Detection and Ranging. It is a detection system that use radio waves to determine the distance, angle or velocity of an object. It can be used to detect the aircraft, ships, vehicles, weather, and terrain etc.
→ The full form of SONAR is Sound Navigation and Ranging. It is a technique that is used for determining the distance and direction of underwater objects with the help of sound waves.

10. Vibrations
To and fro motion of movement of object/particle.

11. Oscillations
Movement of object/particle

12. Frequency
Number of times the object vibrates per second

13. Time period
Time taken to complete one full vibration/oscillation

14. Amplitude
Maximum displacement of an object from its mean position

15. Forms of energy
→ Wind
→ Light
→ Sound
→ Heat
→ Chemical
→ Nuclear

16. The frequency is expressed in hertz (Hz). The loudness of a sound is expressed in decibel (dB). The amplitude of a sound is expressed in mm, cm, m, km etc.

17. When we pluck the string of an instrument, like the sitar, the sound that we hear is not only that of the string. The whole instrument is forced to vibrate, and it is the sound of the vibration of the instrument that we hear. Similarly, when we strike the membrane of a mridangam, the sound that we hear is not only that of the membrane but of the whole body of the instrument.

18. The vocal cords in men are about 20 mm long. In women these are about 15 mm long. Children have very short vocal cords. This is the reason why the voices of men, women and children are different.

19. Infrasonic, Ultrasonic and Supersonic sounds
→ Sound of frequency lower than 20 Hz is called the infrasonic sounds.
→ Sounds of frequencies higher than 20000 Hz are called ultrasonic sounds.
→ Supersonic is used for objects which travel at a speed greater than the speed of sound.

20. Three types of instruments
→ String musical instruments: These instruments produce music by stretched string. Example: Guitar, Sitar
→ Wind musical instruments: These instruments produce music by air column. Example: Flute, Shehnai
→ Membrane musical instruments: These instruments are also known as percussion instrument. These instruments produce music by stretched membranes. Example: Tabla, Drum

21. Lightning and thunder take place in the sky at the same time and at the same distance from us but lightning is seen earlier and thunder is heard later because light travels faster than the sound so we see the lightning first and then the sound.

22. Music and Noise
Music is a sound that is pleasant to hear. Example: Sounds from piano, flute etc are pleasant to hear. Noise is a sound that are unpleasant to hear. Example: Sounds from car horns, construction are unpleasant to hear.

23. Sources of noise pollution
Sources of noise pollution in the surroundings are
→ Car horns
→ TV at loud volume
→ Firecracker or Fireworks bursting
→ Transistors

24. How noise pollution is harmful to humans?
Noise pollution is harmful to humans in following ways:
→ It can cause headache.
→ Lack of sleep
→ Hypertension (high blood-pressure)
→ Anxiety
→ It can also cause temporary or permanent impairment of hearing
→ Stress

25. Why should we plant trees?
Plant parts such as stems, leaves, branches, wood etc absorb sound. The barks of healthy tree are particularly effective at absorbing sound due to dynamic surface area and helps in reducing noise pollution.

26. Sound produced by humans
In humans, the sound is produced by the voice box or the larynx. Voice box is at the upper end of the windpipe. Two vocal cords, are stretched across the voice box or larynx in such a way that it leaves a narrow slit between them for the passage of air. When the lungs force air through the slit, the vocal cords vibrate, producing sound. Muscles attached to the vocal cords can make the cords tight or loose. When the vocal cords are tight and thin, the type or quality of voice is different.

27. Sound can’t travel through vacuum
Vacuum is an empty space in which there are no matter or particles present. As sound is a wave, it needs a material medium to propagate. Sound cannot travel through vacuum as there is no particles present for vibrations to take place.


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