Sound Class - 8th CBSE Notes

Class 8 Science: Sound

1. How Sound is Produced?

➥ Sound is a form of energy that is produced by vibrating objects.

➥ When an object vibrates, it creates disturbances in the surrounding medium (air, water, etc.), producing sound waves.

➥ These vibrations travel through a medium and reach our ears, where we perceive them as sound.

2. Characteristics of Sound

➥ Pitch: It determines how high or low a sound is. It depends on the frequency of the vibrating source. Higher frequency means a higher pitch and vice versa.

➥ Loudness: It is related to the amplitude of the vibrations. Larger amplitudes produce louder sounds.

➥ Quality (Timbre): It helps us to distinguish between sounds of the same pitch and loudness produced by different sources.

3. The Propagation of Sound

➥ Sound needs a medium (solid, liquid, or gas) to travel. It cannot travel in a vacuum.

➥ The speed of sound varies in different mediums:

• Fastest in solids

• Slower in liquids

• Slowest in gases

➥ Speed of sound in air: 343 m/s at 20°C.

4. Audible and Inaudible Sounds

➥ Audible sounds: Sounds that humans can hear, within the frequency range of 20 Hz to 20,000 Hz.

➥ Inaudible sounds:

• Infrasonic: Below 20 Hz.

• Ultrasonic: Above 20,000 Hz (used by bats and in medical imaging).

5. Reflection and Absorption of Sound

➥ Reflection of Sound: When sound waves hit a hard surface, they bounce back. This phenomenon is called echo.

➥ Echoes are used in sonar systems to measure distances.

➥ Absorption of Sound: Soft materials, like curtains or carpets, absorb sound and reduce noise in a room.

6. Sound Produced by Humans

➥ Human vocal cords produce sound by vibrating as air passes through them.

➥ The length and tension of the vocal cords determine the pitch of the sound.

➥ Men generally have thicker and longer vocal cords, producing deeper voices, while women and children have shorter and thinner vocal cords, producing higher-pitched voices.

7. Musical Sound

➥ Musical sound is pleasant to hear and has a regular waveform.

➥ It is produced by musical instruments and is controlled by:

• Frequency: Determines the pitch.

• Amplitude: Determines the loudness.

• Overtones: Give the sound its unique quality.

8. Noise

➥ Noise is an unpleasant sound with irregular vibrations and waveforms.

➥ Continuous exposure to loud noise can cause:

• Hearing loss

• Stress and other health problems