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