Ch 13 covers how sound is produced (vibrations), how it travels (needs a medium), and its characteristics — frequency (pitch), amplitude (loudness). Students also learn about noise pollution.
Sound is produced when objects vibrate. These vibrations travel through a medium (solid, liquid, or gas) as waves. Sound cannot travel through vacuum. Voice box (larynx) produces sound in humans when vocal cords vibrate.
Frequency: number of vibrations per second, measured in Hertz (Hz). Higher frequency = higher pitch (thin wire vs thick wire). Amplitude: the extent of vibration. Higher amplitude = louder sound. Humans can hear 20 Hz to 20,000 Hz (audible range). Music has pleasing patterns; noise is unpleasant.
Sounds above 80 dB are harmful. Sources: loudspeakers, firecrackers, factory machines, vehicles. Effects: hearing loss, irritability, sleep disturbance. Prevention: plant trees (absorb sound), use silencers, reduce honking, use ear protectors in factories.
Download: https://ncert.nic.in/textbook/pdf/hesc113.pdf | Complete book: https://ncert.nic.in/textbook/pdf/hesc1ps.zip
Humans can hear sounds with frequencies between 20 Hz and 20,000 Hz. Sounds below 20 Hz are called infrasonic (elephants use these). Sounds above 20,000 Hz are ultrasonic (bats and dolphins use these for navigation).
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