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CLASS: VIII :: Sound

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CLASS: VIII NCERT (CBSE)
Physics
sound

SUMMARY

Production and Propagation of sound

Sound is a form of energy that causes the sensation of hearing. Sound needs a medium to travel. Sound travels through gases, liquids and solids. The speed of sound is the maximum in solids, less in liquids and the least in gases. Sound cannot travel through vacuum.

In humans, sound is produced by the voice box or the Larynx. Vocal cords in the larynx vibrate and produce sound. We hear sound with our ears. The ear drum has a vibrating membrane like a stretched rubber sheet. When a sound note reaches the ear, the ear drum vibrates, and thevibrations get converted into signals that are carried to the brain to get a sensation of hearing.

To produce pleasant sounds, a number of musical instruments have been developed. Some instruments produce sound due to the vibration of membranes, some due to the vibration of strings, and some others produce sound due to the vibration of an air column. The to and fro or back and forth motion of an object is called vibration. The sitar, veena, violin, guitar and ektara are some stringed instruments.

The tabla, cymbals, ghatam, kartal and manjira are some instruments that work on the vibration of a membrane, and instruments like the flute and the trumpet produce sound due to thevibration of an air column present in them.

Characteristics of sound

Sound is a form of energy produced by a vibrating body. Sound requires a medium for its propagation. Sound does not propagate in vacuum. Sound is produced due to the vibration of an object.

The to and fro or back and forth motion of an object is called vibration. To produce pleasant sounds, a number of musical instruments have been developed. Some instruments produce sound due to the vibration ofmembranes, some due to the vibration of strings, and some others produce sound due to the vibration of an air column. T

he maximum displacement of a vibrating particle from its mean or equilibrium position is called itsamplitude. The time taken by the vibrating particle for one full vibration or oscillation is called the time periodof vibration. The number of vibrations per second is called the frequency. Frequency is measured hertz (Hz).

Sound produced by any means has the following characteristics, namely, loudness, pitch or shrillness,and quality or timbre.

The loudness of sound depends on its amplitude. The loudness of sound is proportional to the square of the amplitude. A roar of a lion is louder than a woman’s voice. The pitch of sound depends on its frequency. If frequency is more, then the pitch or shrillness is more.

The pitch of a woman’s voice is more and it is shrillerthen a man’s voice. The loudness of sound is measured in Decibel (db). If loudness exceeds 80 db, then the sound becomes physically painful.

Not all sound produced by vibrating bodies is audible. The human ear can only recognise sounds of frequencies in the range of 20 Hz and 20,000 Hz. This range of frequency is called audible sound. Some animals like dogs and snakes can hear sounds of frequencies greater than 20,000 Hz. Sounds of frequencies less than 20 Hz are called infrasonic sounds, while sounds of frequencies greater than 20,000 Hz are called ultrasonic sounds.

Noise and Music

Any intolerable and irritating sound is called noise. The word noise comes from the Latin word nausea, meaning seasickness.

Music refers to any sound that is pleasant to the ear. Sound produced by musical instruments is pleasing to the ear. But if the intensity of the sound exceeds a certain limit, then it becomes intolerable and is noise.
Undesirable sounds and disturbances cause noise pollution.

Noise pollution may cause high blood pressure, panic attacks and lack of sleep among those exposed to it. To reduce noise pollution, trees should be planted along roads and in residential areas, factories should not be set up in residential areas, vehicles should not blow horns around schools, hospitals and residential area, TVs and music systems should not be played at high volume.

Continuous exposure to loud noise may cause temporary or even permanent hearing impairment.
Sign language is used by the hearing disabled to communicate.

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