8th Class Science Sound Introduction


Category : 8th Class

*        Introduction


CRASH! BANG! We often hearthat sound. These are actually made by vibrating air. The sounds made by musical instruments are also of same nature. The difference between NOISE and MUSIC is that, mDsical sounds are organized into patterns that have pitch and rhythm. Noise is just random, disorganized sounds. Sounds are made and travel in the same way, whether they are musical or noise.

A pleasing sound is called tone. It is produced by air vibration, occurs certain number of times per second. These vibrations are called waves. The sound wave is controlled, so that the loudness, quality of the tone, and duration of the plays is controlled. Most musical instruments have string, or some other devices that create sound waves, when vibrated. Different sounds are produced because of harmonic. The higher and lower sounds are mixed in to produce a tone of different pitch. The number of times that a sound wave vibrates per second, is called frequency. The frequency of the sound is measured in cycles and called hertz. High notes have a higher frequency and the lower notes have lower frequency. The image given below shows the 'look' of sound waves:



From above, we observe that even if the pitch and volume changes, the shape of the sound wave remains the same.

Some instruments produce sound when blown into, such as, the brass instruments. The vibration of the player's lips make the column of airto vibrate; whereas the woodwinds have reeds, which vibrate when the player blows on it, making the column of airto vibrate. The sound is produced in the flute when the player blows across the mouthpiece, causing vibration of the column. The image below shows the 'look' of the sound waves of different instruments:




*           Let us See the Following Activities

(1) Wrap some stiff tissue paper around a comb. Hum on the paper and feel the vibration as it shivers.

(2) Beat an empty metal tin with a spoon and feel its vibration as it makes a sound.

(3) Fill an empty glass jar with water upto the brim. Take a pencil and gently tap it near the rim. Also fill other jar with different amounts of water and hear the sound in both case. You will find that the sound produced in both the case is different. Thus, we can say that more the water in the jar, lower is the pitch, because the vibrations are slower. On the other hand, less the water in the jar, higher is the pitch, because the vibration is more.

We can, therefore, define sound as the form of energy that produces the sensation of hearing.





        There are different types of sound, some are pleasing and some are irritating. The different sounds are given different names. What do we call a sound that is pleasing to us?

(a) Tone

(b) Loudness     

(c) Pitch

(d) Node

(e) None of these


Answer: (a)


The sound has two effects; pleasing and disturbing . The pleasing effectof the sound is called tone.



           The sound is produced by the vibration in the medium. The vibration of the sound depends on the nature of medium. The number of times a waves vibrates in one second in a medium is called its:

(a) Amplitude

(b) Crest

(c) Trough

(d) Frequency

(e) None of these


Answer: (d)


The number of times a wave vibrates in one second in a medium is called its frequency.



         We can hear music or some one speaking to us, because of the sound produced in the medium. If we go to the space, we will not be able to hear anything. How is sound produced in the medium?

(a) Movement of particles

(b) Hitting of object

(c) Vibration of particle

(d) Particle at rests

(e) None of these


Answer: (c)


The sound produced in the medium is due to the disturbance produce in the medium by the vibration of the particles in the medium.



       Take four jars with different amount of water. Hit the rim of the jars with the pencil. In which one of the following case, the intensity of sound will be maximum?

(a) The jar filled completely

(b) Jar half filled

(c) Jar filled less than half

(d) Jar filled more than half

(e) None of these


Answer: (c)


More the air column more is the vibration, hence more will be the sound..

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