7th Class Science Light NCERT Summary - Sound

NCERT Summary - Sound

Category : 7th Class



  • We hear many sounds around us. For example, the sound of buses and cars running on the street, the chirping of birds on the trees, the rustling of leaves in the breeze, the sound of rain falling on the roof and the sound of thunder in the monsoon.
  • In each case, the sound is produced when the object undergoes a rapid to-and-fro motion. Such motion is called vibration or oscillation. In some cases, the vibrations are easily visible to our naked eyes such as the rustling of tree leaves. But in some cases, they are so small that we feel them with our palms. For example, we can feel the vibrations of a transistor, radio or television by placing our hands on the speaker. If we grab the vibrating tree leaves, their vibrations stop and so does the sound.



  • A vibration is a repeated to-and-fro motion. This motion is also called an oscillation.
  • The distance an object travels from its central position to an extreme position is called the amplitude of the oscillation.


  • When the object goes from one extreme position "B" to the other extreme position "C" and then back to "B", we say that it Mid-position of object completes one oscillation. The rime taken to complete one oscillation is called "time period".
  • The number of oscillations per second is called frequency of the oscillation. Frequency is measured in hertz. If an object makes 10 oscillations in a second, we say that the frequency is 10 hertz (10 Hz).



  • Any vibration or oscillation has an amplitude and a frequency. The amplitude tells us how far the object travels from its central position. The frequency tells us how fast it repeats its position oscillatory motion about its central position.
  • Sound is produced when objects vibrate. Due to vibrations of the object, the molecules of air close to it also start vibrating with the same frequency. The motion of these molecules causes the neighbouring molecules to vibrate similarly and so on. Soon all the air molecules in the vicinity begin to imitate the vibrating object and start oscillating. If we place our ear in the vicinity of vibrating air molecules, we can feel the vibrations as sounds.
  • The loudness of a sound depends on the amplitude of vibration. When the amplitude of vibrating air molecules is large, we say that the sound is loud.
  • The frequency of vibration gives a sound its shrillness or pitch. If the frequency of vibration is high, we say that the sound has a high pitch.
  • Our ears cannot hear a sound if its amplitude is too small, i.e., they do not respond if the frequency of oscillation is less than 20 hertz nor can they hear this sound if its frequency is greater than 20,000 hertz.
    • Sounds of frequency greater than 20,000 hertz are called ultrasonic sounds—ultra means beyond limit and sonic refers to sound.
  • The human voice can produce sounds with a frequency between 60 hertz and 13,000 hertz.
  • Some animals like dogs, leopards, monkeys and deer can hear ultrasonic sounds. Some animals can also produce ultrasonic sounds. The bat, for example, screams at a very high frequency much beyond the limit of our hearing.
  • Ears are the sensory organs that aid us in hearing. Sound from the outside is collected by the outer ear and reaches the eardrum which is situated in the middle ear. When this sound strikes the eardrum, it vibrates tb-and-fro. This vibration causes a delicate set of bones to move. The nerve connected to this region, called the auditory nerve, picks up this motion as a signal and sends it to the brain.

Note: Damage to the eardrum can make a person deaf.

  • Sound needs a medium to be heard. If there is no air between a vibrating object and our ears, we would not hear any sound at all. This means a medium is needed for sound to travel. It can travel through any medium—solids, liquids and gases but not in vacuum.
  • Velocity of Sound: Sound takes some time to travel from a vibrating object to our ears. The speed of sound depends on the medium through which it travels. For example, in dry air the velocity of sound is 330 metres per second (m/s). Its velocity in water is 1500 m/s.


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