Sound Class 8 Notes - Chapter 13

Sound plays an important role in our life. It is through sound we know that a period in school is over or if someone is approaching you by just listening to the footsteps. Vibrating objects produce sound. Vibration is the to and fro or back and forth movement of an object. Sound needs a medium to travel. Hence, it cannot travel in a vacuum.

Sound In Humans

In human beings, the sound is produced by the larynx. Two vocal cords are stretched are stretched across the larynx in a way that it leaves behind a narrow slit between the two for the passage of air. When the lungs force the air through the slit, it causes the vocal cords to vibrate producing sound. Muscles attached to the vocal cords make the cords either loose or tight. The quality of voice is different depending on whether the vocal cord is held firmly or loosely.

Humans hear sound through the ear. When sound enters the human ear it travels down to the eardrum through a canal. The sound vibrations cause the eardrum to vibrate. The eardrum then sends a signal to the inner brain which then sends a message to the brain.

Frequency, Amplitude and Time-Period of a Sound

Frequency, amplitude and time-period are important properties of sound that help us differentiate between various sounds. Their definitions are given below:

    • Frequency – The number of oscillations per second is known as frequency.
    • Amplitude – The number of molecules displaced by sound refers to the amplitude of a sound wave
    • Time-Period – Time required for a complete cycle of vibration to pass. The time period is the inverse of frequency.

Higher the frequency, higher the pitch and shrillness of voice. The loudness of a sound depends on the amplitude of the sound. Higher the amplitude of sound, louder the sound. Sounds that are pleasing to the ear are known as music while sound that causes discomfort to the ear is known as noise. For a human ear, the range of audible frequencies of sound varies from 20 Hz to 20,000 Hz. Excessive noise leads to sound pollution and can pose health risks for human beings.

Sound Class 8 Extra Questions

  1. The sound from a mosquito is produced when it vibrates its wings at an average rate of 500 vibrations per second. What is the time period of the vibration?
  2. What is the difference between noise and music? Can music become noise Sometimes?
  3. List sources of noise pollution in your surroundings

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Practise This Question

Which among the following is a ductless gland?