Information About 8th Class Physics Sound Chapter

Sound is a form of energy that creates the sensation of hearing. The speed of sound in the air under normal conditions is 344meters. Sound is produced by vibrations so that all vibrating bodies produce sound. When a person speaks, the molecules in the air near the mouth get disturbed due to that the molecules start vibrating, in to-and-fro motion so that sound is heard.

  • Oscillation: The body movement from one extreme position to another and back is known as an oscillation.
  • Amplitude of oscillation: If the displacement of a body is maximum from its mean position then it is known as amplitude of oscillation. For a body oscillating in the air, the amplitude of oscillation gradually decreases because of the air in resistance.
  • Time period: Time period is nothing but the time taken to complete one oscillation is known as time period. Time period is denoted by T. Whenever the amplitude of oscillation is small, the pendulum takes equal intervals of time to complete one oscillation.
  • Frequency of oscillation: The number of oscillations made by an oscillating body in one second is known as frequency of oscillation.
    Frequency, v=1/T
    The unit of frequency oscillation is hertz(Hz)
    1Hz=1 cycle per second
  • Audible and inaudible sounds: The audible range of human hearing sound frequency is between 20Hz to 20000Hz is known as audible range.
    1. Sonic sound is nothing but the sound audible range for a human is 20 to 20000Hz is known as sonic sound.
    2. Ultrasonic sound is nothing but the sound frequencies will be greater than 20000Hz.
    3. Subsonic sound is nothing but the sound frequencies will be lower than 20Hz
  • Technological/Industrial applications of sound: Ultrasonic waves have short wavelengths. From smaller objects these short wavelength sound waves reflect back. Hence, ultrasound can be detected or can see smaller objects( Less than 1cm size ). The ultrasonic waves can’t get scattered.

Uses of ultrasonic waves

  • Ultrasonic waves are used to drive away the rats, cockroaches etc.
  • Ultrasonic waves are used for detection of any deformity in unborn babies.
  • Ultrasonic waves are used even for determining the depth of sea.
  • Ultrasonic waves are used for detecting the person’s icebergs, submarines etc in the sea. This process is known as SONAR(Sound navigation and ranging)
  • Speed of sound: In air the speed of sound is 330m/s, in water the speed of sound is 1500m/s, in steel the speed of sound is 6000m/s. So here, is the formula for speed of sound
    Speed of sound=Distance travelled by the sound
    Time taken
  • Characteristics of sound: Loudness, pitch, quality or tone
    1. Loudness: Loudness of the sound depends on the amplitude of the vibration producing that sound. If there is greater amplitude of vibration, then the sound produced by it is louder.Loudness of sound is measured in decibel(dB)units.
    2. Pitch: The shrillness of sound is called pitch. The pitch of a sound depends upon the frequency, if the frequency of sound is high then the pitch is also high. The stretched membrane of a tabla or mridangam produces sound of higher frequency.
    3. Quality: Quality of sound is also known as tone. We can easily find differences between sounds produced by different sources.
  • Noise: Harsh and loud sounds are called noise. Noise is produced by irregular vibrations. Noise pollution is nothing but the disturbance caused by undesired loud sounds by various sources is known as noise pollution. Generally noise pollution is caused by motors, loudspeakers, trains etc.
  • Effects of noise pollution:
    1. Fatigue
    2. Hearing loss
    3. Extreme emotional behaviour
    4. High blood pressure
  • Ways to reduce noise pollution
    1. Planting more trees
    2. Restricted use of loudspeakers, amplifiers, horns etc
    3. Reducing noise level

Problem: A ship out a sound wave and receives an echo after 1 second. If the speed of sound in water is 1500m/s. Then what is the depth of the sea at that point?

Sol: Given:
Total time taken by the sound wave=1sec
Speed of the sound in the sea water=1500m/s

Then, depth of sea=1xspeed of sound total time taken
Depth of the sea=1x1500m/sx1s=750m


This is the information about the sound chapter of class 8. It’s very important to learn all from which we can gain knowledge and by learning this chapter we can know many things about this sound. It will be very useful for higher studies.

Leave a Comment