Sound Class 9 Notes Science - Chapter 12

Chapter: 12

What Are Sound ?

  • Sound is a mechanical energy which produces sensation of hearing.
  • Sound wave propagates as compressions & rarefactions in the medium.
  • Sound is produced due to vibration of different objects.
  • Sound waves are longitudinal waves.
  1. Sound needs a material medium to travel.
  2. It cannot travel in vacuum.

Speed Of Sound

Range Of Hearing

Solids Liquids Gases
Maximum Less than solids Less than liquids
In steel speed of sound is approx 5100 m/sec The speed of sound in water is approx 1500 m/sec The speed of sound in air is approx 343 m/sec
  • The audible range of sound for human beings extends from 20 Hz to 20000 Hz.
  • Sound of frequency less than 20 Hz is called infrasonic sound, whereas sound of frequency higher than 20 KHz is called ultrasonic sound.


  • Ultrasounds are high-frequency waves. These waves are able to travel along well-defined paths even in the presence of obstacles.
  • Ultrasounds are able to travel freely in solids and liquids, but in gases, their intensity reduces considerably.
  • Application: Echocardiography, SONAR (Sound Navigation and Ranging)

Reflection Of Sound

When sound travels in a given medium, it strikes the surface of another medium and bounces back in some other direction, this phenomenon is called the reflection of sound. The waves are called the incident and reflected sound waves.


  • Phenomenon of hearing back our own sound is called an echo.
  • It is due to successive reflection of sound waves from the surfaces or obstacles of large size.

Condition for an echo

  • Time gap between the sound must come back to the person after 0.1 second.
  • For above condition, the reflecting surface must be at a minimum distance of 17.2m. It also depends on temperature.