The longitudinal waves whose frequencies lie below 20 Hz are:

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  1. infrasonic waves
  2. ultrasonic waves
  3. hypersonic waves
  4. audible waves

Answer (Detailed Solution Below)

Option 1 : infrasonic waves
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Concept:

  • Sound waves are longitudinal mechanical waves and frequency can be defined as the number of wave cycles happening in one sec.
  • The types of sound waves whose frequencies are lower than the audible limit of human hearing are called Infrasonic waves.

 

Frequency can be divided into three categories based on their frequency range:

  • Audible sound waves: The frequency range of this wave is 20Hz - 20000Hz. Humans can easily detect these types of waves.
  • Example: Sound produced by Vocal cords.
  • Infrasonic waves: The frequency range of these types of waves is below 20Hz. Humans cannot detect it. 
  • Example: Sound produced by Earthquake, Volcanic eruption and ocean waves, etc.
  • Ultrasonic waves or Ultrasound waves: The sound frequency above 20,000Hz is known as ultrasonic waves. Humans cannot detect it too. 

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