Question

Which of the following are infrasonic waves?
A. 5KHz
B. 25KHz
C. 10Hz
D. 15000Hz

Answer 1

Hint: The audible sound to human ears has frequency between 20Hz and 20KHz. The sound waves with below 20 Hz frequency are infrasonic waves and the sound waves with above 20Hz frequency are ultrasonic waves.

Complete step-by-step answer:
First, let us understand what sound waves are. Sound waves are the waves produced when the air particles vibrate or we can also say that when there is a disturbance caused in air, a sound wave is formed. For example, when you clap your hands, the process of clapping the hands sets the air molecules into vibrations and this causes the sound waves.
From the above example, you may notice that to produce the sound waves, you have to clap your hands, which means that a source is needed to produce these waves. The vibrations of the source set the neighbouring air molecules into oscillations(vibrations), which set the further molecules into oscillations and in these way the vibrations travel and reach your ear to vibrate your eardrum too and the vibration of the eardrum is sensed by the brain and this is how you hear a sound.
Sound waves are formed by compression and rarefaction of air. One cycle of sound wave means one complete compression and rarefaction of the air particles. This compression and rarefaction are pressure differences in the air.
For waves, we have something known as frequency. Frequency of a wave is the numbers of cycles it completes in one unit of time. The unit of frequency is ${{s}^{-1}}$ or Hertz (Hz). The sound that is audible to the human ear has the frequency range of 20Hz to 20000Hz (20KHz). The sound with frequency below 20Hz is called infrasonic wave and the sound with frequency above 20KHz is called ultrasonic wave.

Hence, the correct option is (C) 10Hz, since it is less than 20Hz.

Note: Humans cannot hear both, ultrasonic and infrasonic waves. However, some animals can sense these waves. They can sense the ultrasonic waves generated by the displacement of plates under the earth’s crust. That is why some animals like cows act weirdly before an earthquake.