Complete the following sentence :
Infrasonic sound is of frequency …………….

Explanation

To complete the sentence "Infrasonic sound is of frequency …………….", we need to understand the concept of infrasonic sound.

1. **Understand the audible range**: The average human can hear sounds in the frequency range of 20 Hz to 20 kHz (20,000 Hz). This range is known as the audible range.

2. **Identify infrasonic sound**: Infrasonic sound refers to sound waves that have frequencies below the lower limit of the audible range. Therefore, infrasonic sounds are those frequencies that are less than 20 Hz.

3. **Complete the sentence**: Based on the information above, we can complete the sentence as follows: "Infrasonic sound is of frequency less than 20 Hz."