What frequency is categorized as infrasonic?

Infrasonic refers to sound waves that fall below the range of human hearing, which is generally recognized as the frequency range from 20 Hz to 20 kHz. Frequencies that are classified as infrasonic are those that are lower than 20 Hz. Therefore, the correct categorization for infrasonic frequencies is any sound wave with a frequency below 20 Hz. This range is often associated with natural phenomena like earthquakes and certain animal communications, which can produce sounds that resonate at low frequencies not detectable by human hearing.

In contrast, the other options include frequency ranges that either include audible sounds or extend above the threshold of 20 Hz, which is why they do not fit the definition of infrasonic.