Q. Sounds of frequencies below 20 Hz are called _____.
Answer: infrasonic sound
Notes: Infrasound, also called infrasonic sound, is sound that is lower in frequency than 20 Hz or cycles per second, the "normal" limit of human hearing. Hearing becomes gradually less sensitive as frequency decreases. Sources of infrasound in nature include volcanoes, avalanches, earthquakes and meteorites.

This question is part of 40000+ General Studies MCQ Series Course on GKToday Android app. Question Number: 79 in Physics GK 16 - Sound Waves in above course in App.