Multiple choice general knowledge science & technology The frequency which is below the human range is called as ultra sound infra sound ultra sonic infra sonic Reveal answer Fill a bubble to check yourself B Correct answer Explanation Infrasound (or infra sound) refers to sound frequencies below the human hearing range (below 20 Hz). Ultrasound refers to frequencies above the human range (above 20,000 Hz). The prefix 'infra-' means below.