Question

Sound of frequency below 20Hz is called
A. Audio Sounds
B. Infrasonic
C. Ultrasonic
D. Supersonic

Answer 1

Hint: The audible frequency range humans are used to is from 20 Hz to 20000 Hz. So this is called the audible range of sound.

Complete step by step answer:
The range of frequencies for audio signals ranges from infrasonic, audible to ultrasonic frequency range. A sound is a vibration that propagates as an acoustic wave in different kinds of media ranging from air, water or gas. It always needs a material medium for its propagation. Sounds are produced by a source which vibrates as a result producing waves in the material medium. The waves are transferred through the material medium. The particles of the material medium vibrate about their mean position in order to transfer the waves from the source to a particular point in the medium. We classify sounds on the basis of frequency. It is defined as the number of vibrations per unit time. The SI unit for frequency is Hertz (Hz). Usually, the frequency range for sound is split into three main parts,
First one is the infrasonic frequency which is all the frequencies below 20Hz, which is not audible for humans but some animals like elephant whales etc… can hear these frequencies.
Next is the audible frequency range which is audible for humans. The frequency ranges from 20Hz to 20000Hz.
The third one is the frequency range above 20KHz which goes till a few gigahertz known as the ultrasonic frequency range.
So the answer to the question is option (B)-Infrasonic

Note: Sources of infrasound in nature include volcanoes, avalanches, earthquakes and meteorites.
Monitoring of infrasound seems to be one of the best ways for detecting atmospheric nuclear tests, avalanches, storms, earthquakes etc.