Let me first describe you what is frequency. The number of rotations completed by an object in one second is called frequency. In other words, we can say, reciprocal of time period is also known as frequency The unit of frequency is hertz (Hz).
Do you know in which frequency range Human can hear?
Human beings can hear a sound between the frequency 20 Hz to 20,000 Hz and this is referred to as a Audible sound. Sound with frequency less than 20 Hz is called InfraSonic Sound and the sound with frequency above 20,000 Hz is called UltraSonic Sound.
Infrasonic sounds are generally produced by sources of bigger size such as Earthquakes, Volcanic eruptions etc. Elephants can hear the Infrasonic sound, which is the most unique because a handful of animals have the ability to detect Infrasonic waves.
Ultrasonic waves are produced by a transducer, which can both emit Ultrasonic waves, as well as detect the ultrasound echoes reflected back. Certain creatures such as dog, cat, a bat can hear this kind of sound.