Question

Define : pitch, audible sound, infrasonic sound.

Answer 1

shrillness is explained using a term pitch.
sound which we can hear are called audible sound
sound waves of frequencies below 20 Hz are called infrasonic waves 

Answer 2

Pitch: the pitch of the sound depends on its frequency.........audible sound: not all sounds produced by vibrating bodies is audible. The human ear can hear or recognize the sounds of frequencies in the range of 20hertz. The range frequency of sound is called audible sound.......infrasonic sonunds: sounds of frequencies lesser than 20hz are infrasonic sounds........ultrasonic sounds: sounds more than frequencies of 20hertz.