The human ear can detect sounds in the range of 20Hz-20KHz. These are called Audible Sounds. Eg: Music, Instruments, Crackers.
Frequencies below 20Hz and greater than 20KHz cannot be heard by the human ear. These are called Inaudible Sounds. Eg: It is believed that bat calls are in the inaudible ultrasonics range. Dogs can hear frequencies greater than 20KHz, so they are used by the police to respond to such whistles. Ultrasound scans produce sounds in this range.