Frequencies above 20,000 Hz are called ultrasounds or ultrasonics. When we speak, our vocal chords vibrate to emit sound waves. These travel through air. These sound vibrations are also transmitted through the structure of our body. In the human ear, there is thin membrane called the eardrum.
Sound waves travel down a canal in the ear and reach the eardrum.
The eardrum vibrates with the external stimulus like the sound waves. The inner ear has structures that can amplify the sound waves. Thus an acoustic signal frequency is made into mechanical oscillation and is amplified in the inner ear. The inner ear has receptors which send the auditory sensation to our brains. The auditory area of the brain receives all of these signals and processes them into recognizable patterns.