Why are ultrasonic sounds outside the human hearing range?

Ultrasonic sounds are defined as vibrations with frequencies greater than the upper limit of the human audible range, which is approximately 20,000 hertz (20 kilohertz).12

These sounds are outside the human hearing range because the human ear is physiologically limited in its ability to process and perceive high-frequency vibrations. The process of hearing involves converting sound waves into nerve impulses through a series of mechanical transformations: air vibrations move the tympanic membrane and ossicles, which then create vibrations in the fluid of the cochlea. These fluid vibrations stimulate hair cells in the organ of Corti, which finally transmit signals to the brain.32

Human hearing is most sensitive to frequencies between 1,000 and 4,000 hertz. As frequencies increase toward the upper limit of the audible spectrum, the ear becomes less sensitive, and the mechanical and neurological systems are not structured to detect or interpret vibrations beyond the 20,000 hertz threshold. While humans cannot hear these frequencies, many other mammals and insects possess the biological capability to perceive them.231