Why can't humans hear ultrasonic frequencies?

Humans can't hear ultrasonic frequencies because our ears are not designed to detect sounds above 20,000 Hz.

The human ear is a complex organ that allows us to perceive a wide range of sounds, from the softest whisper to the loudest explosion. However, our hearing range is limited. The average human can hear frequencies from about 20 Hz to 20,000 Hz. This range is known as the 'audible range'. Sounds with frequencies below this range are called 'infrasonic', while those above it are called 'ultrasonic'.

The reason we can't hear ultrasonic frequencies lies in the structure of our ears. The ear is made up of three parts: the outer ear, the middle ear, and the inner ear. Sound waves enter the outer ear and cause the eardrum in the middle ear to vibrate. These vibrations are then transferred to the inner ear, where they are converted into electrical signals that our brain interprets as sound.

The inner ear contains a spiral-shaped organ called the cochlea, which is filled with fluid and lined with tiny hair cells. These hair cells are responsible for converting the vibrations into electrical signals. However, they can only respond to a certain range of frequencies. If a sound's frequency is too high, like an ultrasonic frequency, the hair cells can't detect it, and we can't hear it.

It's worth noting that some animals, such as dogs and bats, can hear ultrasonic frequencies. This is because their ears are structured differently and their hair cells can respond to a wider range of frequencies. For example, bats use ultrasonic frequencies for echolocation, a method of navigation and hunting that involves emitting sounds and listening for the echoes to determine the location and distance of objects.