We perceive sound through our sense of hearing, which is sensitive to a range of frequencies typically between 20 Hz and 20,000 Hz. Frequencies below 20 Hz are considered infrasonic, and frequencies above 20,000 Hz are called ultrasonic.
The inability to hear ultrasonic and infrasonic sound waves is primarily due to the limitations of the human auditory system. Our ears consist of three main parts: the outer ear, the middle ear, and the inner ear. The outer ear collects sound waves and directs them into the ear canal, where they eventually reach the middle ear.
In the middle ear, the sound waves cause the eardrum to vibrate. These vibrations are then transmitted through three tiny bones called ossicles: the malleus, incus, and stapes. The stapes bone connects to the oval window, which is a membrane that separates the middle ear from the inner ear.
The inner ear contains the cochlea, which is responsible for converting mechanical vibrations into electrical signals that can be interpreted by the brain. The cochlea is lined with tiny hair cells that are sensitive to specific frequencies. When sound waves enter the cochlea, they cause these hair cells to vibrate, generating electrical signals that are transmitted to the brain through the auditory nerve.
The range of frequencies that our auditory system can perceive is limited by the characteristics of the cochlea and the sensitivity of the hair cells. Hair cells are specialized to respond to specific frequency ranges, and those in the human ear are most sensitive to frequencies between 1,000 Hz and 4,000 Hz.
Ultrasonic sound waves have frequencies above the range of human hearing, typically starting from 20,000 Hz and extending into the megahertz (MHz) range. These frequencies are too high for the hair cells in our cochlea to detect and transmit to the brain.
Infrasonic sound waves, on the other hand, have frequencies below the audible range, typically starting from 20 Hz and extending into the subhertz (mHz) range. The low frequencies of infrasound are also outside the range of sensitivity of our cochlear hair cells, so we are unable to perceive them as sound.
While humans cannot directly perceive ultrasonic and infrasonic sound waves, certain animals, such as bats and dogs, have hearing capabilities that extend into these frequency ranges. Additionally, specialized equipment can be used to detect and measure ultrasonic and infrasonic waves for scientific and technological purposes.