The ear can only detect sound over a limited frequency range. A young person with healthy hearing can hear from about 20 Hz to 20 000 Hz. Sound below 20 Hz is called infrasound and sound above 20 000 Hz is called ultrasound, and neither can normally be heard.
The range is limited because the process of turning sound waves into vibrations depends on solid parts: the eardrum, the ossicles and the structures in the cochlea. These parts have mass and cannot speed up and slow down quickly enough to follow a very high frequency, so they do not vibrate properly at very high frequencies. At very low frequencies they also respond poorly, so the vibrations are not detected.
Hearing changes as people get older. The eardrum and the other parts of the ear become stiffer, and the hair cells in the cochlea that detect vibrations become damaged. As a result the upper limit of hearing falls. An older person may only hear up to 12 000 Hz, or lower, so high-pitched sounds that a teenager hears easily can no longer be heard.