Most adults are unable to hear high-frequency ring tones because their ability to hear high frequencies lessens as they get older. Some ring tones operate at a frequency that is too high for adults to hear, allowing teenagers to leave them on during classes.
The human ear is capable of hearing sounds that range between 20 and 20,000 hertz. As people age, their ability to hear sounds at the higher end of the range drops. People are able to detect sounds when mechanical changes in the air pass through the ear canal and cause vibrations in the three bones in the inner ear. The resulting vibrations cause fluid in the ear to move, which then stimulates small hairs that send nervous impulses. When the nervous impulses reach the auditory areas of the brain, it interprets them as sounds. People are unable to hear certain sounds, such as the high-frequency whistles dogs can interpret and the low-frequency sounds elephants rely on for communication.
Manufacturers may use the fact that older adults lose their ability to hear high-frequency sound to create ring tones that only younger people can hear. Such ring tones are almost always audible to children and teenagers, as well as some people in their early twenties. Middle-aged adults may struggle to hear them.