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The human ear is said to have an effective range of 20Hz to 20,000Hz (20kHz). That is too say that the overwhelming majority of people can’t hear any sound whose frequency doesn’t fall between these two extremes. What’s more, most of us adults can’t even hear anything remotely close to 20kHz on the upper end of the range.

Once we hit our teen years, most of us begin to experience hearing loss. By the time we reach our 21st birthdays, we are lucky if we can still hear any sound with a frequency as high as 14kHz. Since most of what we have to deal with is below this level, this amount of hearing loss doesn’t interfere with our daily lives. The human voice, for example, is largely relegated to the area below 3500Hz although certain harmonics in the human voice may go above this range.

When our hearing deteriorates to the point where we can no longer hear above 4000Hz or so, we start to have difficulty in understanding human speech. This is because many consonant sounds, the part of our language that separates and defines words, fall in the upper end of the vocal range. Likewise, we may have trouble hearing higher pitched noises like bells, whistles and some warning alarms as our ability to hear higher frequencies fades with age.

High frequency hearing loss is almost universal in the human population with very few exceptions, however, the degree or amount of loss varies greatly between individuals. Certain environmental factors can accelerate hearing loss or cause permanent damage. Chief among these is exposure to loud noises. Except for extremely loud sounds, our ears are able to tolerate loud noises in small doses without significant damage, but prolonged exposure to noise above 90 decibels can cause serious and irreversible hearing loss. Rock concerts are typically much louder than this and last long enough to cause damage. Repeated lengthy exposures increase the amount of damage, though we may not notice any change until a very large portion of the normal frequency range of our hearing has been lost. By then, it’s usually too late.

On the other hand, modern technology also makes use of the difference between the young person’s audio sensing capabilities and that of adults. This benefit can work both for and against either group. For example, cell phones can be equipped with Mosquito Ringtones. These sounds are pitched above 14kHz and although virtually every student in a typical high school or junior high school class can hear them clearly, the vast majority of teachers and parents won’t hear anything at all when a Mosquito Ringtone equipped cell phone rings. On the other side of the coin, shopowners who want to discourage teen loitering can equip their stores with noise generators that produce loud sounds above 14 or 15kHz. Teenagers and children will find these noises uncomfortable and unpleasant motivating them to find somewhere else to hang out, while older patrons of the store will hear nothing out of the ordinary.