How High-Pitched Tones Target Teens but Evade Adults

4 min read

Our hearing picks up a limited range of sounds, limits that were used successfully to keep teenagers from congregating outside British grocery stores by a device called the “teenage sound repeller.” This invention generates high-frequency sounds in the high-pitched range of human hearing, composed of sound waves between 15,000 and 20,000 cycles per second, which can annoy younger people but are not heard by adults (cycles per second is also termed Hertz, abbreviated as Hz).

Source: Lexx/iStock

Illustration of sound waves and the ear.

Source: Lexx/iStock

The normal range of hearing for younger people goes from a low pitch, corresponding to a lower sound frequency, of about 20 cycles per second up to a high of 20,000 cycles per second. But as people grow older, even at mid-life, especially with exposure to loud sounds, they lose the ability to hear the higher range of frequencies, whether due to changes in the microscopic sensory hair cells that respond to sound waves or the auditory nerve fibers activated by them. The teenage sound repeller, and a similar device known as “The Mosquito,” take advantage of these biological differences.

It’s not only in Great Britain but in the U.S., as well. For example, in Philadelphia, according to National Public Radio, “…30 parks and recreation centers are outfitted with a small speaker called the Mosquito. It blares a constant, high-pitched ringing noise all night long — but one that only teenagers and young adults can hear… Anyone over age 25 is supposed to be immune because, basically, their ear cells have started to die off.”

But the adult human voice covers a limited range of frequencies and their corresponding pitches, about 100 cycles per second on the low end for men, up to about 17,000 cycles per second for women. That covers the fundamental voice frequencies on the low end, up to the harmonics at the high end. These sound waves act on a long basilar membrane in the inner ear, with higher pitch acting on one end, and lower pitch traveling along the membrane to the other end.

Instrumental music can have an even more limited range than normal hearing. The piano, for example, has 88 keys that span the frequency range of 27.5 up to 4,186 cycles per second. This absence of higher frequency notes has been explained by the difficulty telling apart notes that are higher than this range, so that higher-frequency notes would not be useful. Another reason might be that if there were many more keys, a pianist might need a much longer arm-span (some primates, like the gibbon, have a longer arm-span than humans, but none are concert musicians).

Spoken words, like piano music, can be detected when they are within a limited frequency range, up to about 4,000 cycles per second. The sound waves differ according to different parts of each word as well. Consonants like “t,” “k,” and “p” contain high-frequency sound waves, and can more easily be confused with each other than the lower-frequency vowel sounds, especially with the loss of high-frequency hearing with age. They also can be more easily confused in a noisy environment, such as a restaurant or pub.

The teenage sound repeller or “Mosquito” can help businesses that do not want teenagers congregating outside their front doors, and it is a clever use of biological differences in hearing that change with age. The device has been opposed by people nearby who find it uncomfortable, or concerned about the discomfort for people sensitive to these sounds. So though it meets some opposition, if you hear a high-pitched mosquito-like buzz outside your neighborhood supermarket, it might be an example of our personalized auditory system at work.

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