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The Mosquito

en.wikipedia.org/wiki/The_Mosquito

The Mosquito or Mosquito alarm is an electronic device used to deter loitering by young people ... The risk to the target group of teenagers and young adults is relatively low. They can leave the ar...

www.ask.com/youtube?q=Sound Adults Can't Hear&v=RG0O478CQ-E
Oct 5, 2008 ... This is a high-pitched sound that people over the age of 25 shouldn't hear. The sound seems to work better in a higher quality.
www.ask.com/youtube?q=Sound Adults Can't Hear&v=VoFGKwZxT_o
Nov 21, 2009 ... This is the sound adults over 30 cannot hear.(some may not qualify)Enjoy!plz comment,rate and subscribe!

Try It: Can You Hear These Sounds Only Young People Hear?

www.yahoo.com/health/try-it-can-you-hear-these-sounds-only-young-112627654778.html

Mar 3, 2015 ... On the flip side, teens are striking back by employing these pitches as ringtones, so they can get calls that their parents and teachers can't hear.

Can You Hear This Sound That Only Young People Can Hear

www.buzzfeed.com/daves4/how-young-are-your-ears

Mar 2, 2015 ... Apparently, there's a noise that only people under 25 can hear. ... As you get older, the hearing process (sound waves travel into your ear, hit your eardrum, and vibrate three tiny bones, ... No, I can't ... PHOTO: Adult Swim.

Stealth Tone on the App Store - iTunes - Apple

itunes.apple.com/us/app/stealth-tone/id314839075?mt=8

Jul 20, 2010 ... Stealth Tone is a special tool that most 20+ year olds cannot hear! Due to a special frequency, you can play this annoying sound in stealth ...

5 Sounds You Probably Can't Hear | Mental Floss

mentalfloss.com/article/64572/5-sounds-you-probably-cant-hear

Jun 2, 2015 ... Think you have perfect hearing? There are still plenty of sounds in the world that you can't detect.

The Teenager Audio Test - Can you hear this sound? - The Oatmeal

theoatmeal.com/quizzes/sound/

... and sounds similar to a buzzing mosquito. The elderly and people with hearing damage often cannot hear the sound. Can you hear this sound? Click to play.

Sonic Science: The High-Frequency Hearing Test - Scientific American

www.scientificamerican.com/article/bring-science-home-high-frequency-hearing/

Teens are effectively driven away, but many adults can't hear this sound at all! Things got even more interesting when some kids realized they could turn this ...

Ultrasonic Ringtones - the new ringtones that only kids can hear!

www.ultrasonic-ringtones.com/

This means that they can only be heard by kids and generally can't be heard by adults over the age over 20 or so. See this article at the BBC to find out more.

Answer
The normal human hearing range is between about 12 and 20 kHz, but beginning around age 8 this range begins to shrink, and adults can no longer hear sounds above a certain frequency level.
More Info

What is a high pitch noise that adults can't hear? | Reference.com

www.reference.com

While many teenagers can hear sounds in the 16 kilohertz range or higher, most adults past the age of 25 cannot. For this reason, 17-kilohertz ring tones are ...

Most people over 18 can't hear these sounds | MNN - Mother Nature ...

www.mnn.com

Jul 24, 2015 ... Most people over 18 can't hear these sounds. Don't worry! You probably don't need to hear them anyway! John Donovan. July 24, 2015, 10:59 ...

Stealth Tone - Android Apps on Google Play

play.google.com

The Official Stealth Tone App by StuckPixel, Inc! Stealth Tone is a special tool that most 20+ year olds cannot hear! Due to a special frequency, you can play this ...