The Mosquito
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The Mosquito or Mosquito alarm is a machine used to deter loitering by emitting sound at high frequency. In some versions, it is intentionally tuned to be heard primarily by younger people. Nicknamed
A Mosquito device mounted outside a store in Philadelphia
Ultrasound
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Ultrasound is sound with frequencies greater than 20 kilohertz. This frequency is the approximate upper audible limit of human hearing in healthy young adults. The physical principles of acoustic wave
An ultrasonic examination
Galton whistle, one of the first devices to produce ultrasound
Bats use ultrasounds to navigate in the darkness.
A dog whistle, which emits sound in the ultrasonic range, used to train dogs and other animals