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Ultrasound is sound waves with frequencies higher than the upper audible limit of human hearing.

Echolocation in bats was discovered by Lazzaro Spallanzani in 1794, when he demonstrated that bats hunted and navigated by inaudible sound and not vision.

Upon hearing a bat, some insects will make evasive manoeuvres to escape being caught. In the order Clupeiformes, members of the subfamily Alosinae (shad), have been shown to be able to detect sounds up to 180 k Hz, while the other subfamilies (e.g. An ultrasonic level or sensing system requires no contact with the target.