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Many insects have good ultrasonic hearing and most of these are nocturnal insects listening for echolocating bats.
Lower frequency ultrasound (50–500 k Hz) can also be used to inspect less dense materials such as wood, concrete and cement.
Acoustics, the science of sound, starts as far back as Pythagoras in the 6th century BC, who wrote on the mathematical properties of stringed instruments.
Echolocation in bats was discovered by Lazzaro Spallanzani in 1794, when he demonstrated that bats hunted and navigated by inaudible sound and not vision.
Ultrasound can also provide additional information such as the depth of flaws in a welded joint.
Ultrasonic inspection has progressed from manual methods to computerized systems that automate much of the process.