Eli5: what is ultrasound and what makes it “ultra”
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“Ultra-” as a prefix means “above (a limit or range)”. The opposite is “infra-“, meaning “below (a limit or range)”.
“Ultrasound” means “sound waves *above* the range that we can hear”, and “infrasound” means “sound waves *below* the range that we can hear”.
Similarly: “ultraviolet” means “energy *above* the violet part of the visible spectrum”, while “infrared” means “energy *below* the red part of the visible spectrum”.
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