( Ultrasound ( ( refers ( to ( ( sound waves ) ( having ( ( a frequency ) ( higher ( than ( what ( ( a human ) ( can hear ) ) ) ) ) ) ) ) ) ) . ) )
(ROOT (S (NP (NNP Ultrasound)) (VP (VBZ refers) (S (VP (TO to) (VP (VB sound) (NP (NNS waves)) (S (VP (VBG having) (S (NP (DT a) (NN frequency)) (ADJP (JJR higher) (PP (IN than) (SBAR (WHNP (WP what)) (S (NP (DT a) (NN human)) (VP (MD can) (VP (VB hear)))))))))))))) (. .)))
Ultrasound refers to sound waves having a frequency higher than what a human can hear.
(ROOT (S (NP (PRP You)) (VP (VBP call) (NP (NP (NN sound)) (SBAR (WHNP (WDT that)) (S (VP (VBZ has) (S (NP (DT a) (NN wave) (NN frequency)) (ADJP (JJR higher) (SBAR (IN than) (S (NP (DT the) (JJ human) (NN ear)) (VP (MD can) (VP (VB detect) (NP (NN ultrasound))))))))))))) (. .)))