( Ultrasound ( ( refers ( to ( sound ( waves ( of ( ( a frequency ) ( which ( ( are not ) ( audible ( to ( the ( human ear ) ) ) ) ) ) ) ) ) ) ) ) . ) )
(ROOT (S (NP (NNP Ultrasound)) (VP (VBZ refers) (S (VP (TO to) (VP (VB sound) (NP (NP (NNS waves)) (PP (IN of) (NP (NP (DT a) (NN frequency)) (SBAR (WHNP (WDT which)) (S (VP (VBP are) (RB not) (ADJP (JJ audible) (PP (TO to) (NP (DT the) (JJ human) (NN ear)))))))))))))) (. .)))
Ultrasound refers to sound waves of a frequency which are not audible to the human ear.
(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))))))))))))) (. .)))