( Sound ( ( ( is ( a ( mechanical wave ) ) ) ( because ( it ( ( needs ( a medium ) ) ( ( in order ) ( to travel ) ) ) ) ) ) . ) )
(ROOT (S (NP (NNP Sound)) (VP (VBZ is) (NP (DT a) (JJ mechanical) (NN wave)) (SBAR (IN because) (S (NP (PRP it)) (VP (VBZ needs) (NP (DT a) (NN medium)) (SBAR (IN in) (NN order) (S (VP (TO to) (VP (VB travel))))))))) (. .)))
Sound is a mechanical wave because it needs a medium in order to travel.
(ROOT (S (NP (NP (NNP Matter)) (SBAR (IN that) (S (NP (JJ mechanical) (NN wave) (NN energy)) (VP (MD can) (ADVP (RB only)) (VP (VB travel) (PRT (RP through))))))) (VP (VBZ is) (ADJP (JJ medium))) (. .)))
Matter that mechanical wave energy can only travel through is medium.