( this ( ( was ( followed ( by ( ( ( ( a sound ) ( of ( loud thunder ) ) ) , ) ( never ( heard before ) ) ) ) ) ) . ) )
(ROOT (S (NP (DT this)) (VP (VBD was) (VP (VBN followed) (PP (IN by) (NP (NP (DT a) (NN sound)) (PP (IN of) (NP (JJ loud) (NN thunder))) (, ,) (VP (ADVP (RB never)) (VBN heard) (PP (IN before))))))) (. .)))
this was followed by a sound of loud thunder, never heard before.
(ROOT (S (NP (NP (DT The) (NN sound)) (PP (IN in) (NP (DT a) (JJ loud) (NN classroom)))) (VP (VBZ is) (NP (NP (DT an) (NN example)) (PP (IN of) (NP (NP (DT a) (NN form)) (PP (IN of) (NP (NN energy))))))) (. .)))