( ( Further research ) ( ( In 1967 ) ( I ( ( ( ( experimented ( with ( ( beating ( two ( ultrasonic ( acoustic waves ) ) ) ) together ) ) ) and ) ( produced ( an ( audible ( difference frequency ) ) ) ) ) . ) ) ) )
(ROOT (S (NP (JJ Further) (NN research)) (PP (IN In) (NP (CD 1967))) (NP (PRP I)) (VP (VP (VBD experimented) (PP (IN with) (S (VP (VBG beating) (NP (CD two) (JJ ultrasonic) (JJ acoustic) (NNS waves)) (ADVP (RB together)))))) (CC and) (VP (VBD produced) (NP (DT an) (JJ audible) (NN difference) (NN frequency)))) (. .)))
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(ROOT (S (SBAR (WHADVP (WRB When)) (S (NP (NP (NNS waves)) (PP (IN of) (NP (CD two) (JJ different) (NNS frequencies)))) (VP (VBP interfere)))) (, ,) (NP (NN beating)) (VP (VBZ occurs)) (. .)))