( Beats ( ( are ( ( ( the ( ( ( periodic and ) repeating ) fluctuations ) ) ( heard ( in ( ( the intensity ) ( of ( a sound ) ) ) ) ) ) ( when ( ( ( two ( sound waves ) ) ( of ( ( very similar ) frequencies ) ) ) ( interfere ( with ( one another ) ) ) ) ) ) ) . ) )
(ROOT (S (NP (NNS Beats)) (VP (VBP are) (NP (NP (DT the) (UCP (NN periodic) (CC and) (JJ repeating)) (NNS fluctuations)) (VP (VBN heard) (PP (IN in) (NP (NP (DT the) (NN intensity)) (PP (IN of) (NP (DT a) (NN sound)))))) (SBAR (WHADVP (WRB when)) (S (NP (NP (CD two) (JJ sound) (NNS waves)) (PP (IN of) (NP (ADJP (RB very) (JJ similar)) (NNS frequencies)))) (VP (VBP interfere) (PP (IN with) (NP (CD one) (DT another)))))))) (. .)))
Beats are the periodic and repeating fluctuations heard in the intensity of a sound when two sound waves of very similar frequencies interfere with one another.
(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)) (. .)))