( ( the intensity ) ( ( will ( vary ( with ( ( ( a frequency ) ( -LRB- ( ( known ( as ( the ( beat frequency ) ) ) ) -RRB- ) ) ) ( that ( is ( ( the difference ) ( between ( ( the frequencies ) ( of ( the ( two waves ) ) ) ) ) ) ) ) ) ) ) ) . ) )
(ROOT (S (NP (DT the) (NN intensity)) (VP (MD will) (VP (VB vary) (PP (IN with) (NP (NP (DT a) (NN frequency)) (PRN (-LRB- -LRB-) (VP (VBN known) (PP (IN as) (NP (DT the) (NN beat) (NN frequency)))) (-RRB- -RRB-)) (SBAR (WHNP (WDT that)) (S (VP (VBZ is) (NP (NP (DT the) (NN difference)) (PP (IN between) (NP (NP (DT the) (NNS frequencies)) (PP (IN of) (NP (DT the) (CD two) (NNS waves))))))))))))) (. .)))
the intensity will vary with a frequency (known as the beat frequency) that is the difference between the frequencies of the two waves.
(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)) (. .)))