( ( ( The frequency ) ( ( the observer ) hears ) ) ( ( is ( ( the speed ) ( of ( propagation ( of ( ( ( the wave ) , ) ( divided ( by ( its wavelength ) ) ) ) ) ) ) ) ) . ) )
(ROOT (S (NP (NP (DT The) (NN frequency)) (SBAR (S (NP (DT the) (NN observer)) (VP (VBZ hears))))) (VP (VBZ is) (NP (NP (DT the) (NN speed)) (PP (IN of) (NP (NP (NN propagation)) (PP (IN of) (NP (NP (DT the) (NN wave)) (, ,) (VP (VBN divided) (PP (IN by) (NP (PRP$ its) (NN wavelength)))))))))) (. .)))
The frequency the observer hears is the speed of propagation of the  wave, divided by its wavelength.
(ROOT (S (NP (NP (DT The) (NN product)) (PP (IN of) (NP (NP (NP (DT a) (NN wave) (POS 's)) (NN wavelength)) (CC and) (NP (PRP$ its) (NN frequency))))) (VP (VBZ is) (NP (PRP$ its) (NN speed))) (. .)))