<Dd> f = (c + v r c + v s) f 0 (\ displaystyle f = \ left ((\ frac (c + v_ (\ text (r))) (c + v_ (\ text (s)))) \ right) f_ (0)) </Dd> <P> An interesting effect was predicted by Lord Rayleigh in his classic book on sound: if the source is moving toward the observer at twice the speed of sound, a musical piece emitted by that source would be heard in correct time and tune, but backwards . The Doppler effect with sound is only clearly heard with objects moving at high speed, as change in frequency of musical tone involves a speed of around 40 meters per second, and smaller changes in frequency can easily be confused by changes in the amplitude of the sounds from moving emitters . Neil A Downie has demonstrated how the Doppler effect can be made much more easily audible by using an ultrasonic (e.g. 40 kHz) emitter on the moving object . The observer then uses a heterodyne frequency converter, as used in many bat detectors, to listen to a band around 40 kHz . In this case, with the bat detector tuned to give frequency for the stationary emitter of 2000 Hz, the observer will perceive a frequency shift of a whole tone, 240 Hz, if the emitter travels at 2 meters per second . </P> <P> The siren on a passing emergency vehicle will start out higher than its stationary pitch, slide down as it passes, and continue lower than its stationary pitch as it recedes from the observer . Astronomer John Dobson explained the effect thus: </P> <Dl> <Dd> "The reason the siren slides is because it doesn't hit you ." </Dd> </Dl>

According to the doppler effect the sound of a police car siren is