Or you`re going mutt and jeff.
This is another reason many emergency services started using "white noise" rather than sirens which are affected by the "Doppler " effect.
Doppler effect
n.
An apparent change in the frequency of waves, as of sound or light, occurring when the source and observer are in motion relative to each other, with the frequency increasing when the source and observer approach each other and decreasing when they move apart.
Alarm
March 2014