The Doppler Effect is something you notice when a sound source, like an ambulance, moves toward or away from you. It changes the way we hear the sound. You can hear this shift clearly with emergency vehicles as they get closer or drift away.

Understanding the Effect

1. Sounds Coming Closer:

   • When an ambulance drives toward you, the sound waves get squished together. This makes the sound higher in pitch and shorter in length.
   • For example, if the siren normally makes a sound at 800 Hz, you will hear it differently when it approaches. The formula for figuring this out is: $f' = f \frac{v + v_o}{v - v_s}$ Here’s what those letters mean:
     • $f'$ = frequency you hear,
     • $f$ = original frequency (800 Hz),
     • $v$ = speed of sound (about 343 meters per second at 20°C),
     • $v_o$ = speed of you (0 if you are standing still),
     • $v_s$ = speed of the ambulance (for example, 30 m/s).
   • So, when it's getting closer, the frequency you hear changes to: $f' = 800 \frac{343 + 0}{343 - 30} \approx 800 \cdot 1.096 \approx 877 Hz$

2. Sounds Moving Away:

   • When the ambulance passes and goes away, the sound waves stretch out. This means the pitch goes lower and the sound gets longer.
   • Using the same formula, but changing $v_s$ to show it’s moving away, we get: $f'' = f \frac{v + v_o}{v + v_s}$
   • The frequency you hear as it moves away is: $f'' = 800 \frac{343 + 0}{343 + 30} \approx 800 \cdot 0.936 \approx 748 Hz$

In Summary

The Doppler Effect causes the sound of the ambulance's siren to change. When it comes closer, it sounds higher; when it moves away, it becomes lower. This creates a sound pattern that you can easily recognize.