Emergency vehicles, like ambulances and police cars, are great examples of something called the Doppler Effect. This is important for understanding how waves behave in physics.

What is the Doppler Effect?
The Doppler Effect is all about how sound changes as something moves.

When an emergency vehicle gets closer to you, the sound waves it makes get squished together. This means the sound is higher in pitch.

But when the vehicle moves away from you, the sound waves stretch out. This makes the sound lower in pitch.

You can even do some math to see how this works! You can use a formula to find out the changed sound frequency you hear:

$$f' = f \frac{v + v_0}{v - v_s}$$

Here’s what each part means:

• $f$ is the original frequency of the siren.
• $v$ is how fast sound travels in the air.
• $v_0$ is how fast you are moving (usually 0 if you are not moving).
• $v_s$ is the speed of the emergency vehicle.

Example in Real Life
Imagine an ambulance that has a siren with a frequency of $650 \, \text{Hz}$. If it’s driving toward a person standing still at a speed of $30 \, \text{m/s}$, and the speed of sound is about $343 \, \text{m/s}$, we can figure out how the frequency changes as it comes closer.

Using our numbers:

For $v = 343 \, \text{m/s}$ and $v_s = 30 \, \text{m/s}$, we can calculate:

$$f' = 650 \, \text{Hz} \frac{343 + 0}{343 - 30} \approx 650 \, \text{Hz} \times \frac{343}{313} \approx 710 \, \text{Hz}$$

When the ambulance drives away, the pitch of the sound goes down again. This shows how the sound changes in our day-to-day lives.

In Summary
So, when you hear the siren of an emergency vehicle change as it moves, that's the Doppler Effect in action. It’s a cool way to see how waves and sounds work in real life!