Your intuition tells you that sounds behave funny when the object making the sound is moving relative to where you are. As the honking horn movies showed us, either the object can be moving while you are stationary or you can be moving while the object is stationary.

Let's see if you can start putting some definition to that intuition. Maybe answering the following two questions will help.

  • If the object is coming toward you how do you expect the pitch to change?
  • If the object is moving away from you how do you expect the pitch to change?

To investigate this further we're going to make a Doppler Ball, a sponge or nerf ball that has a battery operated siren tucked inside of it. You can't buy a doppler ball, so you're going to have to make one. When the battery is connected the horn/siren is activated and emits a sound with a constant pitch.

Here's how to make a doppler ball

Toss the ball around the room. Try these four things with the ball. Don't forget to record your observations:

  1. What is the pitch of the siren when the doppler ball is not moving?
  2. What does the pitch of the siren do when the ball is moving toward you?
  3. What does the pitch of the siren do when the ball is moving away from you?
  4. Leave the ball stationary. What does the pitch of the siren do when you run toward and away from the ball?

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