The doppler effect

HOW

1. Pump air into the system by turning the large wheel clockwise. The pressure gauge reading should increase.
2. Press the button that is located on the perimeter of the device.
3. Tighten the smaller wheel and spin it clockwise.
4. Listen to the generated sound.

WHY

When the button is pressed, the previously compressed air reaches the two pipes. Sounds of the same pitch are created there. When the pipes are stationary, you hear a uniform sound.

As you spin the smaller wheel, one of the pipes moves closer to you and the other moves away. As a result, the pitch of the sounds heard changes – increasing as the source approaches and decreasing as the source moves away. These two sound waves overlap so that you hear a pulsating sound.