When a sound source moves in relation to you, its pitch changes. From this effect you can determine whether the source is moving toward or away from you, and you can estimate how fast it’s going.
- Tennis ball or wiffle ball (something you can cut open)
- Knife (not shown)
- A 9-volt battery and connector
- A 9-volt buzzer (a high-pitched one works best)
- Scrap paper to pack inside the ball (not shown)
- Heavy rubber bands or tape (not shown)
- Strong string
- Optional: On/off switch (available at hardware stores)
- Cut a slit halfway around the ball with a sharp knife.
- Connect the wire from one terminal on the battery to the wire from one terminal on the buzzer. If the buzzer has a (+) and (–) terminal, be sure to connect the buzzer terminal to the matching battery terminal.
- There will be a wire connected to the remaining terminal on the battery and another wire connected to the remaining terminal on the buzzer. Place both battery and buzzer inside the ball, leaving these two unconnected wire ends sticking out of the ball. (Click the diagram below to enlarge.)
- Pack the ball loosely with paper, leaving the buzzer close to the outside edge.
- Twist the remaining two wires together to turn the buzzer on, then close the ball and secure the wires with rubber bands or tape. You may want to wire a switch into your circuit so you can turn the buzzer on and off more conveniently.
Attach the ball securely to a string and twirl it around your head, or toss the ball back and forth with a partner. You might also have a group of students toss the ball around. Notice how the pitch of the buzzer changes as the ball approaches you or moves away from you.
When an oscillator (the buzzer) moves toward you, in effect, it is catching up slightly with its own sound waves. With each successive pulse of the buzzer, the sound source is a little closer to you. The result is that the waves are squeezed together, and more of them reach your ear each second than if the buzzer were standing still. Therefore, the pitch of the buzzer sounds higher. As the buzzer moves away from you, fewer waves reach your ear each second, so the resulting pitch sounds lower. The frequency of the buzzer itself does not change in either case.
For your ears to detect this effect—called the Doppler effect—the sound source has to be moving toward or away from you at a minimum speed of about 15 to 20 mph (24 to 32 kph). As the source moves faster, the effect becomes more pronounced.
If the buzzer has a frequency of 100 hertz, and it is moving toward you through still air at 35 meters per second, then the pitch you hear will be 110 hertz. This result comes from the equation
pitch = f/(1–v/vs)
where f is the frequency, v is the speed of the sources of the sound, and vs is the speed of sound, which is 350 meters per second. If the object is moving away from you, simply replace the minus sign with a plus sign.
The Doppler effect is also observed with light. In the case of light, it’s the color that changes. If an object is moving away, it becomes slightly redder; if an object is approaching, it appears bluer. This effect allows astronomers to determine whether galaxies are approaching us or moving away from us and even how fast they’re moving: The bigger the “red shift,” the faster they’re moving away from us.