Laboratory - Build A Doppler Buzzer Ball
Instructions
Our idea was to put a buzzer that produces a steady tone on a string that one person could swing around in a circle while someone else stood outside the circle. The person in the middle of the circle should hear a uniform tone. The person outside the circle should hear a rising and falling pitch - higher as the buzzer approaches and lower as it recedes.
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The boy twirls the buzzer ball around in circles above his head. He hears a steady sound with no change in pitch. The girl hears a lower pitch sound than the boy hears while the ball is moving away from her. As the ball approaches she hears a higher pitch sound. |
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| As the ball approaches she hears a higher pitch sound.
You can watch and hear this for yourself. Click on either picture for an animated experiment. |
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How We Built It
We found a small electric buzzer that could be powered by a 9 volt battery. For safety, we enclosed the battery and buzzer in a tennis ball. The idea is to help prevent damage to the components should the ball hit something, and should that something be a person, it would help prevent injury. Still, if you build and use this device, make sure that no one is in the way when you are swinging it in circles.
Materials
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What We Did1. Using a small knife we made a slit about 1/3 of the way around the ball. Opposite the slit we made a small hole for the string to go through. We forced the end of the string through the hole and then pulled it out of the ball through the slit. We left about 20cm of string hanging out from the slit and about 5m from the side with the hole. |
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2. Using a wire stripper we removed about 2cm of insulation from the ends of the wires attached to the buzzer and to the battery clip. |
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3. We put a 2cm length of heat shrink tubing on each wire from the battery clip and then twisted the battery clip and buzzer wires together, red to red and black to black. |
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4. We soldered the connections, trimmed the excess wire and moved the heat shrink tubing over the soldered joints. Then we held the joints at a distance over the flame on a gas range to shrink the tubing. Safety Note: Soldering and working with heat shrink tubing should be done only with adult supervision. You can skip these steps and your device will still work. Just make sure the wires are well twisted together and cover them with tape for insulation instead of heat shrink tubing. |
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5. The next step was to tie the end of the string that emerges from the slit in the ball to the buzzer. This knot must be very secure. Our buzzer has two extensions with holes in them, convenient for tying. |
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6. Next we used two rubber bands to secure the battery to the buzzer. |
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7. OK, now we have all the pieces together. |
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8. Next, we pushed the battery and buzzer into the ball with the terminal end of the battery facing towards the slit. This is so the battery clip can be easily attached and removed from the battery. |
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9. Since the tennis ball muffles the sound of the buzzer we used a knife to cut a hole in the ball so that the sound element of the buzzer is exposed. We also stuffed the excess wire into the ball through the slit. |
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10. That’s it. Now out to the park to give it a try. We pressed the battery clip onto the battery to start the buzzer buzzing and then twirled the buzzer ball around. The cameraperson who took this picture heard a rising and falling tone while the twirler heard a steady pitch. The biker and the runner would also have heard a rising and falling pitch if they had been paying attention. |
