Doppler Buzzer, 3B40.10

Topic and Concept:

• Wave Motion, 3B40. Doppler Effect

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Location:

• Cabinet: Waves and Sound (WS)

• Bay: (A4 Right)

• Shelf: #1

Abstract:

A battery powered buzzer on a string is swung around in a horizontal circle.

Important Setup Notes:

• Check to make sure the buzzer works ahead of time. The batteries may be bad.

Setup and Procedure:

1. Switch the buzzer on by sliding the switch (somewhat burried in the ball - see photo below) into the "on" position.
2. Grab the far end of the string from the ball.
3. Swing the ball over your head in a horizontal motion.

Cautions, Warnings, or Safety Concerns:

• N/A

Discussion:

As the ball spins around in a circle, the direction of the relative velocity with respect to the observer (the audience) is constantly changing. In terms of the component of tangential velocity in the direction of the audience, the ball has relative velocity toward the audience half of the time and away from the audience the other half. This allows us to experience "both sides" of the Doppler effect. When the ball moves toward us, the frequency appears higher than the actual frequency. Likewise, when the ball moves away, the frequency appears lower. We hear the actual frequency produced by the ball at only two points: where the tangential velocity of the ball is perpendicular to the observer's line of sight. The overall magnitude of the frequency shift depends on the speed of rotation. The higher the speed is, the larger the shift will be. The functional form of this is given by

f_observed = (c + u_observer) / (c + u_source) * f_source

where u is the relative velocity of the object or person with respect to some common point of reference, and c is the speed of sound.

Videos:

• Lecture Demonstration's Youtube Channel

References:

• Wikipedia - Doppler Effect

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