Concept

The audience will hear the buzzer’s stationary frequency shifted up as the buzzer moves toward them, and down as the buzzer moves away. They will thus hear a “wobbling” pitch as the buzzer is swung in a circular motion.

The shift in frequency is determined by:

$$f' = \frac{v_s}{v_s \pm v}f \text{ where }v=\frac{2 \pi r}{T}$$

Here, $v_s = 343 \text{m}/\text{s}$, $r=$ 1 meter, $T=$ period of revolution, $f=$2.7kHz, and $\pm$ denotes whether the source is moving towards or away the listener.

Procedure

Turn on Doppler Buzzer by flipping switch on side of cardboard tube.
Hold string at the red marked knot (1 meter from buzzer sound source).
Make sure no one is within 1 meter of you.
Swing the buzzer overhead in a circle with a constant speed while trying to keep your hand in one place.

Equipment

Doppler Buzzer