Doppler Ball

Age

Elementary School, Middle School

Format

Hands-on

Materials

   Doppler Ball
   A working 9V battery + backup 

Safety Precautions

Science Theatre demonstrators must keep the safety of themselves and their audience in mind at all times. All Science Theatre demonstrators must have read through the Safety Training page. The ST Safety Box with first aid kit, fire extinguisher, etc. should always be available to demonstrators. Always wear safety gloves, glasses, and a labcoat if handling chemicals; always perform potentially dangerous demonstrations at a safe distance from the audience; and always keep a very close eye on any volunteers you call from the audience. Depending on the age, judge how hard you should be throwing the ball. A simple underhand toss will suffice for most participants.

Preparation

Ensure that the Doppler ball has a working battery (9V). Make sure there is a large enough space available for tossing the ball (preferably a location quiet enough to hear the ball).

Demonstration

Explain the Doppler effect (see script and scientific explanation below) Toss the Doppler ball back and forth and point out the change in pitch. If the audience is of an appropriate age, you can choose members of the audience to throw to. If they are very young, you should only toss the ball between presenters.

What to Say

Start with asking the participants if they have ever heard the sound a train makes when it whizzes past them really fast (try to imitate the sound with your mouth).

Sound travels as a series of compressions of air. When a sound source is moving, the compressions pile up in front of the object and spread out behind the object. The compression or spreading of sound waves changes the pitch of the sound thus creating the familiar phenomena we hear when a train goes by.

Depending on the age of the audience, you should incorporate material from the "Why It Is" section in your demonstration.

Why It Is

Sound is a wave that travels through a medium (solid, liquid, or gas) by a series of compressions. Since the compressions occur at a variety of regular time intervals, we perceive a variety of frequencies or "pitch" of sound. (the human ear can hear from about 20Hz to about 20,000Hz or 20 compressions per second to 20,000 compressions per second).

When an object is moving and emitting sound, the compressions in front of the object in a sense get piled up. They occur more frequently, which raises the frequency of the sound and makes the sound higher in pitch.

When an object is moving away from the observer and emitting sound, then the waves are allowed to spread out (opposite of what is happening in front of the object). The compressions are spread out and are then heard less frequently which makes the pitch of the sound lower.

A simple equation explaining the change in the frequency is as follows.

In this equation, f0 is the initial frequency of the emitted sound, v is the velocity of sound in the medium (e.g. air), and vs is the velocity of the object emitting the sound. (Note: This equation assumes that the receiver (you) are standing still relative to the medium (i.e. air).) We can see that if the velocity of the object is positive (moving away) then the frequency will be a fraction of the original frequency and if the velocity of the object is negative (moving towards observer), then the frequency will be larger than the original frequency.

Real Life Examples

Often we hear this effect when we listen to a train whistle as the train passes by or when a police car with its siren blaring approaches and then passes and moves away. Another example is a race car that "vrooms" past. The effect happens whenever there is a sound emitting source moving in a radial direction from the observer.

This effect can also occur with light. Light can be Doppler shifted and change color. This is useful in astronomy when calculating the motion of stars and galaxies.