Hearing the Doppler effect

What the Activity is for

A visual and auditory demonstration of shifted frequencies.

Here you can listen to the Doppler effect as the relative velocity between you and a moving object is varied. For this you need a simple piece of apparatus, and a Doppler ball is recommended. There are two ways in which the Doppler effect is important:

• The source can be moving towards or away from you.
• The source can be spinning, so effectively moving.

What to Prepare

• a Doppler ball
• sound processing software

To make the Doppler ball take an old tennis ball and cut it in half. Now make a small square hole, to take a slide switch, in one of the halves (a slide switch is recommended because it is more robust). In the middle of one half make a small circular hole, perhaps with a cork borer. Mount a buzzer up against this hole. Now wire up a 9 volt battery to the buzzer and to the slide switch. Pack the ball with foam and stick the two halves together again. Check that operating the slide switch turns the buzzer on and off.

We'd recommend that you consider two variations to improve this basic set up.

One is to use a larger diameter ball, for the spinning experiment. Mount the buzzer on the equator. Larger is not necessarily better – choose one that you can spin relatively rapidly.

A second is to use a throw toy (shaped much like a rugby football, with fins to prevent tumbling when thrown) to prevent the ball rotating, when thrown. Mount the buzzer on the nose, but do remember to allow for its impact to be cushioned.

Safety note: The tennis ball is relatively soft, but obviously should not be thrown directly at students.

What Happens During this Activity

You need to practice. You should aim to throw the ball without it spinning. And you should be able to spin the ball on the table so that the buzzer spins round and round the axis of rotation (so along an equator).

Start by exploring expectations as the ball moves towards or away from the students – just along a line. Draw on their everyday experience in order to generate predictions. The change in pitch at quite reasonable speeds will be noticeable, but do make sure that the ball does not spin as you throw it. This demonstration will provide a clear experience of the Doppler effect.

More subtly, you can link measurements of Doppler spreading to a tabletop experiment. Here the ball is spun. So sometimes the source is travelling towards the students and sometimes it is travelling away from them. Therefore sometimes the pitch will be higher than if the ball was not spinning and sometimes it will be lower. So the emitted frequency is spread into a small range of frequencies as a result of the rotation. That is Doppler spreading. The faster the ball spins, the wider the range of frequencies. It is this that is used to measure the rate of rotation of distant astronomical objects.

You might like to use a microphone and sound processing software, showing frequencies, as the ball spins. It's much harder to make such measurements with the ball moving in a straight line, unless you're very skilful.