Amazing science demo five: Double Doppler with Train Set

This is the fifth and final instalment of our “five amazing physics demonstrations” presented by science-demo guru Neil Downie and his adept assistant Matthew Isbell.

In a special feature in the April issue of Physics World, Downie describes his five best demos of all time, all of which use everyday equipment to illustrate fundamental physics concepts. Downie describes how his fondness for the five experiments comes from the fact that, with a bit of creativity, each one can be easily adapted to explore physical concepts further. In the digital edition of the April issue, each demonstration is accompanied by a video in which Downie walks you through how you would present each demonstration to an audience. Full details of how to access the digital edition are available at the bottom of this article.

In this final demo of the series, Downie uses a model train set equipped with an ultrasound emitter to demonstrate the Doppler effect. Employing a bat detector to convert the ultrasound signal into an audible sound, Downie illustrates how the pitch increases as the train steams towards him, and then decreases as it moves away. The train set brings a potentially dry subject to life and Downie uses the set-up to explain how bats utilize the Doppler effect when hunting for prey.

Double Doppler with Train Set

So what’s this all about? The Doppler effect is of fundamental importance in physics, not least because measuring the redshift of light from galaxies lets us estimate how far away they are. When it comes to sound, however, demonstrating the Doppler effect is not easy. You could get someone on a passing train to play a trumpet or try whirling an electronic beeper around on a string – but neither is particularly simple. What is more, the Doppler shift is small and can be masked by changes in sound volume. This project is a much neater way of showing the Doppler effect and has the added bonus of bringing ultrasonic waves to life.

What bits and pieces do I need? You will need an electronic bat detector – a hand-held device that converts ultrasound into audible sound. You’ll also need an ultrasonic transducer, such as the kind fitted to cars to help drivers park. Such devices, which emit ultrasound at about 40 kHz, are normally hooked up to a separate microprocessor circuit to create pulses. So to get a continuous source of waves, you’ll need to connect the emitter to an oscillator circuit. Finally, you need to attach the transducer to a moving vehicle – a toy train running on a circular track is ideal.

How do I get going? Lay the track somewhere without too many flat walls or in the corner of a room (this reduces reflections). Turn on the ultrasonic emitter and put it in the train. Then switch on your detector. If you’re standing still, you’ll hear the pitch of the signal rise and fall with a characteristic “heeeEEEeeaaAAAAaaaaw” as the train first approaches and then moves off. There’s a steady high note as the train approaches, and a steady low note as it recedes, with a modulating pitch in-between.

And what physics will I learn? If you’re standing still and the train’s approaching you at speed v, the frequency of the sound received, f_r, will differ from the emitted frequency, f_e, according to the classic textbook formula f_r = f_e[c/(c – v)], where c is the speed of sound. (The formula is f_r = f_e[c/(c + v)] if the train is moving away.) If the sound were at an audible frequency of about 2 kHz, then for a source approaching at 0.5 ms^–1, the detected frequency would rise by just 3 Hz, which would be hard to hear given that a semitone in music is 120 Hz. A 40 kHz ultrasonic system moving at the same speed, however, will have a Doppler shift of about 60 Hz, which is easy to notice if you adjust the bat detector so it emits, say, 1000 Hz waves when it receives 40 kHz ultrasound – it will be about a whole musical tone different. You could also use your equipment to detect other sources of ultrasound, from rubbing fingers and whistling fluorescent lamps to electronics and clanging metals. Or even bats.

• If you’re a member of the Institute of Physics (IOP), you can now enjoy immediate access to the April issue of Physics World with the digital edition of the magazine on your desktop via MyIOP.org or on any iOS or Android smartphone or tablet via the Physics World app, available from the App Store and Google Play. If you’re not yet in the IOP, you can join as an IOPimember for just £15, €20 or $25 a year to get full access to Physics World both online and through the apps