The physics of the didgeridoo

27th February 2004 at 00:00
I teach the Salters-Horners advanced physics (SHAP), which I like because it is context-driven, relating the physics theory to direct applications.

Although I have a physics degree, I have worked in recording studios and theatre. The musicwaves section of the AS course lends itself brilliantly to applications. Physics pervades the whole recording process, from creating pitches, through subsequent effects, to recording, and also includes PA systems.

Take for example a study of waves. A-level students invariably know modern music and sound-processing (from their CD collection) and there are usually a couple of them who are only too happy to bring in their electric guitars.

Using a few simple ideas and students' understanding of frequency, wavelength and velocity, we can explain how fundamental notes can be determined, how synthesisers work and how effects such as "flanging" can be created.

If your school runs music technology courses you could probably borrow a synthesiser, a mike, various FX (such as a phaser or a flanger) and possibly a mixer. I think it's fun to use more exotic instruments - such as a didgeridoo.

Students find didgeridoos fascinating. Usually someone can get sound from it, even if it's only a raspberry. (The real trick is a technique called "circular breathing" - experiments with a straw in a cup of water explore how that works.) The didgeridoo sound is very bassy - magnificent amplified. A simple calculation reinforces this. Being an open tube, the standing wave will be a simple Antinode - Node - Antinode (A-N-A). Its length will therefore be half the wavelength (l2). Let some students measure this with a metre-stick, double it and they have the wavelength, l. Then, using the formula f=vl, it is a simple matter to estimate what the fundamental frequency is.

Electric guitars can be dealt with in a similar fashion, although there's more scope for reinforcing v = V(T5), with direct application to the tuning and why some strings are necessarily thicker.

Although this can be equally well done on acoustic guitars (and often is), the electric versions can be used to demonstrate more exciting effects, such as "phasing" and "flanging". The sounds are very obvious to students and are caused, basically, by splitting the sound, delaying it fractionally, and recombining it. The subsequent interference effect is called "phasing" and sounds cool! "Flanging" sounds like an extreme version, although it has a similar origin and is traditionally attributed to the Beatles.

Chris Gidzewicz, teacher, Norton Hill School, Somerset

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