Lockdown Challenge: Making music with the saxotron

Lockdown Challenge #43: The Saxotron – the semi-electronic saxophone

Adolphe Sax invented the saxophone as a kind of clarinet that would be easier to play. That’s because clarinets jump a tricky octave-and-a-fifth when blown hard, whereas the sax jumps an easy octave. The saxophone is easier to play, but it’s very hard to make. The Saxotron, however, is easy to make. Here’s how:

Now there are slide saxophones, although they are rare beasts. But it’s complicated doing a keyed instrument like a standard saxophone, so use a slide resonator instead.

Two tubes of the same length, 30-100cm, one fitting inside the other and 20-40mm in diameter are needed. A 32mm plastic drainpipe inside a 40mm drainpipe works, for example, and a 3m length of both – enough for four or five Saxotrons – is a bargain at £5.

On top of this, you need a microphone, an amplifier, and a small loudspeaker. More or less any variety of those will do, but the amplifier needs enough amplification or ‘gain’. It can be a simple unit, something like the TBA820 kit sold by various suppliers for schools. A microphone is conveniently the common electret type. You feed them a tiny current via a resistor (e.g. 10k), which goes to the 9V battery on the amp. The loudspeaker should be a small type that matches the amp, often 8 Ohms will do, and can be fitted inside a soda-bottle neck glued into the pipe (see image below).

Fit the microphone at one end, the loudspeaker at the other, then switch on your amp. Try the volume control if there is one, to see what it does. You can put a push-switch in the battery connector lead or the loudspeaker wires to turn a note on and off easily.

Play around and test it out. Load a sound spectrum app like FrequenSee (here are both Apple and Android versions) on your phone and check out the harmonics of the Saxotron at different positions. If your small pipe is white, you can easily mark the positions of different musical notes with a permanent marker pen.

Try reversing the loudspeaker connections: What do you hear now? With a bit of luck, you will hear the Saxotron change register: the Saxotron should play either the 1st harmonic or the 2nd harmonic, which is about an octave above the 1st. You could put a double-pole changeover switch in to do that. Now brush up on your playing skills. Duke Ellington, Gershwin, or Charlie Parker?

The Saxotron sound is generated by feedback. The microphone gives a tiny bit of random noise, it is amplified, and the loudspeaker gives 10x the noise back to the microphone, and then 100x the original noise comes back to the microphone, and so on... The process stops when the amplifier can’t give out more than the 9V battery allows. The frequency of the note is given by the to and fro time of the sound wave, which is set by the length of the resonator and the 340 ms^-1 speed of sound. At that frequency – or multiples of it – the wave is amplified.

Could you put three tubes inside each other to make an extended slide range? Does it work if you put the microphone nearer to the speaker end? And what about some filtering like some absorbent cloth lining the inner tube, so that lower harmonics are favoured over higher ones? Or an electronic filter: capacitors and resistors to filter the sound being fed back?

If you liked this, you will find lots more fun science stuff in Neil Downie’s books, like ‘The Ultimate Book of Saturday Science’ from Princeton University, and for lots of other things (and a free copy of the ‘Exploding Disk Cannons’ book), visit www.saturdayscience.org

There is the back catalogue of Lockdown Challenges from the past year to choose from if you are looking for more options. The IET also has a host of resources that adults can use to engage children with the world of STEM.