Lockdown Challenge: Making sweet music with the swanee banana

Flutes and whistles have been made since we all lived in caves. Tucked away in a museum in Stuttgart is the world’s oldest musical instrument, a 40,000-year-old bone flute found in a cave. It was made from a previous owner of the cave: a cave bear.

Generally, when you make a whistle bigger, it makes a lower sound. So, when you go from a pea whistle to an ‘owl whistle’ – made from a Golden Syrup tin with ping-pong ball – you go from a shrill 2 or 3kHz to a ghostly 600Hz. You can see this in most other whistles too.

Whistles have a ‘fipple’, where a sheet of air impinges on an edge, and the air can go above or below the edge, flicking to and fro generating sound waves. Whistles have a resonator tube that reflects a sound wave back to the fipple, and this reflected wave, going at 340ms^-1 (750mph) takes time to get back, when it will reinforce a fipple wave. After a few cycles, only fipple waves which are related to that wave travel time and multiples of it are amplified and we hear tones at frequencies corresponding to (1/travel time) and its multiples, the harmonics. So a 17cm whistle has harmonics at 1000, 2000, and 3000Hz. Twirl-a-Tune pipes do this too, as we once proved when a group of us took Twirl-a-Tunes of different lengths onto the stage at the Lyceum Opera House in Crewe and played a tune.

In a similar way, the Swanee whistle frequency goes up and down as the piston shortens and lengthens the tube. Its harmonics are different because the piston end is a closed-end, unlike the open end of most whistles. So the 17cm Swanee has harmonics at 500, 1500, and 2500Hz. But the tube length effect remains.

Now let’s make two entirely new musical instruments: the Swanee Banana, and the Swanee Banana Split. These achieve an adjustable length resonator in a different way to the usual Swanee.

In the Banana Split version, a 20mm diameter x 150mm or so resonator tube – plastic conduit – is added to a tin-whistle fipple. Before gluing or taping the tube on, though, you have to put a split in it. Cut down the length of the tube with a Stanley knife (watch your fingers). Spread the far end – the open end – of the tube out so that it will sit at 6 or 7mm apart. Now try blowing it, and widening and narrowing the slit by squeezing the sides together. You should get a range of notes, lower as you close the split, e.g. from 800 to 1100Hz or 1600 to 2200Hz. Download an audio spectrum app like FrequenSee onto your phone and take a look at the frequencies and the harmonics of your Swanee Banana Split.

To make a Swanee Banana is even simpler: just glue a 300 or 400mm length of garden hosepipe onto a tin whistle fipple. Now blow to get the different harmonics, and try pinching the hosepipe sides together partially at different positions along the hosepipe. Look at the harmonics on FrequenSee when you pinch the tube closed.

Now the Swanee Banana Split whistle does what you expect. Wider split → shorter resonator effective length → higher tone. But the Swanee Banana doesn’t do this. You can get a higher tone when you squeeze the tube. But you can also easily get a lower tone when you squeeze the tube. This is the Swanee Banana mystery. What is going on here?

The pics below show the Swanee Banana spectrum playing first, then second harmonic.

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 also the back catalogue of Lockdown Challenges from 2020 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.