The Gong Nghiem theory

This sliding whistle experiment is a nice one (one day  will have to disinfect those whistles I think). The note from the whistle is analysed using audacity. The original worksheet has instruction on how to analyse the actual waveform but that was before I discovered the frequency spectrum which makes the analysis much easier. This morning two students were doing this lab (Nghiem and GongPei) and they noticed something interesting. Actually I'm not sure if they noticed it or if it was me. It might have been that I noticed it as I was explaining the spectrum on their computer scree. So, who owns the discovery, the person who spots the interesting feature or the one who took the data and has it stored on their computer? Without the data I suppose it's difficult to publish the findings, can't really report that I saw the data as I was passing the computer screen but don't have it any more. Anyway problem solved, I photographed the data with my phone and here it is.

It's actually just a bit of the data but it shows the interesting feature. The whistle is a closed pipe so has only odd harmonics as expected, and pretty clear ones they are too. The first big peak is at about 800Hz the second big one is at about 2400 Hz etc. But what are those small peaks doing there? They are even multiples of the 1st harmonic which would mean that there would be an antinode at the closed end. GongPei and Nghiem came up with the theory (joint honours) that since the plunger doesn't fit exactly into the tube then the end could be acting as an open end as well as a closed end. This theory was supported by the lack of even harmonics when the plunger was fully withdrawn. At this position the plunger seals the tube so acts as a more perfect closed end. The Gong Nghiem theory.