I did not see it clearly in the video: the 'wa wa wa' effect he refers to is due to beat frequency [1] that is produced when two frequencies are present. Also

I did not see it clearly in the video: the 'wa wa wa' effect he refers to is due to beat frequency [1] that is produced when two frequencies are present. Also, the video alludes to this briefly but worth repeating: if the bell (or the pipe) is perfectly circular, i.e. radially symmetric, there wouldn't be two separate modes as demonstrated (higher frequency harmonic modes would still exists at lower amplitudes, but not relevant to discussion here). Grinding the bell down on the sides is breaking the symmetry, and thus, separate modes. Coincidentally, Joseph Fourier studied harmonics using a metal ring and a heat source, analogous to the circular bell shown here. With a heated metal ring, you can visually see the different modes (including higher harmonics). [1] https://en.wikipedia.org/wiki/Beat_(acoustics) https://en.wikipedia.org/wiki/Beat_(acoustics)