It’s not mentioned, but the bourdon pipe (used for the lowest pitch pipes) is a stopped pipe, which means it overblows at the twelfth rather than the octave, wh

It’s not mentioned, but the bourdon pipe (used for the lowest pitch pipes) is a stopped pipe, which means it overblows at the twelfth rather than the octave, which is why you don’t see the second harmonic in the frequency analysis like you do with the two shorter pipes.