Who knows the physics behind the existence of hard D? What factors go into its existence? I imagine it was purely accidental that it came about. I couldn’t imagine any pipemaker thinking ‘I’m going to invent a way of making the bottom note sound in two different ways’. More likely someone flicked their A gracenote onto low D and it gave a hint of hardness and so perhaps he went on to try and refine its existence. But what actually happens to the air column and reed head ‘stature’ to produce it?
That’s interesting because I have noticed that when I have a good reed, I can stop the chanter on my leg, lift it, and it will immediately play a hard D which I can repeat over and over - stacatto Hard D. On not-so-good reeds, stopping an lifting like this results in the high A ‘squeak’. This suggests that the not-so-good reed is not capable of locking into this 2nd harmonic but goes straight to the third harmonic. Hmmmm…
Cheers,
Ah now David, there are some who would take issue wit yerself for speaking such tings! As we all know the proper bottom note is in fact always a HARD D.
I think there’s some question about whether it’s a true “multiphonic” or not. My favorite pet theory about it is that it’s a chaotic phenomenon, rapidly flipping between two modes.
That’s reasonable theory. Given that to play a hard D you have to put the reed under more stress initially than with the soft D (the propper D ) my guess the hard D is occurring under some duress and that could account for its chaotic rapid flipping between the two (or more) harmonics.
I looked at the difference between hard and soft D using the TRANSCRIBE software (the very useful “slowing down” and “shifting pitch” software), which can be used to do a spectral analysis of the note. (Very useful by the way for determining exactly the pitch of each note). It shows that a soft D has primarily D harmonics, but the hard D also has a strong A harmonic, as far as I remember. I suppose this would explain why the hard D comes from an A gracenote. (Wish I could remember more of my acoustics!).
things certainly happen in the upper bore of the chanter. 
Hard "D"s they don’t come from the land of down under, they come from the land of all over. 
