I notice that on some of my whistles are very reluctant to let me play a low D#, or its cognate on a non-D whistle. But there’s generally no problem producing that D# an octave up. The Parks Ghost whistle is quite friendly this way (a Friendly Ghost, you see); the Mellow Dog is more truculent. I don’t understand why half-holing this particular note should present any special difficulty, but for some reason it does.
Ubizmo
The sixth hole is in an unfortunate position, it is far higher up the tube than it would be good for optimal acoustic tone. It is up there so we can reach it. But this compromise means that the hole is a lot smaller than would be good. So the second note (E on D whistle) is suffering from poor venting. It is usually the weakest note because of this. Half holing the sixth hole results in an even poorer vented D#/Eb. Some whistles won’t like this. Compare that with half holing a F nat, which works so much better because of the big fifth hole.
Whistles with a relative big sixth hole are easier for a half-holed D#/Eb.
I suspect the problem may have something to do with the hole’s proximity to the open end of the tube. There’s nothing about the position of that tonehole on a Mellow Dog that’s been adjusted to try to move it further up the tube for ease of fingering (not an issue on a high D whistle anyway).
However, in working on designs for other whistles, I’ve observed that the performance of the two bottom toneholes can be improved by using an eight hole design based on the design of eight hole flutes. The eight hole design has two bottom tuning holes that are not played. They move the bottom two toneholes that are fingered, further away from the open end of the tube, which gets them to behave like toneholes further up the scale.
On a Mellow Dog, the bottom notes are good and strong, and no one’s ever had any issue with them, so I’ve never thought about doing them differently. You’re the first person ever to mention this particular concern.
Best wishes,
Jerry
I’m not sure I buy the premise, ubizmo. I just checked the low D# (xxx xx@) on a bunch of my own whistles in different keys - including my Parks Walkabout and Freeman Mellow Dog. And I can’t find a single one where it is a problem.
The bell D is a “delicate” note on any whistle, and the half-holed D# shares roughly the same tonal quality as the low D or E. Otherwise, it sounds just fine, half-holing the side edge of the B6 hole with either the fingertip or knuckle.
To check Jerry’s interesting observation, I used my SZBE D whistle, which is convertible from 6 holes to 9 holes with the extended foot. And I find no difference in the clarity of the low D# in either configuration.
In the absence of other information, ubizmo, I’d suspect that the problem is with your fingering, not the whistles. If you’re “flat-fingering” the half hole, try the tip/knuckle technique I described above.
I just attempted to reply, but for some reason my post didn’t “take.”
In my attempted post, which was written before I read this, I also considered that the phenomenon could simply be an artifact of my technique. I don’t want to elevate it to the level of a “problem” or even a “concern.” Since I don’t know any whistle music that requires that low D#, my question is mainly academic.
Yes, I realize that the low D is a fragile note on most or all whistles. My observation is that on some, at least, the low D# is more fragile, and that’s puzzling to me. On my Kerry low D, I find it just about impossible to play, but I have no trouble getting the low D, or low E, or half-holing other notes (including higher D#). I just tried again with the Mellow Dog, and was able to get the note, but still pretty anemically. My Generation Bb seems worse. On the Ghost high D, however, the low D# sounds just like the low D and E. The holes on the Ghost are bigger than the holes on the Mellow Dog, but not bigger than the holes on the Generation Bb.
At any rate, low D# may be the least important note on a whistle, and maybe no one else has any difficulty. One reason for posting was to find out.
Ubizmo
Well, there’s certainly nothing wrong with exploring the issue by both experimenting for yourself and asking around. 
Again, I tried my Bb Gens, and the D# sounds fine to me. I don’t have a Kerry to check.
You could do this: Try taping over the B6 holes with electrician’s tape in various positions, and see if you can find placements that give you a good D#. If so, then you know that the problem is with your fingering.
And actually, that D# is not so rare in my own playing. For example, it seems to come up often enough in E minorish tunes - as a lower neighboring tone, or as a substitute when the melody drops to B below the range (as in Lads of Laois).
I think any “delicacy” of the bottom note varies widely from whistle to whistle. There are no design reasons not to have a strong bottom note, a note which can be pushed without much fear for it to slip into the next octave. I have not observed a whistle for which the half-holed D# is not weaker than the D or the E in the bottom octave, so I would not say that it shares the same tonal quality than it’s neighbours.
I can play D# (B in fact) fine on a Generation Bb, but it is weaker than the bottom bell note, it is a bit quieter and can’t be pushed as the neighbouring notes can. It works well enough as a transient note, but not as a principal note.
The weaker D# is accentuated on low whistles, for which the sixth hole is smaller than acoustically desirable. I can understand that half-holing D# on some low whistles can fail, because of this, as it can and does on D flutes quite often, particularly if the low whistle is designed to accommodate shorter hole spacings for smaller hands.
Hmm. I suspect a connection with the fact that D# is sufficiently unsatisfactory on a flute to have made that the first extra key that people found necessary.