Brian,
I
think the emphasis on getting the harp deep in the mouth is not
necessarily only to promote internal resonance, but also to simply
physically seal off more holes on the harp from the outside air. If
someone wants a fully dark and compressed tone, they need to use some
combination of their lips, tongue, face and hands to completely seal
off all the mouthpiece holes and the coverplate openings. It's not
necessary to use all four--that's why lip blockers can get a good
tone--but it's all gotta be relatively sealed up to create the darkest
end of the tonal spectrum, and getting the harp deep in the mouth takes
care of a lot of holes that then don't need sealing off by the hands,
cheek, etc.
That's
especially important if someone wants that full-on Big Walter/Dennis
Gruenling tone. But Kim Wilson's a good example of someone who leaves a
bit of the coverplates open on the end and works the remaining tonal
spectrum well. Ditto for Little Walter, I think; the clue is whether
the high end of the harp sticks out of the hands or not. Filisko
discussed these two schools at his tone seminar at the Minneapolis
Summit in 2000. Either way you do it, closing off mouthpiece holes is
one key to creating the dark end of your tonal palette, and getting the
harp well into the mouth does that automatically. The teeth are really
shaped like a half-circle in front, so pushing the harp deeper
automatically creates a bigger chord (geometry, not music) across that
semicircle, covering up more holes along the mouthpiece (it's part of
blocking octaves too, for example).
The way I
understand it, the short-wavelength high freqs travel mainly through
the air, so you are trying to develop a relaxed position where you use
your body to damp out that "acoustic-sounding" element of harp tone by
sealing off the instrument from the open air, yet can open your cup for
brighter tonal shadings as part of your phrasing when you want to. The
dark, deep, compressed end of the amplified harp tonal spectrum is the
part that's "artificial," if you think about it, not what the general
public thinks of as the natural sound of the harmonica, so it's the
part that will require more conscious effort when you first start to
learn it: how you are holding the harmonica, how deep it is in your
mouth, your internal resonance, how you are sealing things up. The
bright end of the spectrum sort of takes care of itself if you reverse
that: open your hands, let the focus of your internal resonance move
forward in your mouth toward your front teeth, and things will brighten
right up. I'm not sure whether I habitually move the harp to a less
shallow position in my mouth to get brightness. All this stuff, at some
point, will be taken over by your body and executed unconsciously if
you focus on the tones you are getting. It's kind of a Zen thing: Play
the music, not the harmonica, not the mic, not the amp. Imagine sounds
you want to hear, and you'll be surprised how your body can deliver
them without conscious directions. You are just trying to train the
habits that deliver you to that point.