Design is comprised of many features. In lutherie there is no "silver bullet" that guarantees success. Rather, success is brought about by hundreds of design details and methods all working in concert toward the same end.
The basic acoustic dynamic of the Appalachian dulcimer, the primary producer of sound, is the vibration of air inside the body, according to my friend and famed guitar luthier, Alan Carruth. The primary influences on this body of air are volume and shape. In what follows here, I will be discussing the dynamics of the Bear Meadow design, which I know intimately, and do not intend these remarks as a general critique of other dulcimer builders' practices and views.
This air volume is driven by vibrations from the top and fretboard, acting as a system. The fretboard/top system also drives, in a secondary manner, vibrations in the sides, back, and peghead. One can experience this secondary drive by grasping the peghead during playing, or touching any part of the instrument.
The back is an important component of this secondary dynamic. Proper bracing of the back will produce an Appalachian dulcimer which is not damped by the legs when played on the lap; it will sing with the same projection and volume as when played on a table.
To return to the fretboard/top system dynamics, the top is primarily driven by the fretboard, which, in turn is driven by the strings. The raised fretboard is an evolutionary step in the development of the Appalachian dulcimer, and changes the secondary acoustic dynamic. Such a fretboard touches the top with "feet" at defined places, thus constraining the top to move in the same vibrational mode as the fretboard is vibrating at that point. Additionally, in the top braces, consisting of a strap brace at the waist and bow tie braces at the major and minor bouts, stiffen the top at these points, giving emphasis of bass volume (in the major bout) and mid-range volume (in the minor bout).
The ability of the fretboard/top system to drive the air volume is greatly enhanced, in the Bear Meadow design, by isolating this system from the mass of the tailblock and peghead. This is done by
Another key factor in the Bear Meadow design is that every element is under stress. Ribs are cold-formed. Back braces have curved bottoms, putting a slight bow in the back. The fretboard has a bottom convex profile to slightly bow the top both laterally and longitudinally. And, finally, the fretboard and top are bent against each other during assembly, in a calibrated manner, so they must be held in a slight arch by weights and clamps while being joined. The effect is an exciting responsiveness in the Bear Meadow Appalachian dulcimer. One can experience this by tapping the sides with a finger: the strings will begin to audibly vibrate. You cannot adequately and completely evaluate a Bear Meadow instrument without touching it and playing it. Until then, you will be unaware of its responsiveness, dynamic range and projection.
Taken together with a general lightness of building (tops, ribs and back are typically .080" thick) these are, to my knowledge and practice, the key elements responsible for the rich voice of the Bear Meadow design.