A crown is a slight convex shape given to a soundboard and it's synonymous with archtop. A crown gives the soundboard springiness and responsiveness.
Greek, Turkish, Persian and Hungarian makers have been putting a crown to their soundboards by building a gentle curve on the top and bottom rails, where the soundboard comes in contact with the body.
American dulcimer makers add two steel rods under the soundboard, at the approximate location of each bridge. This raises the center portion of the soundboard and creates a crown.
The rods allow the player to voice the instrument by slightly adjusting the relative angle of the rods to the bridge above. That doesn't mean that the instrument is readily adjustable. Adjusting the rods requires loosening the strings, moving the rods by no more than an eighth of an inch, retuning the instrument and testing the voice. The procedure can be repeated for as long as the player has the patience to retune.
Also, the specific timbre of the instrument is determined by its design, construction and materials. Voicing through the rods has a rather subtle effect. Tuning has the greatest effect. An even slightly out of tune instrument will sound awful regardless of timbre, voicing or anything else.