The mountain or lap dulcimer is a very traditional instrument whose popularity has come from simplicity and - please no immediate howls of rage - low expectations. As usually constructed, it violates just about every rule in the book; and if you try to apply too many "rules" you end up with something not quite a dulcimer - at least to the traditionalists.

The tone you'll get depends quite simply on the internal volume and the total cross-section area of the holes you put in it. The bigger the internal volume, the bigger you can make the holes. The bigger the holes, to some extent, the louder it will be. Unfortunately, if you make the holes bigger to get more volume, it is quite easy to "de-tune" the thing so it sounds more like you're beating on a log than playing music.

Of course, to get sound out of the holes, they have to be "pumped" by flexure of some - preferably large - surface of the box. The traditional method of gluing the fingerboard solidly, along its full length, to the top of the box (we don't call it a soundboard at this point) ties the "best available" pumping surface into a rigid bar.

A traditonal lap dulcimer has no bridge in any conventional sense. The string can transmit vibration into the nut, or into the fret that you press it against. Either of these points is far too rigid to "move" in the conventional construction, so the sound must be transmitted as a "conduction wave" through the wood until it hits something that can bend.

You can make a significantly louder dulcimer - all things else being equal - simply by relieving the bottom of the fingerboard so that it contacts the top of the box at only one, or a few, localized points, thereby actually letting the top of the box move (in flexure) more freely. Moving the nut an inch or two out onto the top of the box, with a small contact directly under it should also help some.

Be aware that gluing the fingerboard down, as in the conventional configuration, contributes a lot more than one might expect to the stiffness of the fingerboard. If you leave it unattached (localized attachemts), you will need to add a lot of thickness to the fingerboard to keep it from warping under string tension. (Because its not uniform in thickness if you relieve much under it, the fingerboard won't just bend - it will assume some obscene and unexpected shape guaranteed to make fingering a torture test. Also makes it difficult to use a truss rod to compensate.)

In a conventional construction, it makes little difference what wood you use in the box, since the attachment of the fingerboard prevents the top board from doing much. If you can come up with a configuration that actually leaves the top board free to move in bending then you can call it a soundboard, and can start thinking about quarter-sawn spruce (which does work quite well - with the above mentioned modifications).

Be aware that what you have, if you proceed as above, won't be a mountain dulcimer to the traditionalists. If you really want loud rather than observing the traditional charm of the instrument - get a banjo.

John