Any acoustic musical instrument can be considered a finely tuned machine. The fascination of instrument building is creating a "machine" that is also a unique character with a unique voice that will change and appreciate as it ages and is played. The mechanics of the guitar are such that : as finger strikes string , imparting energy, the string vibrates creating a wave that has its nodal points at 12th, 5th, 7th fret etc.( i.e. where you find your harmonics) Regardless the energy of the string activates the bridge bone, rocking the bridge itself, which in turn activates the soundboard and to a lesser extent the guitar body itself. Additional energy is transferred through the nut and neck. Neck materials affect the sound of a guitar marginally. Vibrations travel through the soundboard, dispersed through the bracing pattern and along the longitudinal winter wood (grain lines). Along the way energy is lost, absorbed by the mass of the wood. All joinery and fitting is precise and perfect to minimize this loss. Finishes are kept lightweight to avoid loading the soundboard. The vibrating soundboard in turn, agitates the air column within the guitar body. Sound waves bounce about the interior of the guitar being absorbed by the pore structure and reflected by its contours. (think of clapping one's hands in a newly drywalled room and in the same room furnished with carpet etc.) Mario Maccaferri's innovation was to create a second soundbox to contain treble notes and to bounce them off a reflector and into the audience thereby cutting through the sound of an orchestra. The sound waves eventually bounce out the soundhole to reach us as auditory sensation.
Harps are a little different. The harp is characterised by a large sound box and a relatively narrow sound board. String tension is massive. When your fingers activate the string the energy is transferred directly to the soundboard which activates the air column within the soundbox. The massive tension associated with the harp means that it is an instrument characterised by volume and a rapid decay of the note.
All wood has hydroscopic properties that have to be accounted for when building an instrument. The effects of humidity are accounted for by the use of book matched pieces for the soundboard, back and sides. Changes in humidity change the moisture content of the thin wood of the instrument. Extended periods of wet weather may raise the moisture content in the air. Wood will expand under these conditions. Heat from a furnace , baseboard or woodstove dries the atmosphere. Wood contracts under these conditions. The effects of humidity are minimized by using well seasoned vertical grain timbers and a stable shop environment during construction. By utilizing book matched pieces, each side is structurally similar and reacts similarly. The chances of distortion or cracking being minimized. It is also a great decorative opportunity, creating a mirror image. Any instrument made of solid wood (as opposed to plywood) will react according to these principles. They should be kept in their case and extremes of temperature and humidity should be avoided. This means don't set your prized and much loved instruments next to sources of heat (fireplace, baseboard heaters, heat ducting , car interiors in summer etc.) because its asking for trouble!
All my instruments are designed with the eventual "sound " in mind. I strive for a robust , well balanced instrument with a clear voice. The soundboard is about 80% responsible for the eventual character of the instrument. In the case of guitars, each soundboard is manipulated and selectively thicknessed by hand to achieve good balance between bass and treble notes. This process is continued through out the construction of the soundboard. Braces are scalloped, pared and polished. Mass is pared away from the soundboard and braces. The reaction of the soundboard is monitored through constant testing with tap tones. Lightness of construction must be balanced with structural integrity. There is a tremendous amount of string tension focussed on a very thin soundboard.
Harp soundboards are tapered so that the lighter stringing of the upper registers has less soundboard to activate. Similarly the more extreme tensions associated with heavier stringing require a thicker sound board. Different woods have different strength to weight ratios and this also must be accounted for in the soundboard construction.
I use a Spanish joint at the neck and body of the guitar. This means the guitar is built up side down and the neck is joined integrally with the body. The sides of the guitar slide into slots in the neck block . I believe this joint is superior acoustically and structurally to the use of a dovetail joint.
I favour French polish or a hand rubbed oil finish. I feel these finishes are superior acoustically. The French polish is a process of applying multiple thin layers of shellac. When complete and properly done it has no equal for providing a very tough protection for the instrument, minimally inhibiting its response and providing a deep mirror like gloss. Its refraction allows one to feel as if you were peering deep into the grain of the wood. The down side is it is not impervious to alcohol.
Oil finishes (as pertaining to instruments) come in two types those that penetrate and those that build and bond to a surface. Tung oil is a finish that builds on top of the surface. It also greatly enhances the grain of any wood it is applied to. It can be finished to a high gloss or to a more subdued matte finish. Again multiple thin layers create a tough "wet looking " finish that minimally inhibits the soundboard.
Danish oil is a finish for necks. It penetrates and polishes, enhancing the wood, rendering it smooth for fast slick playing.