A guitar soundboard and soundbox act as a transformer between the string vibrations and the human ear. It is not an amplifier, as there is no external energy being added. The tiny vibrations of the strings are coupled to a large, softwood plate which moves the surrounding air, while the hard back and sides form the air chamber of the soundbox,support and stiffen the soundboard and separate the vibration of its inner and outer surfaces.
This is all very similar to a loudspeaker, but while in Hi~Fi we try to reproduce the quality of an instrument that exists elsewhere, in a guitar, the soundbox IS the instrument, and it is the structure and materials of the soundbox that determine important parts of the music itself.
Each component part can provide a filter, modulation or resonance that modifies the overall sound of the guitar, and the individual character of even one piece of wood can colour every note that a guitar plays.
To understand part of what is happening with different timber combinations, consider stick and ball games. Those of you who play badminton or table tennis will know that the “ball” will fly away very fast with very little effort, but no matter how hard you try, you cannot get the ball to travel very far. Now think of golf ~ given a decent swing and timing, there isn’t a limit to how far you can hit the ball. That is one of the differences between a Mahogany guitar with a Cedar top, and a Rosewood guitar with a Spruce top. A light responsive instrument, or harder work but more possibilities. Horses for courses, not better or worse.
When it comes to choosing a particular piece of wood for a soundboard, neck or body, every maker works differently. Some tap and scrape the timber, listening for a particular clue in its sound. Others flex it, check the grain width and slope and proportions of summer to winter growth. Some measure the speed of sound and damping coefficients in the wood. Some are looking for particular properties, others have already chosen the wood, and are assessing how to use that particular piece. Some will be looking for the perfect piece of wood, while others will accept some small flaws if they believe it will otherwise make a good guitar (its not the trees fault if it has a beauty spot). Some will be looking for a visual and tonal match with the other components. I like to use engineering and scientific principles as much as I possibly can, but if it was purely a science, I could explain it more clearly. It isn’t and I can’t.