The dimensions or adjusted dimensions of the inner bore create or decide most of the sound of a shakuhachi. For example, if a maker crafts a shakuhachi in one-piece and leaves the inside mostly or all-natural (jinashi) then nature determines most of the sound, though it’s up to the maker to bring that sound out and balance it. Alternatively, if the length is adjusted by cutting and jointing and or if the inner bore is shaped by applying paste, then the character of the original bamboo can be slightly augmented or completely changed. Finger-hole placement and size determine the pitch, what techniques will work or not work, how well techniques will work, the volume, and a good portion of the sound. The dimensions of the blowing-edge or “utaguchi” also plays a large role in the overall sound and play-ability with mere millimeters of change to this area having a profound effect. In the end, it takes a skilled player with intimate knowledge of traditional music and of what a balanced and authentic shakuhachi is and is not.
As for the material a flute is made from, science has proven that flute bodies do not resonate in such a way that contributes to the sound, however, many people believe the opposite. This snippet of a debate from a shakuhachi forum is a good example of the kinds of discussions on this topic that will most likely continue for as long as humans play flutes http://shakuhachi.wikidot.com/material