From The Art and Popular Culture Encyclopedia
A sound hole is an opening in the upper sound board of a stringed musical instrument. The sound holes can have different shapes: round in flat-top guitars, F-holes in instruments from the violin or viol families and in arched-top guitars, rosettes in lutes. Bowed Lyras have D-holes and Mandolins may have round or oval holes. A round or oval hole is usually a single one, under the strings. F-holes and D-holes are usually made in pairs placed symmetrically on both sides of the strings. Some electric guitars, such as Fender Telecaster Thinline and the majority of Gretsch guitars have one or two sound holes.
Though the purpose of sound holes is to help acoustic instruments project their sound more efficiently, the sound does not emanate solely (nor even mostly) from the location of the sound hole. The majority of sound emanates from the surface area of both sounding boards, with sound holes playing a part by allowing the sounding boards to vibrate more freely, and by allowing some of the vibrations which have been set in motion inside the instrument to travel outside the instrument.