What is "sound hole"

n. a hole in a stringed instrument that is shaped like an "f"

n. a hole in a soundboard (as of a violin) designed to resonate with the tones

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 and traditional bowl-back mandolins; F-holes in instruments from the violin family, archtop mandolins and in archtop guitars; C-holes in violas da gamba, and rosettes in lutes. Bowed lyras have D-holes and mandolins may have F-holes, 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. Most hollowbody and semi-hollow electric guitars also have F-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.

In 2015 researchers at MIT published an analysis charting the evolution and improvements in effectiveness of violin f-hole design over time.