Sound Hole — Lutherie Encyclopedia

Sound HoleType: glossaryThe opening in the soundboard that couples the internal air cavity with the outside air, enhancing bass response and projection.## Definition The sound hole is an opening cut through the soundboard of an acoustic instrument, most commonly a circular hole centered on the upper portion of the lower bout in flat-top guitars. The sound hole serves a critical acoustic function: it couples the air inside the body cavity with the outside air, enabling the Helmholtz resonance that is primarily responsible for the instrument's bass response. The body cavity acts like an air spring, and the sound hole is the port through which air moves in and out, amplifying low-frequency energy. The size and placement of the sound hole significantly affect the instrument's tonal balance and volume. A larger sound hole generally boosts bass and overall projection but can weaken the structural integrity of the soundboard. A smaller hole reduces bass but can increase midrange clarity. ## In Practice Sound holes are cut during soundboard preparation, typically after the rosette has been installed. The luthier uses a circle cutter, router, or sharp knife to cut a clean, precise opening. The standard diameter for a steel-string acoustic guitar sound hole is approximately 100mm (about 4 inches), though builders experiment with different sizes. Some modern builders have moved the sound hole to alternative locations, such as the upper bout or the side of the body, to maintain structural integrity in the soundboard's most acoustically active zone while still providing the necessary air coupling. Sound hole covers or plugs are sometime…