Why Is There a Hole in a Guitar? Letting the Box Breathe

The body of an acoustic stringed instrument is a box of air with a hole in it, and that arrangement is a resonator in which the air in the opening moves as a mass against the springiness of the air in the cavity. Driven by the vibrating top, it resonates at a particular frequency determined by the volume of the box, the area of the hole and its effective depth, and that resonance provides much of the instrument's output at the bottom of its range, where the top plate alone radiates poorly. The same principle explains the note produced by blowing across a bottle, which is why the arrangement carries the name of the physicist who analysed it. Sound emerges from the hole and from the vibrating plates, but making an exit for sound is not what the hole is for.

The parameters interact and instrument makers adjust them deliberately:

• •A larger hole raises the resonance frequency and lets air move more freely
• •A larger body volume lowers it, which is why bigger instruments have deeper voices
• •A thicker top around the hole raises the effective depth and lowers the frequency
• •Sealing the hole entirely leaves the instrument quiet and thin at the bottom
• •Multiple smaller holes behave differently from one large one of the same total area
• •The hole also weakens the top, so its position is a structural decision as much as an acoustic one

The curved slots on a violin family instrument look decorative and are doing the same job in a different way. Their long narrow shape means that for a given open area they sit alongside a greater length of free top edge, which allows the plate between and around them to move more freely than a round hole would permit, coupling the plate's motion to the air more effectively across a range of frequencies. Their position and length also define the free vibrating area of the top and thereby its stiffness. The shape evolved over centuries from earlier circular and crescent openings, and analysis has suggested that the gradual elongation across the classical period tracked improvements in acoustic output, with the length of the opening rather than its area being the quantity that mattered.

Comparing instruments that work differently isolates what the opening contributes. A solid-body electric guitar has no cavity and no opening, produces almost no acoustic output and relies entirely on sensing string motion electrically, which is why it can be played at volumes that would make any acoustic instrument feed back uncontrollably. A banjo uses a stretched membrane rather than a wooden plate, with an open or partly enclosed back, and gets its characteristic sharp decay from the low mass of that membrane. A piano has an enormous soundboard and no enclosed cavity at all, radiating directly. A harp likewise. Each arrangement trades output, tone and controllability differently, and the enclosed box with an opening is simply the solution that suits an instrument that must be portable and loud without amplification.

Several practical and modern considerations attach to the opening. It is the access point for repairs, for fitting internal pickups and for inserting braces, which is a genuine constraint on where it can go. On amplified instruments it is also the path by which sound from a loudspeaker re-enters the body and drives the top, producing feedback, which is why players block it with a rubber plug when playing loud and why some instruments are built with offset or omitted holes for that reason. Makers have experimented with side-mounted openings that direct sound towards the player, with multiple small holes, and with adjustable ports. The label is traditionally placed inside where the hole makes it readable, which is how many historical instruments are identified.

The hole and the enclosed air form a resonator, with air in the opening moving as a mass against the springiness of the air inside, which supplies much of the low-end output that the top alone radiates poorly. Hole area, body volume and top thickness all shift that resonance. The elongated slots on a violin free a greater length of plate edge for the same open area.