The Science Behind Acoustic Guitar Soundholes

The Soundhole Is a Helmholtz Resonator

An acoustic guitar body is a Helmholtz resonator, a closed air cavity with a single opening, and the soundhole is that opening. The body resonates at a fundamental frequency determined by air volume, soundhole diameter, and the stiffness of the top plate. On a standard Martin dreadnought, this frequency lands around 90 to 110 Hz, which corresponds to the low G or A note on the guitar.

This resonance is why dreadnoughts have their characteristic low-mid punch. The body air column reinforces frequencies around 100 Hz, boosting them by 3 to 5 dB compared to a smaller-bodied guitar like an OM or concert. Understanding the physics is not academic: it tells you why certain notes on your guitar sound louder or duller than others, and what happens when you change the soundhole.

Soundhole Diameter Directly Controls Resonance

A standard Martin dreadnought has a soundhole diameter of 101mm (4 inches). A Taylor Grand Auditorium uses 95mm (3.75 inches). A classical guitar uses 85mm (3.35 inches). Each 5mm reduction in diameter drops the Helmholtz resonance by roughly 3 to 5 Hz, which is why classical guitars have deeper, rounder low-mid voices than steel-strings.

Taper the hole toward the top edge and the effective diameter drops further, producing an even warmer tone. Some boutique builders like Ervin Somogyi experiment with elongated or tear-drop soundhole shapes specifically to tune the resonance independent of the diameter, achieving body responses that cannot be replicated on a standard round hole.

Soundhole Position Shapes the Top's Vibration

On most acoustics, the soundhole sits roughly 90mm below the upper bout and 150mm above the bridge. This position is not arbitrary, it marks the natural node line of the top's dominant vibration mode, where the top surface has minimal movement. Cutting the hole at this position preserves the top's ability to vibrate in its most efficient pattern.

Modern builders like Manzer have experimented with moving the soundhole to the upper bout (called an 'upper bout soundhole' or 'side soundport') to open up different parts of the top for vibration. These designs produce a distinctly different voice, more articulate, slightly brighter, at the cost of reduced volume directed at the listener.

The Rosette: Decoration or Structure?

A rosette is the decorative inlay around the soundhole, typically made of wood or plastic strips, mother-of-pearl, or abalone. On high-end guitars, the rosette is purely decorative, it adds visual complexity without structural function. On cheaper guitars, the rosette sometimes serves as a reinforcement because the soundhole area is structurally weak.

The most famous rosettes in acoustic history are the rings of colored wood on prewar Martins and the herringbone on pre-1947 D-28s. These became identifying marks rather than just decoration, and modern reissues carefully replicate the exact strip spacing and colors. A Martin HD-28 with correct herringbone is worth $300 to $500 more than one without.

Side Soundports: The Modern Innovation

Side soundports, small holes cut into the side of the guitar body facing the player, have become a luthier favorite since the 2000s. Breedlove, Greenfield, and many boutique makers offer them as standard or optional features. A typical side port is 30 to 50mm in diameter and is positioned on the upper bout side near the neck heel.

The player hears a fuller, louder sound with a side port because they are positioned in the direct sound path of an air column that would normally exit only through the main soundhole. Audiences in front of the guitar hear essentially the same tone; the benefit is mainly for the performer's monitoring. Tommy Emmanuel's Maton EBG808 has a side port, and he credits it with making on-stage monitoring much more accurate.

Modifications and Feedback Control

The most common soundhole modification is a feedback cover, a rubber or foam plug that reduces the effective hole diameter to control stage feedback. The McPherson Cover and Planet Waves Screeching Halt drop low-frequency feedback by 8 to 12 dB by reducing the Helmholtz resonance strength, essentially detuning the body's natural howl frequency.

Permanent soundhole modifications are rare and almost always reduce the guitar's value. Enlarging or relocating the hole changes the resonance irreversibly and cannot be undone without serious repair work. If you experiment with modifications, use removable covers or clips rather than cutting or drilling.