Native" works, though if you replace it with the word "Resonant" you will get less sideways glances and raised eyebrows from the this-is-our-club-o-phile crowd.
The Resonant Frequency of an acoustic guitar, specifically, the natural vibrating frequency of the body, the frequency at which the body of the guitar vibrates the most in response to, is typically somewhere between 80 Hz and 120 Hz, with the "nicer, more pleasing" sounding instruments ranging between 95 Hz and 105 Hz (G# on the 6th string). Hum that note (or thereabouts) into the soundhole of your guitar (privately, or you could elicit very concerned looks from well-meaning loved ones, friends, or passers by), vary the note up and down, and you will experience it for yourself when you feel the guitar vibrate in unison as you nail that pitch. This number is referred to as the First Fundamental Resonant Frequency, and is a primary contributor in determining whether we like or dislike the overall sound of a particular guitar. (For those who care) there are actually two additional frequencies associated with the "Resonant Frequency" of a guitar; namely, the Helmholtz Frequency, also called the "Anti-resonant" Frequency, and the Second Fundamental Resonant Frequency. But for most arguments... er... discussions, the term "Resonant Frequency' is generally used to refer to the First Fundamental Resonant Frequency.
And here is where things get fun, as there are so many rabbit trails to explore:
Hertz measures vibrations per second. What/which vibrations are we measuring? Short answer: lots a' stuff, together. When attempting to convey sound generation of an acoustic guitar to the unwashed hordes, the soundboard, specifically, the area of the lower bout from the bridge to the tail, is often likened to a speaker cone, with the sides and back of the guitar being likened to the speaker cabinet. The metaphor works insofar as everyone is realizing that, unlike the speaker cabinet, the body of the acoustic guitar is also vibrating generously along with the soundboard. Various sections of the body will vibrate distinctly from other sections, moving in conjunction with the energy applied to them (plucked string, hand tapped surfaces, etc). Various shapes (called mode shapes or eigenvectors) are formed as these structures vibrate. The collective relationship between these constantly changing shapes, and our appreciation or disdain for the role they play in "conditioning" the overall sound of a given instrument, is a fascinating study.
There was a time when it was believed that, in order to achieve "proper" sound for a guitar, it was necessary to try to isolate the soundboard for vibration (like the speaker in the cabinet). So-called "ladder bracing" (the four braces glued to the inside of the back, running perpendicular to the string path) formed a very rigid plate (though that plate still vibrates!). Like most guitar manufacturers, Guild ladder braced the backs of its guitars and opted not to brace the sides. However, in an exploratory break from tradition, we discovered that Guild's Archback guitars have a distinct and welcome sound, don't they? The laminated arched back vibrates differently, forming different mode shapes, when compared to a ladder-braced sibling.
Where builders are concerned, the factors that most govern the overall "native" frequency, our First Fundamental Resonant Frequency, are the volume inside the box (determined by length, width and height), the size of the soundhole, and the material(s) used for construction. For a given body shape, such as a Guild dreadnought, altering the depth of the body will dramatically alter the Resonant Frequency. Likewise, leaving the depth alone but altering the size of the soundhole will dramatically alter the Resonant Frequency. The relationship between these two factors, body depth and soundhole size, is crucial. To a lesser extent, though still a factor, altering the wood species will affect the Resonant Frequency.
Loosening the bracing on the rigid backs, making the backs more "responsive," also affects the Resonant Frequency, though this moves quickly into the more subtle realm of the Second Fundamental Resonant Frequency and the corresponding Helmholtz Frequency. Uber-rigid sides and un-coupled backs also make for a super-interesting study. "Squirrel!" {sorry}
When we play the guitar, we complement the Resonant Frequency, more easily measured with the soundboard at rest, with the potential cacophony of frequencies generated when the soundboard is set in motion (some of us appear to be consistently more capable of generating cacophony than others). The bracing on the soundboard (including the bridge and bridgeplate) governs the response of the energy applied from the plucked strings. All that movement of all those sections of wood is producing more than a recognizable note or notes. All those mode shapes, taken together as a whole, provides us with what we generally identify as the overall "sound" of that guitar. Cool, huh?
