The neck and fingerboard

​​A:  I disagree with your hypothesis that the fingerboard's swelling and shrinking causes the neck to back- or forward-bow. The course I audited with Professor Bruce Hoadley, a Wood Science and Technology Professor at the University of Massachusetts Department of Wood Technology, taught me that wood expands and contracts perpendicular to the grain direction, not longitudinally with it.

     So in response to changes in ambient humidity, the fingerboard will expand and contract in width, not in length. Thus, the explanation for what you observe doesn't seem to square with what I've learned about wood's behavior.



     My experience with high or changing action on classics does not point to the neck curving forward as a result of string tension, but of other distortion elsewhere on the guitar which causes the action to rise and fall—and would not be resolved by simply tightening a truss rod (if one were there).

     My initial response that characterized classic truss rods as "silly" referred to the ADJUSTABLE kind of rod. The tension arising from what essentially amounts to six strands of taut nylon fishing line (about 90 lbs. at concert pitch) seems insufficient to significantly distort a foot-long lamination of solid mahogany and ebony.



     I hasten to caution that when you so readily ascribe specific acoustic effects of what you're doing as a fact, ("as the neck absorbs half of the energy of the vibrating string and transfers high frequencies back into the mix of sound in an instrument") when presumably) you're not a trained acoustician. Even an acoustician would hesitate to make this claim because they know just how complicated the system is.

     So, within musical organology and instrument acoustics, there is no universal consensus that stiffening a guitar’s neck unconditionally improves its response. But if you want to believe that, go ahead.

    Granted,  neck stiffness can affect the instrument’s resonant profile. Because the guitar functions as a system of coupled oscillators, modifications to the neck fundamentally rearrange the physics of the entire chassis. But one luthier’s "improvement" is often another’s loss of traditional character.

     Proponents of highly rigid necks—typically achieved via carbon fiber reinforcement rods, multi-laminate construction, or dense tonewoods—base their philosophy on the law of energy conservation. When a string vibrates, it transfers energy not only to the bridge but also to the neck via the nut and frets. A flexible neck yields to this movement, absorbing vibrational energy through internal damping and converting it into heat. By increasing the neck's mechanical impedance through stiffening, the structure resists this compliance. This forces a higher percentage of kinetic energy to remain within the strings and transfer directly to the soundboard, resulting in a sharper transient attack, increased sustain in the upper registers, and the mitigation of "dead spots" caused by sympathetic neck frequencies. 

     Conversely, a significant camp of researchers and traditional luthiers argue that an overly rigid neck robs an acoustic instrument of its organic warmth, yielding a sterile or clinical voice. Ultimately, changing neck stiffness does not inherently perfect the instrument; it merely recalculates the acoustic equation, shifting the balance between focused efficiency and compliant, coupled resonance.

     Indeed, a bonafide acoustician would be far more hesitant to explain ANY acoustic phenomena on a guitar as easily as you apparently can. Don't take it personally--it's a quirk of the profession: luthiers readily and regularly do just that.  I've learned just enough about guitar acoustics from Tim White, editor of the Journal of Guitar Acoustics, to be very cautious about spreading notions about acoustic phenomena around, as if they were fact: there is very little known for certain about how guitars work and the energy interactions within them. Guitars are enormously complex energy transforming devices. Very little is known for certain, and much of what you read about guitar acoustics in the guitarmaking media is just...made up.