If you look at two guitars on the same wall, same model, same year, same strings, some pickups and some shop… you can still have one that rings, and one that dies.
Anyone who has spent an afternoon trying guitars has experienced this, and the explanation usually arrives before the note has finished decaying.
It must be the joint!
Of the wood!
Or the nut!
Somewhere deep inside the guitar that rings there is that magic ingredient that makes sustain. And the one that does no ring simply has less of it.
Now let me tell you a secret: there is no such part.
A plucked string gets all the energy it will ever have at the moment the pick excites it. Nothing after that adds anything. From the instant the string starts vibrating, then the accounting of what takes it it away more slowly starts.
The three exits
A string that vibrates can lose energy in three places.
The air
The string pushes against the air and drags on it, and a thin string does this less than a thick one.The string itself
Steel flexing millions of times turns the motion into imperceptible heat, and the windings on a wound string rub against each other. This is why older strings die earlier. The dirt between windings damps them from the inside.The instrument
This is the real one - the one that really metters.
The string is pinned at both ends to something that can move. The saddle on one end, and the nut or fret at the other. Every time the anchor moves, it takes energy away. If the string was perfectly rigid, an infinitely heavy guitar would dissipate basically nothing. But real guitars are (usually) wooden objects bolted or glued, and they move.
The note that falls through the floor
A neck is essentially a long springy beam, and like any beam it has a resonant frequency of its own. If you play a note whose pitch lands on one of them, the anchor stops doing the job of the anchor. The nick moves in sympathy with the string, and the string’s energy pours into the neck instead of staying in the string. This is why a certain note collapses in a second, while others ring for longer.
This is also known as a dead spot. It is not a defect of cheap guitars. It is a natural consequence of building a stringed instrument out of a beam.
Helmut Fleischer and Tilmann Zwicker, working at the Universität der Bundeswehr in Munich measured this in the late 90s. They drove guitar necks mechanically and created a map of how easily each point moved at each frequency. They then compared this to where the notes died, and found that the dead notes lined up with the peaks. Basically, the neck is simply better at absorbing some frequencies than others, and the notes it absorbs are the notes that die.
The location of the spots varies from instrument to instrument, but it is not random. On many guitars it sits somewhere on the G or B string in the first octave, and all players describe it in the same way: one note that sounds quieter and shorter.
So, it follows that sustain is not a property of the guitar. It is a property of a note on the guitar.
What the joint actually changes
Now that we cleared the basics, let’s look at the never ending joint / sustain argument, in the most honest manner as possible.
A set neck is couple to the body across a large glued surface. A bolt on is coupled through four screws and a machined pocket. It would be weird if they behaved in the same exact way, and in fact they do not. What changes the most is the stiffness and mass distribution of the neck and body, and changing those moves the resonant frequencies of the beam.
Note I said moved them. Not removes them.
Physically, a different join gives a neck that absorbs a different set of frequencies. What the player hears is that some notes now ring longer and others (which are fine on another guitar) have quietly become the shorter ones.
And why that matters is that the 50 years old argument has both sides reporting real experiences and both sides drawing the wrong conclusion!
A player that swaps a neck or shims a pocket, or moves from a bolted neck to a set neck, and reports more sustain is usually telling the truth about the notes he actually plays! What happened is that a dead spot slid off a note in their vocabulary into another one they do not touch so much.
The same logic explains another trick that confused guitarists for decades. If you clamp a heavy brass weight onto the headstock, some players swear the guitar wakes up to a new life, while others hear absolutely nothing. And the beauty is that both are correct. The mass lowers the neck’s resonant frequency and shift the dead spot to a different note. If your dead spot is a note you use, then you suddenly fixed your guitar. It if was on a note you do not really use, then nothing really changed.
The sustain that does not sustain
There is one more reason this argument will never resolve.
Most of the time we judge sustain through an amplifier and gain compressor. Distorted note decays exactly as fast as before, but the amp does the work to level up as the signal falls. So the note appears to hold. Id you turn up further and the speaker starts to feed energy back, then at that point none of that is the guitar’s achievement.
This means that under enough gain, a piece of dead firewood with pickups can sustain beautifully.
The clean test is the honest test. Better if direct input, or even inplugged.
The gap between what the string does and what the rig does is why we built Guitar Earo. All our guitars in the library are recorded in direct input, so you really isolate the tone of the guitar.
An exercise you can run yourself
Here’s a little thing you can do.
Sit in a quiet room with no amp. Play every note on the G and B string from the open string up to 12th fret. Do not rush and pay real attention. Do not pick harder on the ones that seem week, keep a consistent force.
You will find that one or two spots will die earlier than their neighbours. That is your guitar. It is not a fault, and it is generally not fixing. Once you know where the decay leve, you can voice around it. A guitar is a beautiful instrument because of how imperfect it is. This is one of the imperfection that any guitarist, consciously or unconsciously, will find their way around.
Now go and do the same thing on the guitar that you were about to buy, before you decide that the joint is the reason it sustains better.