Graph Tech News

The Science Behind Acoustic Guitar Tone

Dec 11, 2017
What are the magical properties that make a good quality guitar sound so good you get the tingles?

What are the magical properties that make a good quality guitar sound so good you get the tingles? After you strip away the centuries of tradition, the romance, and the tiny bits of luthier voodoo here and there, the answer is grounded firmly in science.

Let’s take a look.

How does a guitar make sound anyways?

When you pluck a guitar string, the string starts vibrating in a complex pattern. But the sound you hear isn’t coming directly from the vibrating string. Rather, the vibration is being transferred to the soundboard through the saddle. 

The saddle actually acts like a filter - it only allows some of the vibrations through. Once they get to the soundboard, its unique characteristics will also have an effect on the sound. Again, some of the frequencies will be suppressed and some will be enhanced. The air on the inside of the guitar cavity also has an influence on the quality of the sound.

When a guitar string is plucked with a pick (or a finger), a little kink is made in the string. As the pick leaves the string, the kink travels down the string and hits the saddle.

The kink transfers some of its energy through the saddle to the bridge, which then spreads out the energy throughout the highly flexible soundboard.

The rest of the energy is retained in the kink and continues to travel back and forth between the nut and the saddle. On each journey, a little bit of the energy is transferred to the saddle (and goes to the soundboard). 

Acoustic-Guitar-Tone-Science-Graph-TechPhoto Credit: Solitary Road

The string eventually comes to a resting point after all the energy has been sent to the soundboard.

The speed with which the kink travels down a string depends on its thickness and tension. And, that speed combined with how long the string is tells us how fast the string will vibrate.

All about that string

The way in which a guitar string vibrates is actually a little more complicated. When that little travelling kink makes 440 round trips (from saddle to nut and back) every second, the string makes the sound for the “A” above middle “C”.

Otherwise known as “A 440”: the A at 440 hertz (Hz). 

But here’s the trickier bit: While the string is vibrating at 440 Hz, it’s also vibrating in halves at 880 Hz.

...And in thirds, at 1,320 Hz. And fourths, at 1,760 Hz. And so on. 

Guitar-String-Vibration-Harmonics-Partials-Overtones-Graph-TechPhoto Credit: University of Toronto

These different modes of vibration are called harmonics, partials or overtones. In a perfect world, strings would love to oscillate as closely as possible to their favourite harmonic partials (or whole number multiples of their fundamental frequency).

Because each string has so many modes of vibration, the saddle must deal with the energy of all partial vibrations, as well as their fundamental “full” vibrations.

The saddle’s game

The saddle is kind of like the gatekeeper to the soundboard. It denies access to certain partials and allows some through. The impedance match between a string and its soundboard is what affects the efficiency of the saddle, and helps create the perfect blend of tone and sustain.

A perfect impedance match would mean there’s no obstruction or reflection from the saddle, and all the energy from that “kink” would transfer to the soundboard at once.

The result? A loud BANG sort of sound, with no sustain. No too pleasant.

On the other hand, a poor impedance match would take so long for the energy from the string to be transferred that no sound would be heard. Too much sustain. Again, not so good.

Think about it this way: The string and the soundboard are partners who have really different opinions. The saddle is their therapist. If the saddle lets either party overpower the other with its opinions about tone and sustain, the partnership falls apart and they sound awful. So, instead, the saddle chooses their impedance match and balances it.

The saddle allows appropriate frequencies to go from the string to the soundboard (to make tone) and stops others from going through easily (to make sustain). 

It's the perfect mix of these two elements that creates the sound we look for.

What’s up with the soundboard?

Turns out the string and the saddle aren’t the only ones making the dream work here. The soundboard also has a lot of sway in how a guitar sounds.

The soundboard is like a loud speaker. It’ll vibrate at most frequencies the saddle gives it, but it also has its own favourites. At these frequencies, the soundboard vibrates extra excitedly and transfers its own special characteristics to the energy it was given from the vibrating string.

The soundboard’s ability and efficiency depend on its shape, thickness, mass distribution, and grain pattern - as well as the characteristics of the bridge and how the bracing on the underside is glued. A good luthier will meticulously sculpt the bracing to give the guitar its “voice”.

Using the air

The wood and the strings are not the only parts of the guitar that move. Because a guitar has a bottom, there’s an amount of air that’s enclosed within it. When the soundboard vibrates, the air inside also vibrates.

And, of course, just like everything else involved, the air also has favourite frequencies (or modes) to vibrate at.

The strings don’t directly interact with the air cavity; only the soundboard’s vibrations can excite an air mode. But it’s a two-way street between the soundboard and the air modes, because air modes can actually also influence the board by acting like an internal spring.

In fact, if the mode shapes between the board and air are super similar and they have similar favourites when it comes to frequencies, they’ll pretty much become best friends and physically couple. 

The magic of the human ear

There’s one last element that lets us hear a guitar the way we do - and that element is something we carry around every day: our own human ear.

One of the characteristics of a good guitar chord is its strong bass. On its own, the regular fundamental frequency of a bass string isn’t very loud. But fortunately, bass strings have a rich structure of strong partial frequencies (or harmonics). Our brain uses these partials to “hear” the bass notes of a guitar.

What’s this ability of ours called? “Heterodyning”.

It’s the same technique our brains use so we can hear that friend with the super deep voice over the phone.

The human ear also does this other strange thing where it “hears” high pitched sounds really well - but in the bass register. Weird, hey? So to compensate, a well-built guitar will actually generate more power in the bass than in the treble.

The result?

We hear a balanced and even distribution of sound from the entire instrument.

Nuts, saddles and bridge pins: It’s time to experience TUSQ

Replacing your standard saddle with a TUSQ acoustic guitar saddle will have a profound adjustment on your instrument’s tone, harmonic content and playability.

The TUSQ line (that includes nuts, saddles, and bridge pins) produces more harmonics (or partials) in the mid and upper range of your guitar.

TUSQ-Man-Made-Ivory-Acoustic-Guitar-Tone-Graph-Tech-Christmas-Gift-

The TUSQ material itself has a more harmonically rich tone than that of ivory, bone, or other natural materials. Bone and ivory have hard and soft spots (grain) throughout each piece, preventing the consistent transfer of vibrations to the soundboard. The TUSQ saddle has been designed to transfer the right frequencies more efficiently from the strings to the guitar’s body.

Once you’ve got a TUSQ saddle in hand, follow these instructions from the Graph Tech workshop to install it!

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