Ever wondered how the acoustic guitar works? How does it generate sound, and what influences and colors the sound it makes?
Below is a quick summary…
When a guitar string is played vibrations from the strings are transferred via the bridge to the soundboard. As the soundboard vibrates air particles around and inside the body are displaced and compressed and dispersed from the soundhole. The materials and design of the guitar, shape the frequency response, giving the guitar its unique tone.
For a more detailed answer continue reading.
Because in today’s article, I’m going to break down, in detail, exactly how the acoustic works, and why not all acoustic guitars sound the same. But first…
How we hear sound
To understand how the acoustic guitar creates sound, it’s useful to know how the ear detects sound, as both involve resonance.
What is resonance?
To describe something as resonating is to describe it as:
Vibrating at its natural frequency, in response to another object vibrating at the same frequency.
This is why we say ~ something ‘resonates‘ with us or we are getting ‘vibes’ from something.
An example of the power of resonance is the classic depiction of the opera singer breaking glass with just their voice.
While this is more difficult than it looks, it’s possible.
And, while you might expect this phenomenon to have more to do with volume and projection, it’s caused by a combination of resonance (mostly) and volume.
For example, if the singer can match the resonant frequency (the natural frequency that the glass vibrates at (you can tap the glass and listen to the pitch to hear the resonant frequency) the air molecules surrounding the glass will begin to vibrate.
If the singer sings with enough volume and projection this increases the force of the air molecules surrounding the glass, eventually causing the glass to break…at least in theory.
When something makes a sound, in our case a guitar string, it vibrates. Air molecules surrounding it are subsequently displaced and compressed, causing the air molecules nearby to do the same.
This is essentially what a sound wave is e.g. a wave of compression and rarefaction.
These pressure waves transfer through the air to our ears and resonate with the eardrum. As the vibrations travel through the ear canal they’re amplified.
From the ear canal, the vibrations resonate with the small bones of the middle ear (the ossicles) which transfer the vibrations between the middle and inner ear.
The fluid within the cochlea (a spiral-shaped bone of the inner ear) resonates with these vibrations which cause tiny hairs within the cochlea to move, converting the mechanical energy of the vibrations into an electrical signal which is then transferred to the brain via the auditory nerve and detected as sound.
I’m skipping over a lot of the detail here, but to summarize, the human ear detects vibrations. The transfer of resonant energy converts the vibrations into an electrical signal which the brain interprets as sound.
How the acoustic guitar produces sound
Based on our understanding of resonance things should start to be getting a little clearer.
All sound produced by the guitar starts with the strings vibrating. The other components of the guitar shape the sound produced.
So how does this work?
An acoustic guitar is made up of two main components, the neck, and the body.
At the end of the neck, we have the nut, the slotted plastic or bone (or graphite) component positioned between the end of the fretboard and the headstock. The saddle (the white plastic, or bone component embedded in the bridge) is located just below the waist of the guitar body.
The guitar’s strings are suspended between these two endpoints (this is the scale length) and when a string is struck the strings vibrate between the two.
The strings themselves don’t displace a great deal of air due to their small surface area, as evident when playing an electric guitar unplugged, and the limited volume produced compared to an acoustic guitar.
However, the saddle is embedded within the bridge and this is attached directly to the soundboard (the top) of the guitar, which like a drum skin, sits on top of the hollow chamber of the body supported by the more rigid back and sides.
The soundboard has a much larger surface area than the strings and when it vibrates is capable of pushing around a lot more air.
This transfer of energy between the strings and soundboard may seem disproportionate given the energy imparted on the strings would seem insufficient to move the much larger surface area of the soundboard.
But there’s no magic involved.
The trade-off in transferring the energy of the strings to the soundboard is a loss of sustain e.g. the length of the note before it decays.
There’s normally a trade-off between volume and sustain, and, in the case of the acoustic guitar if the strings were suspended over a completely rigid object (e.g. stone) they would sustain for longer, as the stone is denser and less able to absorb the vibrational energy from the strings.
The more flexible nature of the guitar’s soundboard means the energy from the strings decays at a faster rate. However, more air particles are affected over a shorter duration, resulting in greater volume. A trade-off worth making.
This concept is also important when considering nut and saddle materials. Nuts and saddles made from plastic (usually found on inexpensive guitars) are more absorptive. Whereas a material such as bone or graphite, being denser, absorbs less of the energy from the strings, transferring more of the energy to the soundboard of the guitar.
What about the soundhole?
The internal chamber of the guitar body contains air. As the soundboard vibrates the air particles within the hollow body of the guitar are compressed, displaced, and released via the soundhole.
As air is released this lowers the air pressure inside the guitar’s body, forcing it to draw in more air to correct the pressure imbalance, allowing the process to repeat. You can read more about this here.
How acoustic guitars sound different from One Another
So, what makes an acoustic guitar sound the way it does?
When it comes to the characteristics of the guitar e.g. tone, volume, projection, and response to the player, there are many factors involved, including, but not limited to:
- Body shape and size
- Materials (tonewoods, nut, and saddle)
- Positioning of the bridge
- Bracing pattern and bracing materials
- Luthier’s construction methods including joining methods and adhesives
- Finishing products and thickness
- Type and gauge of the steel strings
- The type of pick used (if one is used at all)
Not to mention the guitarist’s technique, and the acoustics of the room the guitar is played in.
Below we’ll discuss body shape and size, along with tonewoods, and bracing, but just to further complicate things, keep in mind, that none of the factors listed above work in isolation.
Acoustic guitar bodies are shaped like a figure 8, featuring a top, back, and sides, and an upper bout, lower bout, and waist.
The internal volume of the acoustic guitar body, defined by the depth of the sides, the width of the waist, the surface area of the back and sides, and the width of both the upper and lower bout all shape the frequency response (the range of frequencies an object can produce) of the guitar along with volume and projection e.g. the penetrating nature of the sound produced.
For example, the loudest acoustic guitars are larger body guitars such as jumbos and dreadnoughts, while the quietest are the smaller body parlor guitars.
The larger internal cavity of the body of these guitars means there are more air molecules contained within, and therefore more air particles are affected when the guitar is played and the soundboard vibrates.
The size and body shape of the guitar have a big influence on tone (along with the materials), as this affects how the vibrations are distributed throughout the guitar’s body.
While a jumbo is larger than a dreadnought, due to the wider waist and square shoulders of the dreadnought the bass frequencies are accentuated giving the dreadnought its iconic boomy, bass presence. Alternatively, a jumbo guitar usually sounds balanced, due to its relatively tighter waist.
The smaller more elongated parlor guitar, with its wide waist relative to the lower bout, accentuates mid-range frequencies and can tend to sound boxy, producing focused mids, with less bass and treble present.
Alternatively, a guitar with a tight waist, and rounded, shallower body, such as a concert guitar, offers a more focused, balanced tone, accentuating mid and upper-range frequencies.
The position of the waist is also important.
A guitar with a higher waist increases the size of the lower bout, which further accentuates the bass response.
This makes the dreadnought better suited to strumming and means it has more presence when accompanying other instruments. Whereas the smaller-bodied concert guitar lends itself to more intimate styles of playing such as fingerstyle.
This is also evident in the body shape, from a comfort perspective. The concert guitar is easier to play when seated compared to the larger, blockier dreadnought.
The bracing pattern is the arrangement of the wooden tapered struts fixed to the inside of the soundboard and back of the guitar.
Bracing provides stiffness to the soundboard, and the pattern and materials used to brace the guitar, play a big role in the guitar’s sound projection, volume and responsiveness.
Bracing must balance the need to withstand tension from the strings, allowing the top to be thin and light, without adding too much weight which would otherwise affect its ability to vibrate.
There is a wide range of bracing patterns used in the construction of guitars, the most common are:
The standardization of the classic guitar body shape and size is largely based on the invention of fan bracing by Spanish Luthier Antonio Torres Jurado.
Fan bracing consists of adhering 5 – 7 struts in a fan-like pattern to the inside of the soundboard, with the individual struts pointing toward the 12th fret. The added stiffness this design provides to the soundboard, allowed guitar bodies to increase in size and subsequently volume. This allowed guitars of the time, to accompany instruments such as the banjo and violin.
While Fan bracing was an important step in the evolution of classical guitars, Christian Frederick Martin (C.F. Martin Guitars) invented X-bracing in the 1840s which, while initially developed for catgut strings, eventually led to the invention of steel-string guitars which offered even greater volume and projection and have been replicated by guitar manufacturers ever since.
As the name implies, X-bracing consists of two cross braces intersecting between the soundhole and the bridge in an X shape extending out to the upper and lower bout. A transverse top brace is added across the upper bout, along with smaller struts to stabilize the soundhole.
Sidebar struts are joined to the X-brace, extending out to the edges of the soundboard, and 2 struts are added, angled across the lower bout of the guitar behind the bridge plate, referred to as tone bars.
Most guitar backs use ladder bracing. This was once a more common soundboard bracing due to its cost-effectiveness but now is mostly consigned to the back of the guitar.
As the name implies, the struts are arranged to form a ‘ladder’ pattern which helps distribute the force across the back of the guitar evenly.
Scalloped bracing isn’t a system of bracing, it’s the practice of scooping out the middle of the tapered struts and tone bars, reducing the weight of the individual struts, and increasing the energy and responsiveness of the guitar.
The struts taper toward the edges, so when the strut is scalloped it takes on more of a wave-like shape, as shown in the diagram above.
This affords the soundboard greater flexibility and response to string vibration toward the center of the guitar, and bridge. This results in more energy from the strings being imparted on the soundboard, and more complex overtones emanating from the guitar. This is what, for many, gives Martin guitars their ‘rich’ distinctive sound.
All standard series Martin Guitars (Except the D-28 and D-35) feature scalloped bracing.
I’ve written a fairly detailed article on tonewoods here which I’d recommend reading if interested in taking a deeper dive on the topic of tonewoods, but in simple terms, different species of timber have different acoustic qualities due to their inherent differences in hardness, density, strength, and flexibility.
While the back and sides are important, especially concerning how they are paired with the soundboard, the characteristics of the solid wood used for the soundboard have the greatest influence on how the guitar sounds.
The wood used for the soundboard is selected for its weight, strength, and flexibility. In a practical sense, this means, its flexibility and lightness should respond strongly to the vibrations from the guitar’s strings while retaining structural integrity.
In most cases, higher quality guitars comprise solid wood, however, laminate is regularly used, on less expensive models. Laminate lacks the vibrational energy of solid wood but does offer advantages in terms of handling humidity and of course lower production costs.
Sitka spruce is perhaps the most commonly used soundboard timber. It’s an ideal top wood for acoustic guitars because it is light and strong, contributing a bright, balanced tone with a snappy attack.
Other commonly used top woods include cedar and mahogany. Mahogany provides a warm, balanced sound with a strong fundamental, while cedar offers a wider frequency response and fuller sound.
Denser, harder timbers such as rosewood and maple are less absorptive and more responsive compared to lighter timbers such as mahogany, and so are mostly used for the sides.
Maple, being a denser, harder timber is often used for the neck, while mahogany, also being a hardwood is another popular option.
What about the acoustic-electric guitar?
An acoustic-electric guitar is essentially an acoustic guitar with pickups, giving it the ability to utilize electronic amplification, along with volume and tone controls, much like a solid body guitar.
The most common type of pickups used on acoustic guitars are magnetic pickups e.g. soundhole pickups, piezo pickups that detect pressure changes at the bridge, or small internal microphone pickups (or a hybrid/combination of all three).
In the case of magnetic soundhole pickups, the pickup works the same as an electric guitar pickup. The pickup (which spans the width of the soundhole) creates a magnetic field due to the magnetic pole pieces of the pickup being wrapped in conductive copper.
When a steel string, being ferromagnetic (e.g. able to be magnetized), is plucked, the magnetic field is disturbed, and the mechanical energy of the vibrating string can be converted to electrical energy allowing the signal to be sent to an electronic amplifier.
While any amplifier will amplify the sound of an acoustic-electric guitar, it is preferable to use an acoustic guitar amp as they are purpose-designed for the flatter frequency response of the acoustic guitar.
You can read more about acoustic-electric guitars here.
Unfortunately, when amplifying an acoustic guitar it becomes more susceptible to audio feedback.
The hollow body of the guitar will resonate with the soundwaves coming from the amplifier speaker, (or foldback/front of house speakers). As the guitar is amplified this creates a feedback loop between the guitar and amp.
The best way to avoid this occurring is to adjust the angle and distance the guitar is from the amp, or use a soundhole cover to add additional weight to the top of the guitar affecting its ability to resonate. The use of EQ, and filtering out unwanted frequencies can also be helpful.
You can read more about the causes of feedback, and how to prevent it from occurring here.
How do the notes on a guitar work
As we now know, sound occurs when something vibrates.
But that’s a fairly simple way of looking at things if analyzing something like a performance of Asturias.
Before we delve too far, first some basic guitar theory, as it will help explain the concept.
The chromatic scale consists of 12 notes. This is the scale all western music is derived from e.g. every note, scale, and combination of notes used melodically, or harmonically comes from just these 12 notes:
And each open string of the guitar when tuned to concert pitch and played completes a specific number of vibrations per second:
There are more notes on the guitar’s fretboard than just 12. In fact, most guitars have over 120 notes available, but it is the same 12 notes, just repeated in higher or lower octaves.
The number beside the notes in the table above specifies the octave. This is known as scientific pitch notation.
So, while each note e.g. G3 — G4 will vibrate at different frequencies, they are always logarithmic integers of the note in a higher or lower octave. For example, G3 is 196 Hz, and G4 is 392Hz which is exactly double the completed vibrations per second of G3.
The human ear (remarkably) recognizes this as G, despite one being a higher pitch, hence they are the same note letter value but not of the same frequency.
Each additional higher octave is double the frequency of the next lowest.
12 tone equal temperament
12-TET short for 12 tone equal temperament describes how octaves are divided into 12 notes and is essentially how western music works. Some musical systems around the world, and historically have simply broken down the octave into a different number e.g. 22-Tet
When we apply this to the acoustic guitar, this begins to make more sense.
The octave (e.g. where the first note’s frequency is doubled, as explained above) is divided into 12 e.g. the 12 notes of the chromatic scale, with each fret representing a half-step difference from the notes adjacent to it on the chromatic scale.
As guitarists, we understand this because when we play an open string we can then can play the same note at the 12th fret of the guitar, an octave higher.
But why is this the case?
String Length and Mass
This is again where vibrational energy comes into play and how it relates to string length and subsequently its mass, at least the section of the string that can vibrate.
Tension is increased or decreased, raising the pitch of the strings, using the tuners. The tuners are adjusted until the string is tuned to the correct frequency. For example, as shown in the table above, our open A string is an A2 note and completes 110 vibrations per second, otherwise known as 110.00Hz.
The required tension placed on the string when tuning to concert pitch (the universally accepted tuning standard for all instruments) is dictated by the mass of the string, which being an open string is the full length of the string.
If we then play a note on the same string at the 12th fret, we have shortened the length of the string that can vibrate/ The length of string able to vibrate now is restricted to the length between the 12th fret and the saddle, which greatly reducing the strings mass, allowing it to vibrate at 220Hz, double the rate of the open string.
This is also the case when comparing the 6 open strings and their different thicknesses and subsequent mass. This is why your high E string is a higher frequency than your low E string, despite both being tuned.
The acoustic guitar is a sophisticated instrument, concerning how it produces sound and how its size, body shape, and materials used in the construction of the guitar influence the sound produced and how that sound is shaped to create music.
Considering the earliest stringed instruments probably began appearing somewhere between 2500 and 3000 BC the guitar we know and love today has come a long way. And, while it might be difficult to imagine how the humble acoustic guitar could evolve further, thankfully skilled luthiers/acoustic engineers from iconic companies such as Taylor with its V class bracing system, and Martin with their new modern deluxe series featuring a bolt-on dovetail neck joint and two-way titanium truss rod, continue to push the development of the acoustic guitar onto even greater heights.
Enjoy the article? Be sure to check out my article on how acoustic guitars are built.