When a guitar string is plucked vibrations from the strings are transferred to the soundboard via the bridge. As the soundboard vibrates the air particles inside the internal chamber of the guitar are displaced and compressed resulting in waves of pressure (soundwaves). When a fretted note is played the length of the guitar string able to vibrate is shortened. This increases the frequency of vibration resulting in a higher pitch note.
How does an acoustic guitar make sound
Ever wondered how the acoustic guitar really works?
To address this we really need to answer two questions. How does the acoustic guitar project sound? and, how does the acoustic guitar make music?
In today’s article, I’m going to break down exactly how the acoustic guitar produces sound, and secondly how the guitar works as an instrument and how different pitches are produced depending on where the note is played on the fretboard. We’ll also cover why acoustic guitars sound different in terms of tone, response, and volume based on different aspects such as body shape, and the materials they are made from.
How do we hear sound?
To really understand how the acoustic guitar produces sound, it’s useful to know how the ear detects sound, as both involve vibrations, and ultimately resonance.
What is resonance?
To describe something as resonating is to describe it as:
Something vibrating causes another object to vibrate at the same frequency.
Hence, why we often describe something that ‘resonates‘ with us, as something that we relate to strongly e.g. we ‘vibe’ with it.
Hertz (Hz) is the term used to describe the number of completed vibrations per second e.g. the open D string on your guitar completes 146.83 vibrations per second. The number of completed vibrations that occur per second determines the pitch of the sound produced (more on this soon).
To really understand resonance, consider the opera singer breaking glass with only their voice.
While this is actually more difficult than often depicted it’s definitely possible. And, while you might expect this to be more to do with force e.g. volume, it’s a combination of resonance (mostly), and volume. For example, if the singer can match the resonant frequency aka the natural frequency of vibration of the glass (you can tap the glass and listen to the pitch to hear the resonant frequency) of the glass the air molecules surrounding the glass will vibrate.
If the singer sings at a sufficient amplitude (volume) this increases the force of the air molecules surrounding the glass, eventually causing the glass to break, at least in theory.
In reality, this is more difficult than it sounds and is often the result of a defect in the glass or the glass being quite fragile to begin with.
In simple terms, when something makes a sound e.g. a guitar string, or speaker cone, even our vocal cords it vibrates. Air molecules in its immediate vicinity are displaced and compressed, causing the air molecules near these molecules to also become compressed, increasing their pressure.
This is essentially the definition of a sound wave e.g. a wave of compression.
These vibrations are detected by the ear drum, and as the vibrations travel through the ear canal they are amplified.
From the ear canal the vibrations cause the small bones of the middle ear (ossicles) to vibrate which transfers the vibrations between the middle ear and inner ear.
The fluid within the cochlea (a bone of the inner ear) moves with these vibrations which causes the hairs within the cochlea to move, converting the vibrational energy into an electrical signal which is then transferred to the brain via the auditory nerve.
While the above might sound a little complicated, In simple terms, the human ear detects vibrations and through resonance converts the vibrations into electricity which the brain interprets as sound.
Guitar pickups do a similar thing, they detect vibrations within a magnetic field or through changes in pressure caused by vibrations if using a piezo pickup.
How the acoustic guitar produces sound
Based on our understanding of vibrations and resonance it should now be simpler to understand how the acoustic guitar makes sound.
Put simply, all sound produced by the guitar starts with the strings vibrating after they are plucked or strummed. The other components of the guitar really just amplify the vibrations of the strings and shape the pitch and tone, which we will touch on shortly. But first, let’s take a closer look at the acoustic guitar’s body and how it projects sound.
If we take a typical acoustic guitar, it is made up of two distinct components, the neck and the body.
At the far end of the neck we have the nut, the grooved plastic, bone, or graphite component at the headstock end of the neck. The saddle is located at the bottom end of the body (the white plastic, or bonecomponent embedded in the bridge).
Our guitar strings are suspended between these two endpoints and when a string or strings are plucked the strings vibrate between the two.
The strings themselves don’t displace a great deal of air due to the small surface area of the strings, so this produces only a small amount of sound, as evidenced when playing an electric guitar unplugged.
However, the saddle is embedded in the bridge which is attached to the soundboard of the guitar which has a much larger surface area. And, much like a drum skin, sits atop the hollow chamber of the body supported by the more rigid back and sides.
The wood used for the soundboard is selected for its strength to weight ratio, meaning, due to its lightness it is capable of vibrating more than other many other species of wood but it is also strong enough to maintain structural integrity over time.
Sitka spruce is perhaps the most commonly used soundboard timber for this reason, and it will probably come as no surprise to learn it was also used in the early days of aviation due to its inherent strength to weight ratio.
As the soundboard resonates with the vibration of the strings, more air particles are displaced. This is due to the larger surface area of the soundboard.
This transfer of energy eventually resulting in greater amplitude (volume) may seem strange given the energy imparted on the strings is transferred to the much larger surface area of the soundboard to produce sound. But there’s no magic involved, the trade off in transferring the energy of the strings to the soundboard is a loss of sustain.
There is normally some kind of trade-off between volume and sustain, and, in the case of the acoustic guitar if the strings were suspended between two completely rigid objects (e.g. stone) the strings would vibrate much longer.
The more flexible nature of the guitar’s soundboard means the energy from the strings decays at a faster rate, however, it’s also capable of displacing far more air particles over a shorter duration, resulting in greater volume.
What about the sound hole and hollow chamber within the guitar body?
While it can help to think of the internal chamber of the acoustic guitar body as an amplifier, it’s not entirely the case. The chamber of the guitar contains air molecules. As the soundboard vibrates the air particles within the chamber of the guitar are also compressed, displaced and released via the sound hole. Generally speaking, the larger the chamber, the more air particles affected, but because of the sound hole the air is released in a more focused manner. This is why larger body guitars typically project more volume than a small body guitar, all things being equal, and why if recording we never place the microphone directly in front of the sound hole as the ‘whoosh’ of air as the air particles are displaced are detected by the microphone.
Why different acoustic guitars sound different
All acoustic guitars work the same way, however as you will have noticed guitars often sound quite different when compared e.g. some guitars are described as warm, others might be described as bright, and others may be described as scooped, meaning the mid-range frequencies are less dominant than the low and high frequencies. There are also additional factors including different rates of decay e.g. how long the note sustains, along with projection and volume.
We’ve already covered volume earlier in this article e.g. the larger (and deeper) the guitar body the more air particles contained within the chamber, resulting in more volume being produced.
So a dreadnought (the most popular shape) with its large body size and wide waist offers a fuller sound with more bass and treble response than a parlor guitar which although sacrificing some volume will generally sound more focused due to its relatively small size in comparison.
According to how attuned your ears are, this may be evident when comparing dreadnoughts with cutaways. While it can be hard to tell the difference to the untrained ear, the guitar with the cutaway (due to having less internal space and slightly smaller soundboard surface) is considered, under most circumstances to have slightly less bass response, although in a practical sense the difference is only minor and difficult to observe.
With this in mind, a dreadnought is often recommended for someone who strums the guitar, while the smaller body guitar is often the guitar of choice for a fingerstyle guitarist, although this isn’t always the case, as plenty of fingerstyle guitarists also play dreadnoughts.
Size and volume are one thing, but why do guitars have vastly different tonal characteristics, and different levels of response and sustain?
It’s not all down to the size of the guitar, other factors include the body shape of the guitar and the materials it’s constructed from.
I’ve written a fairly extensive article on tonewoods here but in simple terms different species of timer have different characteristics with regard to density, strength, and flexibility.
Denser timbers such as maple absorb less of the sound, and are considered more responsive compared to a less dense, more absorptive timber such as Mahogany.
While timber with greater flexible strength e.g. Sitka Spruce allows the soundboard to move more, offering a wider frequency range and volume and projection.
It’s also true that as a guitar ages, the soundboard becomes lighter while maintaining its flexible strength. This is why older guitars typically sound better than newer guitars.
Body shape also affects the tone of your acoustic guitar. Acoustic guitars are shaped much like a figure 8, featuring a top bout, lower boat and waist.
The size and depth of the waist, top, and lower bout affect how the air inside the internal chamber of the guitar responds.
Typically a guitar with a wider waist offers greater mid-range and bass response, whereas a guitar with a tighter waist offers a more focused tone.
The position of the waist is also important. For example a guitar with a higher waist increases the size of the lower bout, which would accentuate the bass response of the instrument further.
There are many other contributing factors that influence tone including the strings installed on the guitar, whether the guitar is played with a pick or fingers and the gauge of both the pick and strings to name just a few.
But, all things being equal, the tonewoods used to build the guitar including the soundboard, back and sides, along with the size and shape of the guitar play a large role in the tone produced.
So, that’s how an acoustic guitar makes sound, and how the sound is shaped by the characteristics of the guitar, but sound is not music.
So how do we hear different notes based on where we place our fingers on the fretboard?
That’s also got a lot to do with vibrations, along with some basic music theory which we’ll discuss in the next section.
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 for example.
Before we delve too far, first some basic music theory, as it will help explain the concept. There are 12 notes in the chromatic scale, which is the scale all western music is derived from e.g. every note, scale, and combination of notes that form chords all come from these 12 notes:
And each open string of the guitar when played completes a specific number of vibrations per second:
There are obviously 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 repeated, just in different octaves.
The number beside the notes in the table above specifies the octave. This is known as Scientific pitch notation.
So, while each individual note e.g. G3 — G4 will have different Hz values, they are always integers of one another. For example G3 is 196 Hz, 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.
12 tone equal temperament
This is known as 12-TET short for 12 tone equal temperament. This is 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 starts 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 to 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? Why just because we fret a note higher up the neck does its frequency increase?
String Length and Mass
This is again where vibrational energy comes into play which relates to string length and subsequently its mass, at least the section of the string able to freely vibrate.
If you take an open string, the length of string able to freely vibrate extends the full scale length of the guitar e.g. from the nut to the saddle. We add tension to the string using the tuners until the string is tuned to the correct frequency. E.g. as shown in the table above, our open A string is an A2 note and completes 110 vibrations per second, or is otherwise known as 110.00Hz.
How much tension is applied when tuning is dictated by the mass of the string that is able to freely vibrate, which being an open string is the full length of the string.
If we then play a note at the 12 fret, we have shortened the length of the string that can vibrate to between the 12th fret and the saddle, reducing its mass and allowing it to vibrate at 220Hz, double the Hz of the open string.
This is obviously also the case when comparing the open strings and their different thicknesses and subsequent mass, which is why your high E string sounds higher in pitch than your low E string, despite both being tuned with sufficient tension to prevent the strings from being too tight or too loose.
The acoustic guitar is a sophisticated instrument, with regard to 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. Understandably, even taking into account the electronics, building an electric guitar requires far less expertise than an acoustic guitar for these exact reasons.
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 evolution of the acoustic guitar onto even greater heights.