A chordophone is defined as any musical instrument that uses vibrating strings or strings connected between two points to produce sound, so you can think of things such as harps, violins, and even guitars as falling into this category. The first chordophone that resembled the classical acoustic guitar is said to have appeared about 3,300 years ago in a Babylonian stone carving, with a stark contrast to today’s modern acoustic, electric, and acoustic electric guitars, which have become highly optimized feats of engineering. Today we are going to talk about just a few of the many, vast engineering subjects applicable to the design of the modern guitar that have helped to revolutionize the way we play music today, regardless of what genre it may fall into…Blues, Indie, Latin Funk, Country, Hair Metal, Ska, Garage Punk, even some Hip Hop, and countless other genres and subgenres!
As Keysight Technologies points out in an editorial on the guitar as an example of engineering, “The guitar touches on a rich set of engineering principles, among them: resonant frequency, period, amplitude, distortion, harmonics, wavelength, stress & strain, elastic limit, am, fm, damping coefficient, Doppler effect, step response, coupled oscillations, fft’s and signal processing”. I can assure, this is in no way a complete list as in any engineering project, an engineer generally applies every subject they’ve ever learned about in some way, shape, or form, regardless of whether they realize it or not. Today we are going to pick three of the subjects most prevalent to an understanding of the mechanics of a modern acoustic electric guitar (an acoustic guitar fitted with equipment to increase the volume, so that one can play with the volume of an electric guitar, while still maintaining the sound of an acoustic guitar), and we will utilize a logical progression to do so, meaning we will move from the user’s hand, through the strings, through the body, back out to the atmosphere.
Strings & Frets
When a guitarist plays the guitar, they use both hands actively; generally this consists of strumming or plucking the strings of the guitar with one hand and using the other hand to push the string down at the frets (those raised, generally metal, bars that are transverse to the neck of the guitar) on the fretboard/fingerboard. When the string is disturbed (plucked or strummed), it vibrates, as it is connected rigidly at two ends, or nodes, (one at the saddle of the bridge, and the other end at the nut of the head). This vibration occurs at a resonant frequency that is a function of the string material (due to differences in density), tension, length, and diameter. This is to say that if you were to strum the top string of the guitar soft, medium, hard, whatever, it will produce the same frequency every time (unless the string contacts something, or you pluck it too hard and plastically deform, or stretch in this case, the string or pull the string partially out of the bridge or tuning keys, changing the length and thus frequency). If you want to hear a different frequency, you can move to a smaller (diameter) string for a higher frequency, play a guitar with a longer neck (and thus longer strings) for a lower frequency, increase the tension in the strings for a higher frequency, or change to denser/heavier strings to reduce the frequency (& vice versa for all of these; also note that change in pitch will positively correlate with change in frequency…i.e. lower frequency will produce lower pitch sound).
Now, what happens when we start using our other hand to press down the string at frets? Rather than having the string vibrate between nodes at the saddle and nut, it will be vibrating at nodes between the saddle and fret, changing the length of the vibrating portion of the string altogether! Following from this, a fret closer to the saddle (as opposed to closer to the nut) would produce a shorter string length, and thus a higher frequency or pitch! Now, if we repeat this action, but this time we do not push the string all the way down, so as to not lock the free side of the string from vibrating, we produce a harmonic. In essence, the vibration mode is changed as we create our frequency between the fret (or our finger in this case) and the saddle, and let it propagate down the string, adding to the frequency with each harmonic. In the far right picture below, we see the original first harmonic to the right of the finger, followed by the 2nd, 3rd, and 4th harmonics down the line to the left. If the first harmonic was vibrating at a frequency of 100 Hz, then the 2nd would be at 200 Hz, the 3rd at 300 Hz, and the 4th at 400 Hz. A musician (rather than an engineer) would term the 2nd, 3rd, and 4th harmonics the 1st, 2nd, and 3rd overtones.
Now that we have produced the vibration, it must travel through the body before we can hear the sound it produces. This action begins at the bridge, where the vibration of the strings is transmitted through the saddle (and followingly, the bridge) to the soundboard (the top plate of the guitar body, to which the bridge is attached). This soundboard is lightweight and has a large amount of surface area associated with it, which is important, because now that the soundboard is vibrating, we want to be able to transmit that vibration to the air inside the body as efficiently as possible. Lightweight materials that are relatively strong, yet also are fairly springy (to use a non-technical/scientific term) are very good at transmitting these frequencies with little loss/damping through the material itself; therefore, materials such as Spruce and Cedar (both are types of wood, if that wasn’t clear here) are used. If we were talking about solid-body or semi-solid electric guitars, here would be a good place to comment on the choice of center-body material, as the vibration would have to be transmitted through significantly more material before getting to the devices that convert the vibration to sound (generally electromagnetic pickups), thus body material has an extremely significant effect on the sound of the guitar.
Back to acoustic guitars, once we have begun to vibrate the soundboard, the air inside the guitar begins to vibrate and resonate as it is pressurized by the downwards movement of the soundboard, and depressurized by the upwards movement of the soundboard. This fluctuating or resonating air now travels through the body of the guitar to the sound hole, where it exits back out to the environment as sound, largely through the phenomena associated with Helmholtz Resonance (which we discussed a while back, as it relates to automotive side-window buffeting and blowing across the top of beer, or soda, bottles).
It’s also interesting to note the how the shape of the body effects the sound. As seen above, there is a lot of structural bracing in the guitar body itself (which makes sense as the body is made from lightweight materials, but must withstand its own weight, use, and sometimes a little abuse. As you might imagine, all of this bracing will effect the vibration and sound and so it must be designed properly to have minimal effect on the sound of the guitar…with the exception of at least two vastly important features, which are not labeled above. The first is the lower bout, which is the lower part of the body that balloons out; this portion attenuates (or reduces/weakens/cuts out) lower tones. The second is the upper bout, which is the upper body portion that balloons out and attenuates higher tones, making the acoustic guitar what we term a band-pass filter, meaning it filters out lower and higher tones (frequencies), only allowing a specific bandwidth of frequencies to pass through.
So far we have seen how an acoustic guitar produces its sound, and this is fine for sitting around the campfire, but what if you want to take your acoustic guitar and play at your friend’s wedding in front of a hundred or so people? Do you expect them to all crowd around silently to hear you play? Hopefully not, and this is where the electric part of electric acoustic guitars comes in. Pickups are used to pick up the vibration of the strings/body and convert it to an output electrical signal that can be fed into an amplifier to produce a much louder sound. In electric guitars, this is generally done using magnetic pickups that utilize the effects of vibrating steel strings over the magnetic pickups mounted on the top of the guitar directly below the strings to produce a signal. In acoustic guitars this is usually not done (for a few reasons, but mainly due to the difference in sound produced, as the sound is decidedly more electric than acoustic when using magnetic pickups), instead piezoelectric pickups are used, which have an added benefit of being able to bypass the interference (that annoying buzzing/static-y sound you hear) often heard when using electromagnetic equipment. The piezo pickups (as they are often called) place a sensor on the soundboard; as the soundboard vibrates, the pressure or applied stress on the face of the sensor changes as the soundboard moves in and out. The change in stress creates a change in strain of the sensor material, leading to a really small change in size and thickness of the sensor material, which sees the change in the material’s geometry as a change in electrical resistance, and registers it a an electrical signal, which is then fed to a pre-amp which essentially just gets the signal ordered up and ready for the amplifier, where the volume is increased, just as with the electric guitar.
So there you have it, just three of the countless things an engineer must consider in the design of an acoustic electric guitar. As always, hopefully everyone learned something new today and now you can go out and rock out (or stay in and serenade) like an engineer! Lastly, take a second and apply what you just learned when you look over the infographic below!