Ed note: Kevin Gillies is Sole Proprietor and Luthier at ‘Oi Acoustics. He is a residential architect, land use planner, woodworker, and luthier. In his words, he "strives to achieve elegance through simplicity, reducing the number of visual elements to the minimum required. The focus is on ergonomics and tone to make an instrument that speaks for itself and you won't want to put down." This multi-part series delves into Kevin's design philosophy and what he considers important elements when it comes to instrument architecting and selection. This is Part II of a multi-part series. © 2022 www.oiacoustics.com.
In order to address sound quality, we have to start with a basic agreement on how we hear sound. A critical notion is that we all hear differently—most of us have lost some portion of the frequency range, and everybody's unique, so we actually don't hear the same thing. We don't know how other people experience any of their senses—we make an assumption that others hear what we hear, or see what we see, but they don't!
Various genres utilize different sound textures. If an instrument is within a band or group, it has to hold its own place or frequency range to be heard within the group, each instrument having a layer within the overall sound. For instance, a gypsy jazz rhythm guitar intentionally has a constricted tonal range compared to a fingerstyle guitar, as it serves the function of a timekeeper or percussionist in addition to guitar player, and tight sustain.
Finally, and rarely discussed, is that what the player experiences as the sound of an instrument, particularly with acoustic and to a lesser degree but important with an electric, is different from what the audience hears.
Acoustic guitar players hear a "sound," but what are they hearing? The guitar sound is comprised of the fundamentals—the open string notes, and the partials—all the harmonics above that which are produced by a complex relationship of materials. Each note is comprised of many frequencies; if it were a single one it would just be an uninteresting monotone.
"It turns out that ~1500hz is the lowest frequency of sound that can escape the guitar box through the sound hole". That is note G6 or the second octave above a high G string; therefore ALL the fundamentals on an acoustic guitar are produced by the vibration of the top and back of the instrument, in turn moving the air all around it, and omni-directional.
Schematic and nomenclature for 4-DOF Model: Gore & Gilet "Contemporary Acoustic Guitar Design
The interrelationship between the top and back—the "Helmholtz coupling" created by containing the air between them with the sides—is critical as no fundamentals actually come out of the soundhole, making the top and back the creator of all the low sounds.
What is extremely important and makes the guitar sound like a guitar, is the harmonic content, which above G6 escapes via the soundhole and we hear up to 18000 hz or higher with great ears. That is the texture or voice of the instrument. If you put your hand over the soundhole, it mutes the high harmonics, not the lower fundamentals, and changes your perception of tone as it is altering the balance of frequencies.
The acoustic guitarist is hearing a blend of low notes produced by the vibration of the instrument itself in a close or near field and the harmonics above G6 through the soundhole (which with a conventional center hole are pointed away, towards the audience, who hear them differently than the guitarist in a small unamplified setting). They are created by all the strings and everything else that is vibrating.
If the instrument produces too much of one frequency, it is heard as taking away another. So if you need to balance out the sound, you may have to reduce some tones to allow others to shine. This is the interrelationship between the string choice and guitar construction. It is also, in essence, the core of audio engineering, recording and mastering. It's a complex gumbo!
A pickup in essence is a circuit that is effectively magnetizing the string, turning it into a moving magnet relative to the stationary coil. The tone is controlled by the coil materials, which determine the inductance (in Henries), impedance and resistance (in ohms). As such, it can't possibly "hear" the tone of the wood. Any marketing that a certain wood sounds a certain way on an electric is confusing the near field tone, which you may hear unplugged, and the amplified signal.
The audience hears the amplified signal which has been altered by the particular environment (room, concert hall, etc.). The player hears some of that reflected back, the same or modified signal through monitors, the near field sound of the instrument vibrating if the volume isn't too loud, and may feel it through their bones if played on the lap. Then the amp and various electronics offer multiple ways to alter and balance the tonal content.
How all this affects the sound, and what the listener perceives but isn't necessarily accurately identified as to the cause, leads to a lot of confusion in describing what a certain instrument sounds like, depending on your perspective. When you play an instrument, especially an acoustic in a quiet setting, the near field acoustics have an impact which is not transferred through the air or pickup & amp (the signal chain) to the larger far field. An electric player feels the sound, but again that is not shared by the audience.
This suggests that the body material is less important for amplified tone production than advertisers would claim, especially for the professional musician under amplification where it overwhelms most if not all of the near field tones you would get at home or on a very small stage. You frequently hear comparisons of the tonal qualities of various woods in terms similar to fine wines—woody, sweet, mellow, overtones, etc. This is stated primarily for acoustics, but also somewhat with electric amplification which can't possibly be true. While there may be a perception of difference to some subjective degree, they seem to be primarily marketing; the tone of the body material in the overall context of the guitar is too limited a factor when everything else is considered. (However, this clearly does not apply to unamplified acoustic classical instruments such as the violin, which is played close to the ear.)
This has lead to the overarching goal of producing instruments that have great "resonance", which is a very distinct concept from "tone".
"Resonance is an action/reaction equation. When vibrations occur in one location, and then transfer to another location (including the air) through some degree of filtering, this is resonance. Generally speaking, resonance is something considered to occur both in real-time, and depending on the amplitude of the original vibration source (and environmental acoustics), will occur across time after the original vibration occurs. In other words, you play a note, and some filtered element of the original note continues for some duration afterward. This can be reverberation in a room, it can be the decay of an acoustic guitar top still ringing moments after the note is played, and a lot of the way it impacts what we hear and feel is by blending and adding to the original note."Frank Falbo
Resonance applies to our whole auditory world: not only music but the single human voice, a church choir, clapping your hands, or even in the stillness of meditation where there is a prevalent background of resonance, and is inseparable from the environment. It is the presence that is underneath and in addition to the original sound.
Instrumental resonance is dependent on four main components: how it is played (plucked vs. bowed), the sound production of the instrument, the output via the acoustic and electric signal chains, and the environment it is played in.
There are many resonant qualities we might associate with instruments: long vs. short sustain, warm vs. cool tone, complex harmonics vs. dry, full vs. thin, and so on, including the concept of the "voice" of the instrument.
There is no one "best" guitar resonance; there are many depending on the type of music to be played, and the context—for example solo or in a band, rhythm or lead, and the genre. What's important is that the luthier produce the desired sound intentionally, and that the musician pick or order an appropriate instrument to their style, or perhaps more than one to cover different situations, as one can't possibly do everything.
The two major materials that Hawaiian guitars have been made from are steel, as with a frypan, and wood. I'm a wood guy, so my comments are specific to that and not to metal, with which I don't have the expertise.
After the pickup, different materials do determine one of the most fundamental determinants of good sound for the steel guitar in that the two anchor points of the strings must be as solid and stable as possible. The body material and composition, which largely determine the body rigidity, has the primary role in accomplishing this.
This leads us into next chapter's topic: Strings & Intonation.Feature
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