
A closer look at how an acoustic guitar’s soundhole and interior air volume work together to shape its voice.
In a simplified formula of a guitar’s sound, three primary resonators—the back, the top, and the air inside the body chamber—all work together (and occasionally against each other) to produce its tone. Much has been said about the top, and even the back, and we see these described loosely with specifications as to material and perhaps the bracing pattern. The air inside the “lungs” of each guitar is far more difficult to describe, although it is a significant part of each guitar’s sound and function.
When we think of the volume of an acoustic guitar, what typically comes to mind is the loudness of sound we hear and feel when we hit a chord. We often describe it with terms like “projection” and “dynamic range,” and we’ve all heard a loud guitar referred to metaphorically as a “cannon.” While these are important qualities for any instrument, there is a second definition of volume when it comes to an acoustic guitar—one of quantity. To be more specific, the volume or quantity of air inside the guitar’s body.
Imagine blowing across the top of a bottle. It produces a tone that is sustained as long as air is moving across the opening. Consider what is going on there. As air is blown across the opening, some is forced into the chamber and compresses the air already inside. At a certain point, the air cannot be compressed any further at that pressure, and it springs back toward the opening. Gaining momentum, it overshoots and makes a lower pressure in the bottle than the surrounding air, which then sucks air back inside—again overshooting and compressing air into the bottle. This cycle will continue with just a little help from additional air being input into the cycle. The cycle occurs rapidly: typically hundreds or even thousands of times each second for audible range. The exact number of cycles each second or frequency depends on a few factors: the size of the bottle, the length of the neck, and the size of the opening. It’s easy to hear the effect of changing the bottle’s size. We could take a half-filled bottle and blow across the top to get a reference sound. After emptying the bottle, blowing across the top would generate a tone about an octave lower.
As critical as this aspect is in the acoustic voice of the guitar, it can easily turn into a nightmare onstage, where sound pressure is pushing at the body from every direction.
The body of a guitar acts as the chamber of a bottle, and the soundhole as the opening. The thickness of the guitar’s top is equivalent to the bottle’s neck, and in this case has little influence. Increasing the size of the body, and resulting air box, will lower the resonant frequency of this tone as long as the soundhole size stays the same. It is tempting to try measuring this by adding up the dimensions of a guitar body—the width at the upper and lower bouts, the waist, the length, and the depth—but doing this provides an inaccurate picture. That’s because the curves of a guitar can impact the overall air mass even more than mere dimensions.
For most acoustic guitars, this air-mass resonance will tend to live in the lower register of the guitar. It’s a large part of what makes up a particular instrument’s low-end response and has further impact across the entire register. As critical as this aspect is in the acoustic voice of the guitar, it can easily turn into a nightmare onstage, where sound pressure is pushing at the body from every direction. When this occurs at a frequency near one of the guitar’s natural top, back, or air resonance notes or overtones, we experience some form of feedback or oscillation as the notes fight for dominance. At this point, we turn to electronic manipulation to remove offending frequencies, shift their timing location via phase corrections, or reach for a mechanical solution in the form of a soundhole plug.
Covering up the soundhole of an acoustic guitar is remarkably effective at feedback control. After all, this completely upsets the guitar’s ability to act as an air chamber. Air no longer moves back and forth, resonating freely. Once the air inside the guitar does not have free passage with the air outside the guitar, even the resonance of the top and back is reduced a great deal, preventing them from responding to the sound pressure assaulting the guitar. While the guitar may sound like a stifled and congested version of itself, that sometimes is exactly what we need.
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AI, which generated this image in seconds, can obviously do amazing things. But can it actually replace human creativity?
Technology has always disrupted the music biz, but we’ve never seen anything like this.
AI has me deeply thinking: Is guitar (or any instrument) still valid? Are musicians still valid? I don’t think the answer is as obvious as I’d like it to be.
As a professional musician, I’ve spent the vast majority of my days immersed in the tones of tube amps, the resistance of steel strings under my fingers, and the endless pursuit of musical expression. Each day, I strive to tap into the Source, channel something new into the world (however small), and share it. Yet, lately, a new presence has entered the room—artificial intelligence. It is an interloper unlike any I’ve ever encountered. If you’re thinking that AI is something off in the “not-too-distant future,” you’re exponentially wrong. So, this month I’m going to ask that we sit and meditate on this technology, and hopefully gain some insight into how we are just beginning to use it.
AI: Friend or Foe?
In the last 12 months, I’ve heard quite a bit of AI-generated music. Algorithms can now “compose,” “perform” (with vocals of your choosing), and “produce” entire songs in minutes, with prompts as flippant as, “Write a song about__in the style of__.” AI never misses a note and can mimic the finer details of almost any genre with unnerving precision. For those who are merely curious about music, or those easily distracted by novelty, this might seem exciting … a shortcut to creating “professional” sounding music without years of practice. But for those of us who are deeply passionate about music, it raises some profound existential questions.
When you play an instrument, you engage in something deeply human. Each musician carries their life experiences into their playing. The pain of heartbreak, the joy of new beginnings, or the struggle to find a voice in an increasingly noisy and artificial online world dominated by algorithms. Sweat, tears, and callouses develop from your efforts and repetition. Your mistakes can lead to new creative vistas and shape the evolution of your style.
Emotions shape the music we create. While an algorithm can only infer and assign a “value” to the vast variety of our experience, it is ruthlessly proficient at analyzing and recording the entire corpus of human existence, and further, cataloging every known human behavioral action and response in mere fractions of a second.
Pardon the Disruption
Technology has always disrupted the music industry. The invention of musical notation provided unprecedented access to compositions. The advent of records allowed performances of music to be captured and shared. When radio brought music into every home, there was fear that no one would buy records. Television added visual spectacle, sparking fears that it would kill live performance. MIDI revolutionized music production but raised concerns about replacing human players. The internet, paired with the MP3 format, democratized music distribution, shattered traditional revenue models, and shifted power from labels to artists. Each of these innovations was met with resistance and uncertainty, but ultimately, they expanded the ways music could be created, shared, and experienced.
Every revolution in art and technology forces us to rediscover what is uniquely human about creativity. To me, though, this is different. AI isn’t a tool that requires a significant amount of human input in order to work. It’s already analyzed the minutia of all of humanity’s greatest creations—from the most esoteric to the ubiquitous, and it is wholly capable of creating entire works of art that are as commercially competitive as anything you’ve ever heard. This will force us to recalibrate our definition of art and push us to dig deeper into our personal truths.
“In an age where performed perfection is casually synthesized into existence, does our human expression still hold value? Especially if the average listener can’t tell the difference?”
Advantage: Humans
What if we don’t want to, though? In an age where performed perfection is casually synthesized into existence, does our human expression still hold value? Especially if the average listener can’t tell the difference?
Of course, the answer is still emphatically “Yes!” But caveat emptor. I believe that the value of the tool depends entirely on the way in which it is used—and this one in particular is a very, very powerful tool. We all need to read the manual and handle with care.
AI cannot replicate the experience of creating music in the moment. It cannot capture the energy of a living room jam session with friends or the adrenaline of playing a less-than-perfect set in front of a crowd who cheers because they feel your passion. It cannot replace the personal journey you take each time you push through frustration to master a riff that once seemed impossible. So, my fellow musicians, I say this: Your music is valid. Your guitar is valid. What you create with your hands and heart will always stand apart from what an algorithm can generate.
Our audience, on the other hand, is quite a different matter. And that’s the subject for next month’s Dojo. Until then, namaste.
Our columnist’s bass, built by Anders Mattisson.
Would your instrumental preconceptions hold up if you don a blindfold and take them for a test drive?
I used to think that stereotypes and preconceived notions about what is right and wrong when it comes to bass were things that other people dealt with—not me. I was past all that. Unfazed by opinion, immune to classification. Or so I thought, tucked away in my jazz-hermit-like existence.
That belief was shattered the day Ian Martin Allison handed me a Fender Coronado while I was blindfolded in his basement. (Don’t ask—it’s a long story and an even longer YouTube video if you have time to kill.) For years, I had been a single-cut, 5-string, high-C-string player. That was my world. So, you can imagine my shock when I connected almost instantly with something that felt like it was orbiting a different solar system.
Less than 5 minutes with the instrument, and it was all over. The bass stayed in Ian’s basement. (I did not.) I returned home to Los Angeles, but I couldn’t stop thinking about it. I kept playing my beloved semi-chambered single-cut 5-string, but I sent its builder, Anders Mattisson, a message about my recent discovery. I asked if there was any way we could create something with the essence of a Coronado while still suiting my playing and my music.
That’s when everything I thought I knew about bass—and the personal boundaries I had set for myself—came crashing down.
When we started talking about building a bass with a fully chambered body, much like the Coronado, I was adamant about two things: It needed to have active electronics, and I would never play a headless bass.
Fast-forward three months to the winterNAMM show in California. Anders arrived for dinner at my house, along with a group of incredible bass players, includingHenrik Linder. I was literally in a chef’s apron, trying to get course after course of food on the table, when Henrik said, “Hey, let’s bring the new bass in.”
He came down the stairs carrying something that looked suspiciously like a guitar case—not a bass case. I figured there had been some kind of mistake or maybe even a prank. When I finally got a break from the chaos in the kitchen, I sat down with the new bass for the first time. And, of course, it was both headless and passive.
I should mention that even though I had made my requests clear—no headless bass, active electronics—I had also told Anders that I trusted him completely. And I’m so glad I did. He disintegrated my assumptions about what a bass “has to” or “should” be, and in doing so, changed my life as a musician in an instant. The weight reduction from the fully chambered body made it essential for the instrument to be headless to maintain perfect balance. And the passive nature of the pickups gave me the most honest representation of my sound that I’ve ever heard in over 30 years of playing bass.
I’m 46 years old. It took me this long to let go of certain fundamental beliefs about my instrument and allow them to evolve naturally, without interference. Updating my understanding of what works for me as a bass player required perspective, whereas some of my most deeply held beliefs about the instrument were based on perception. I don’t want to disregard my experiences or instincts, but I do want to make sure I’m always open to the bigger picture—to other people’s insights and expertise.
Trusting my bass builder’s vision opened musical doors that would have otherwise stayed bolted shut for years to come. The more I improve my awareness of where the line between perception and perspective falls, the more I can apply it to all aspects of my world of bass.
Maybe this month, it’s playing an instrument I never would have previously considered. Next month, it might be incorporating MIDI into my pedalboard, or transcribing bass lines from spaghetti Westerns.
No matter what challenges or evolutions I take on in my music and bass playing, I want to remain open—open to change, open to new ideas, and open to being proven wrong. Because sometimes, the instrument you never thought you’d play ends up being the one that changes everything.
Genuine, dynamic Vox sound and feel. Plenty of different tone-sweetening applications. Receives other pedals as nicely as a real amp.
Can get icy quick. Preamp tube presents risk for damage.
$299
Tubesteader Roy
tubesteader.com
The Roy is an exceedingly faithful Vox box that brings genuine tube dynamics to your pedalboard.
This is an interesting moment for amp-in-a-box pedals. It used to be novel to have a little box that approximated the tone signature of an iconic amp. Nowadays, though, modeling pedals and profilers can give you many digital emulations in one package. Nevertheless, there are still worlds of possibility in pedals that copy amp topology in discrete form—particularly when you add a real preamp tube to that mix.
That’s what Montreal builder Tubesteader did with the Roy, their entry in the Vox-Top-Boost-AC30-in-a-box race. The Roy is a 2-channel preamp and overdrive built around a 12AX7 vacuum tube—a design gambit that is relatively uncommon if not totally unique. The tube makes the Roy look much more vintage in spirit at a time when sleek, black Helixes and Fractals are overtaking stages. In some ways, it looks like an antique. It can sound like one in the best way too.
Riding the Tube
The Roy comes in a handsome brownish-red enclosure, with an unsurprising control layout. The rightmost footswitch turns the pedal on and off, and the one at left switches between the identical channels. Each channel has an output volume and gain knob; the controls on the right are assigned to the default channel, and when you tap the left footswitch, you engage the left-side control tandem. The treble and bass controls between the two volume and gain knobs are shared by the two channels, but a post-EQ master tone cut control, which rolls off additional treble frequencies, is mounted on the crown of the pedal beside the power input. The input and output jacks occupy the left and right sides, along with a 3.5 mm jack for external operation. The Roy runs at 12 volts and draws 350 mA, and the included power supply can be reconfigured easily for a range of international outlets.
Tubesteader’s literature says the pedal’s tones are generated via a high-voltage transistor in the first gain stage coupled with the 12AX7, which operates at 260 volts. That preamp tube is nested at the top of the enclosure’s face, underneath a protective metal “roll bar”. Trusty as it looks, when there is a glass element on the exterior of a pedal’s housing, there’s an element of vulnerability, and transporting and using the Roy probably requires a more conscientious approach than a standard stompbox.
Royal Tones
Compared to the Vox's own Mystic Edge, an AC30-in-a-box from Vox powered by Korg’s NuTube vacuum fluorescent display technology, the Roy feels warmer, and more dynamic, proving that the 12AX7 isn’t just there for looks. The Mystic Edge could sound positively icy compared to the Roy’s smooth, even breakup. The Roy is very happy at aggressive settings, and in my estimation, it sounds best with output volumes driving an amp hard and the pedal’s gain around 3 o’clock. That recipe sounds good with single-coil guitars, but with a P-90-loaded Les Paul Junior, it achieves roaring classic-rock greatness. I’ve always felt Voxes, rather than Marshalls, are better vehicles for dirty punk chording. The Roy did nothing to dissuade me from that belief. And the pedals' midrange punch and bark in power-chord contexts lent authority and balance that makes such chords hit extra hard.
Though the Roy creates many of its own tasty drive tones, it really comes to life when pushed by a boost or overdrive, much like a real amp. When I punched it with a Fish Circuits Model One overdrive, the Roy was smoother and less spiky than a cranked AC30, yet there was plenty of note definition, attack, and the harmonic riches you’d turn to an AC for in the first place. A JFET SuperCool Caffeine Boost also brought additional depth and color to the output and broadened the pedal’s voice.
If you’re most comfortable with a real Vox amp, the Roy is a reliable and familiar-feeling stand-in when managing a different backline amp. In at least one way, though, the Roy is, perhaps, a bit toofaithful to its influence’s design: There’s a lot of treble on tap, and it’s easy to cook up tinnitus-inducing frequencies if you get too aggressive with the treble control. Noon positions on the cut/boost tone knobs sound pretty neutral. But I found it difficult to push the treble much past 2 o’clock without wincing—even with the tone cut control set at its darkest. (This quality, of course, may make the Roy a good match for squishier Fender-style designs). The relationship between the Roy’s treble and bass controls also takes time to master. The two don’t just add or boost their respective frequencies, but also add or subtract midrange, which can result in intense and sudden gain-response changes. As a general guideline, a light touch goes a long way when fine tuning these frequencies.The Verdict
The Roy isn’t exactly a bargain at $299. Then again, this Vox-in-a-box can stand in for real-deal Top Boost tones and the 2-channel design means you can move between an AC’s chimey cleans and ripping cranked sounds in a flash. If you’re squarely in the Vox amp camp, you’d be hard-pressed to find a more authentic means of achieving that range of clean-to-crunchy sounds.