Utilizing a rubber soundhole plug is a go-to transfer for decreasing suggestions when plugging in an acoustic guitar. However what have an effect on does it have on the general sound of the guitar?

Q: I’ve two acoustic-electric guitars with under-the-top pickup techniques. When enjoying dwell with ground screens, I typically use a rubber soundhole plug to scale back the possibility of suggestions. I don’t discover it, however I’m questioning if the truth that the soundhole is closed alters the true acoustic sound of the guitar because the sound vibrations are projected by way of the soundhole? —Randy Cummings

A: This can be a nice query, because it combines elementary guitar design with sensible player-level expertise. The soundhole is a vital function with huge implications for the tone and response of the guitar; plugging it can actually have an impact. To grasp precisely how, let’s begin with the necessities.

The guitar is, at its most simple stage, an air pump. The strings’ vibrations distort the highest (in addition to the again and sides, although to a lesser extent). This adjustments the quantity of air contained throughout the physique. As this quantity will increase and reduces, air is compelled out and in by way of the soundhole, creating the sound you hear. That’s why calmly constructed guitars with extra versatile tops are typically louder—their surfaces are capable of distort additional, creating better web adjustments within the airspace throughout the guitar physique.

Like all enclosures, the physique may have a sequence of resonant peaks the place it vibrates most effectively. For those who’ve ever blown throughout the mouth of a glass bottle, you’ve found the resonant peak of an air chamber. Because of their advanced form and versatile surfaces, guitar our bodies have a couple of peak—the air resonance, which is akin to the bottle, in addition to an ascending sequence of high and again resonances. Trendy makers have began to make use of FFT (Quick Fourier Remodel) software program to investigate these peaks in actual time whereas voicing their tops, backs, and assembled our bodies as a solution to each measure and predict the final word voice of the guitar.

The principle air resonance is most immediately affected by two predominant variables—the general quantity of the airspace, and the dimensions of the soundhole(s). Speaker cupboard designers play with this equation continually when attempting to design enclosures that may complement the actual qualities of a given speaker driver. Guitars are pretty distinctive of their number of sizes—you’ll nearly by no means see two identically tuned violins that had been as completely different in measurement as a parlor guitar and a jumbo—so the airspace quantity can fluctuate extensively. 

A cautious guitar designer will scale the soundhole to enhance the physique measurement, in an effort to keep up a balanced response between the bottom and highest ranges. Some adventurous corporations and luthiers have even explored the sonic results of various soundhole geometries—for instance, D’Addario’s O-Port is an aftermarket soundhole insert with a flared horn form, purported to enhance projection and readability (outcomes might fluctuate). The legendary Spanish luthier Antonio de Torres Jurado constructed lots of his devices with a tornavoz, which is a cylindrical tube extending beneath the soundhole into the guitar physique. Just a few trendy makers reminiscent of Tom Ribbecke and Mike Kennedy have reintroduced this method into the steel-string area with very promising outcomes.

Since we’re doing a deep dive, let’s additionally contact briefly on soundhole form and place. The spherical, centrally situated soundhole is the most typical by far, however some designers have performed with alternate shapes and positions fairly efficiently. On easy air chambers, altering the opening’s place doesn’t change the air resonance. The consensus at current amongst most of my colleagues is that an untraditional soundhole place can most simply contribute to altering the tone of the guitar by permitting the builder to brace the guitar in another way. Soundhole form is a extra advanced and controversial topic, however some experiments point out that lengthy, slim holes will have an effect on air chambers in another way than spherical ones. As soon as once more, speaker cupboard design displays this—some cupboards have slit-shaped or shelf ports, whereas others have spherical or tube ports, and every has its personal specific habits.

Right here’s a enjoyable experiment to shortly and simply illustrate the impact of soundhole measurement: Start by thumping on the bridge of your guitar. You’ll hear a muffled thud, which ought to have an identifiable pitch. It’s usually straightforward to search out the corresponding pitch on the decrease strings—I’ve discovered it wherever from first-fret low F as much as Bb or B on smaller guitars. Muting the strings with a bit of fabric or foam may also help with making the pitch extra audible. Subsequent, proceed tapping the bridge whereas transferring your different hand steadily over the opening, a bit just like the moon throughout an eclipse. You’ll clearly hear the pitch get decrease as you cowl extra of the opening.

This experiment demonstrates one of many typically counterintuitive realities of guitar physique tuning—a bigger gap will elevate the air chamber resonance, whereas a smaller one will decrease it. Since bass frequencies are giant waves, it’s usually assumed {that a} bigger gap will sound deeper, however the physics present in any other case. I’ve had a number of events to debate this with bluegrass guitarists who had been impressed to widen the soundholes on their dreadnoughts, in an try and seize the epic tone of Tony Rice’s well-known Martin D-28 with its enlarged gap. The result’s usually a guitar with a extra strident higher vary, which isn’t all the time what the participant expects.

Getting again to your query: What occurs when the soundhole is closed completely? On this case, the air contained in the guitar is prevented from transferring in/out of the opening, and as a substitute acts as a pneumatic spring, which counteracts the vibration of the physique. A tough analogy could be to the air inside a balloon—as you squeeze the balloon, the air strain inside it will increase.

If no air is popping out of the opening, why can we nonetheless hear the guitar? That is primarily as a result of the vibrations of the highest and again additionally displace air that’s outdoors the guitar, creating strain waves that our ears obtain as sound. Nonetheless, with the soundhole sealed up, the inner air spring restricts the highest and again’s free motion. That is why soundhole plugs assist scale back suggestions—they harness the captive air contained in the guitar as a device to scale back the highest’s skill to vibrate. Additionally they lock up the principle air resonance, which is the frequency at which the guitar can (and can!) feed again with most effectivity when amplified.

Clearly, there’s lots to say a few easy gap in a guitar! I hope this provides only a style of the complexities that guitarmakers take care of when designing and constructing. This is just one of an extended, lengthy listing of variables that add as much as create the tones we now think about basic. Fortunately, the common soundhole is simply giant sufficient for me to stuff my arm inside to repair free braces and cracked bridge plates—which may be very fortunate certainly!

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