From the September/October 2022 issue of Acoustic Guitar | By Martin Keith
Using a rubber soundhole plug is a go-to move for reducing feedback when plugging in an acoustic guitar. But what affect does it have on the overall sound of the guitar?
I have two acoustic-electric guitars with under-the-top pickup systems. When playing live with floor monitors, I generally use a rubber soundhole plug to reduce the chance of feedback. I don’t notice it, but I’m wondering if the fact that the soundhole is closed alters the true acoustic sound of the guitar since the sound vibrations are projected through the soundhole? —Randy Cummings
This is a great question, as it combines fundamental guitar design with practical player-level experience. The soundhole is a very important feature with big implications for the tone and response of the guitar; plugging it will certainly have an effect. To understand exactly how, let’s start with the essentials.
The guitar is, at its most basic level, an air pump. The strings’ vibrations distort the top (as well as the back and sides, though to a lesser extent). This changes the volume of air contained within the body. As this volume increases and decreases, air is forced in and out through the soundhole, creating the sound you hear. That’s why lightly built guitars with more flexible tops tend to be louder—their surfaces are able to distort further, creating greater net changes in the airspace within the guitar body.
Like all enclosures, the body will have a series of resonant peaks where it vibrates most efficiently. If you’ve ever blown across the mouth of a glass bottle, you’ve discovered the resonant peak of an air chamber. Due to their complex shape and flexible surfaces, guitar bodies have more than one peak—the air resonance, which is akin to the bottle, as well as an ascending series of top and back resonances. Modern makers have started to use FFT (Fast Fourier Transform) software to analyze these peaks in real time while voicing their tops, backs, and assembled bodies as a way to both measure and predict the ultimate voice of the guitar.
The main air resonance is most directly affected by two main variables—the overall volume of the airspace, and the size of the soundhole(s). Speaker cabinet designers play with this equation constantly when trying to design enclosures that will complement the particular qualities of a given speaker driver. Guitars are fairly unique in their variety of sizes—you would almost never see two identically tuned violins that were as different in size as a parlor guitar and a jumbo—so the airspace volume can vary widely.
A careful guitar designer will scale the soundhole to complement the body size, in an effort to maintain a balanced response between the lowest and highest ranges. Some adventurous companies and luthiers have even explored the sonic effects of different soundhole geometries—for example, D’Addario’s O-Port is an aftermarket soundhole insert with a flared horn shape, purported to improve projection and clarity (results may vary). The legendary Spanish luthier Antonio de Torres Jurado built many of his instruments with a tornavoz, which is a cylindrical tube extending below the soundhole into the guitar body. A few modern makers such as Tom Ribbecke and Mike Kennedy have reintroduced this approach into the steel-string field with very promising results.
Since we’re doing a deep dive, let’s also touch briefly on soundhole shape and position. The round, centrally located soundhole is the most common by far, but some designers have played with alternate shapes and positions quite successfully. On simple air chambers, altering the hole’s position does not change the air resonance. The consensus at present among most of my colleagues is that an untraditional soundhole position can most easily contribute to changing the tone of the guitar by allowing the builder to brace the guitar differently. Soundhole shape is a more complex and controversial subject, but some experiments indicate that long, narrow holes will affect air chambers differently than round ones. Once again, speaker cabinet design reflects this—some cabinets have slit-shaped or shelf ports, whereas others have round or tube ports, and each has its own particular behavior.
Here’s a fun experiment to quickly and easily illustrate the effect of soundhole size: Begin by thumping on the bridge of your guitar. You’ll hear a muffled thud, which should have an identifiable pitch. It’s often easy to find the corresponding pitch on the lower strings—I’ve found it anywhere from first-fret low F up to Bb or B on smaller guitars. Muting the strings with a piece of cloth or foam can help with making the pitch more audible. Next, continue tapping the bridge while moving your other hand gradually over the hole, a bit like the moon during an eclipse. You will clearly hear the pitch get lower as you cover more of the hole.
This experiment demonstrates one of the sometimes counterintuitive realities of guitar body tuning—a larger hole will raise the air chamber resonance, whereas a smaller one will lower it. Since bass frequencies are large waves, it is often assumed that a larger hole will sound deeper, but the physics show otherwise. I’ve had several occasions to discuss this with bluegrass guitarists who were inspired to widen the soundholes on their dreadnoughts, in an attempt to capture the epic tone of Tony Rice’s famous Martin D-28 with its enlarged hole. The result is often a guitar with a more strident upper range, which is not always what the player expects.
Getting back to your question: What happens when the soundhole is closed entirely? In this case, the air inside the guitar is prevented from moving in/out of the hole, and instead acts as a pneumatic spring, which counteracts the vibration of the body. A rough analogy would be to the air inside a balloon—as you squeeze the balloon, the air pressure inside it increases.
If no air is coming out of the hole, why can we still hear the guitar? This is mainly because the vibrations of the top and back also displace air that is outside the guitar, creating pressure waves that our ears receive as sound. However, with the soundhole sealed up, the internal air spring restricts the top and back’s free movement. This is why soundhole plugs help reduce feedback—they harness the captive air inside the guitar as a tool to reduce the top’s ability to vibrate. They also lock up the main air resonance, which is the frequency at which the guitar can (and will!) feed back with maximum efficiency when amplified.
Obviously, there’s a lot to say about a simple hole in a guitar! I hope this gives just a taste of the complexities that guitarmakers contend with when designing and building. This is only one of a long, long list of variables that add up to create the tones we now consider classic. Thankfully, the average soundhole is just large enough for me to stuff my arm inside to fix loose braces and cracked bridge plates—which is very lucky indeed!
Uncertain about guitar care and maintenance? The ins-and-outs of guitar building? Or another topic related to your gear? Ask Acoustic Guitar’s repair expert Martin Keith by sending an email titled “Repair Expert” to Editors.AG@stringletter.com and we’ll forward it to Keith. If your question is selected for publication, you’ll receive a complimentary copy of AG’s Acoustic Guitar Owner’s Manual.
This article originally appeared in the September/October 2022 issue of Acoustic Guitar magazine.