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Side Located Soundholes and Ports-Theory and Execution

Ray Whitaker

 

Side Located Soundholes

In 1997 I started experimenting with soundholes located in the side of the guitar instead of in the soundboard. I had built several guitars with soundholes in the upper bout of the top and liked the increased amount of sound to the players ears. Putting the soundhole in the side was the next logical step in getting more tone and volume to the player, while regaining more top wood to project sound. This design feature is now beginning to find it's way into the mainstream and I'm glad to see that happening.

Most builders are very familiar with the relationship between the soundhole
size and the so-called "Helmholtz resonance". Guitar designers and builders generally try to find the optimum size soundhole for a particular design, based on the body size (interior volume) and construction. They then settle on that one size soundhole for each guitar, regardless of the types of wood the instrument happens to be built of. While this generally works well with the more commonly used body shapes and soundboard designs, I feel that it is, at best, a compromise.

I have seen pictures of, and read about, many guitars with soundholes in the side. However, those were in addition to, rather than instead of, the standard soundhole or soundholes in the top. While this may work well on acoustic archtop guitars because of their midrange-biased sound, I feel that on flat top guitars, it's the equivalent of throwing out the baby with the bath water. It disregards the Helmholtz resonance and it's effect on the guitars sound. It moves the resonant frequency to a point that will not accent the bass response. This will decrease the bass heaviness of a large jumbo or dreadnaught guitar, which may not be acceptable. Players who favor those types of guitars generally prefer them for the bass-heavy sound. On small bodied guitars it will drastically weaken the resonance of the bass notes.

Builders invest much time and care into tuning plates and assembling the soundbox, only to find that the Helmholtz resonance didn't fall where they expected it to. This might be because the wood density can vary considerably from one guitar to another, the depth of the sides was changed, or because the guitar is a new design still in the development stages. Sometimes the "educated guess" isn't exactly on the mark.

The stiffness of the top, back, and sides can also effect the Helmholtz resonance. A good example of this can be found in the *"Kasha Soundboard Without Waist Bar" article by Gila Eban. In that article she notes that on two almost identical guitars, one with a waist bar and one without, the frequency of the Helmholtz resonance (in real life, a combination of the Helmholtz resonance and the top and back resonances) was significantly lower on the non-waistbar guitar. The waist bar stiffens and adds mass not only to the soundboard, but also to the sides where the supports are attached.

I have developed a reputation for tackling unconventional projects, and because of this I have built many one of a kind designs for players who want something that is not available off the shelf. Most of these instruments begin with a clean slate, with the instrument being designed from the ground up to exactly fit the players needs. It is very hard to estimate where the Helmholtz resonance will end up on a new and untried design. There are mathematical formulas that can help with this process, but they are a simplification. Because the top, back and sides are not perfectly rigid, different wood properties can throw things off considerably.

Most builders concentrating on a few standard body shapes won't have a lot of problems with getting the resonance in a favorable location and can generally fine tune things with a new design in the first few guitars. When you're building only one example of a particular design, the guesswork, educated or otherwise, can result in a lesser quality sound for the player.

When the soundholes are located in the sides, the builder has the advantage of starting with a smaller soundhole than may be nessasary. After stringing up the guitar and allowing it to settle in a little, the soundhole size can be increased until the resonanant frequency is where the guitar produces the best sound. At this stage, it should be remembered that the frequencies of some resonances have been noted to fall by a small amount as the guitar settles in over time. When building a "one off", or any design, being able to fine tune the resonance gives you an additional edge to produce the best sound possible for the player. This process is more work and may not be cost effective for some builders, but I find the end result worth it.

Adjustable Ports

An adjustable port in the side of the lower bout of the guitar is another design feature that I first tried on a guitar in 1997. Varying the opening size allows the resonant frequency to be altered in a controlled fashion any time it is desired. I am certainly not the first luthier to find ways to vary the Helmholtz resonance in a controlled fashion. One noted example of this is the Advance model that James D'Aquisto built for Scott Chinnery in 1994. That guitar, profiled in the Jan. 1995 issue of 20th Century Guitar, varied the resonant frequency with 4 removable inserts perfectly fitted into two large holes in the soundboard. His design proves the old adage to be true. There is more than one way to skin a cat.

The movable panel of my adjustable port consists of thin vinyl coated aluminum sheet stock, with an ebony knob attached. The metal is flexible enough to conform to the side shape and slides in felt lined wood tracks. This allows the player to alter the total soundhole area quickly and easily to fine tune the guitars' sound to suit his or her tastes, or the type of music being played.

One player who now owns one of my guitars with this feature closes the port completely to locate the resonant frequency where the treble response is more accented for playing the music of Django. He opens the port back up for a sound more suited to Travis picking, or any other type of music where more bass is desired.

In order for the adjustable port to work properly, the size of the main soundhole must be reduced to put the resonant frequency below the "sweet spot" while the port is closed. I like to tune it so that sliding the door about 3/4 open raises the resonance up to the "sweet spot" where the bass response is strongest. This point is where the resonant frequency accents the bass the most, and is what most builders "shoot for" with a standard soundhole.

Opening the port fully will push the resonant frequency up past the "sweet spot" where the bass resonance is strongest. This will increase the overall volume and projection slightly due to the increase in air flow allowing the wood to vibrate more freely. The port adjustment is quite sensitive, and moving the sliding door 1/4" inch makes an audible difference to most players ears. It should also be noted that changing the resonant frequency effects every note on the fretboard, not just in the bass frequencies where it is the most noticable.

Another good point to make is that the "rumble in the ribcage" bass response on any bassy guitar is felt and heard more by the player than by those sitting several feet in front of the guitar. The slower bass frequency soundwaves do not project as well and die away quicker. Adjusting the port to where the bass is the weakest may sound the best to the player for a certain type of music, but the player should have someone else play the guitar while he or she sits in front of it for a better idea of what the listener hears.

Another plus with the adjustable port is that you can tune it to fight against feedback problems when using a pickup to play at high amplified volumes. It acts very much like a notch filter since it changes the resonant frequency of the guitar. Many readers with experience using a rubber "feedback buster" soundhole insert will have a clearer understanding of what I mean. Tuning the port can also give you more options when using a microphone for amplification.

Some players choose not to have the adjustable port in favor of a full size soundhole in the side. This gives them the maximum amount of volume possible directed at their ears. Each player will have different tastes and needs. If a player prefers a dreadnaught or jumbo guitar for most of their playing, the port may appeal to them because they can "dial out" the bass heavy sound to play fingerstyle, where more clarity is considered a plus. The advantages of the port diminish as the body sizes get smaller.

On a 000 size guitar I built recently for myself, I decided to put two almost equally sized soundholes in the side with no adjustable port. I generally play in a classical position and the soundholes are positioned where each hole projects sound at one of my ears. I wouldn't go so far as to call it "stereo", but I really enjoy all that sound.

Acoustic guitars are a very personal instrument, with the player usually spending a lot of time playing alone practicing and entertaining themselves. For this reason, I feel that the most important goal in acoustic guitar design is to make the player enjoy the experience as much as possible. Side located soundholes definitely add a great smile factor. If you get a chance to try out a side soundhole guitar, don't pass it up. You may find yourself pleasantly surprised.

Ray Whitaker

*Article by Gila Eban, published in American Lutherie #8 in 1986.

Copyright Ray Whitaker 1999.