Guitarbench Magazine Issue 4 Guitar Anatomy- Soundholes article by Terence Tan

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Guitar Anatomy - Soundholes

If you have been seeing a lot of interesting

modern guitars with a lot of unusual soundholes, you may be wondering what they are all about. Your guitar probably has one big hole in the middle of the guitar. Although most guitars have always had, and still do have, one centred soundhole, that orthodoxy is changing. As with most unconventional ideas, there is much misinformation and conjecture about the purpose and effect of multiple soundholes. I shifted the soundhole to the upper bout initially in the eighties to increase the uninterrupted area of my guitars’ and basses’ tops. Later, after studying the research of Helmholtz, I experimented with dividing the hole into two, unevenly sized, smaller holes. To understand the double soundhole it is useful to first understand the function of the

Harry Fleishman

soundhole of a conventional guitar. The term soundhole has led most people to believe that most of the sound of a guitar comes from the hole in the top. In fact, if one covers the hole only the low register is strongly affected. This is because the soundhole is actually a low frequency port, not so different from the bass port on a bass reflex speaker cabinet. Each aspect of the design of a guitar contributes to its tone. However the cavity resonance is based on a simple mathematical formula and can be set independently of the “voice” of the guitar, though it will, of course, have some effect on the voice. We can either use the following formulae to calculate the Helmoltz resonance of the guitar’s body, or we can make educated guesses based on similarly sized existing guitars. The Helmholtz resonance is the natural lowest range of the cavity, in this case the guitar body.

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Essentially, it is the lowest note the guitar can support with an output that is balanced with the rest of the instrument. There are tricks we can use to fool the ear, but that is for another article. H =(C/2π) x (√a/V)

H is the resonance frequency a is the area of opening V is the volume of the box, the guitar body. C = speed of sound = 344 meters per second (1140 ft./ sec) a = area of soundhole = pi X radius squared V = volume of air in guitar body

Simply stated, (unlike the formulae above) a larger box has a lower resonance frequency, all other things being equal. The soundhole diameter is the second defining feature controlling cavity resonance. A smaller soundhole leads to a lower cavity resonance, which most people find counter intuitive. As we will see, this is due to friction, believe it or not! Multiple soundholes can be added up in area (up to a point) and each will have its own resonance frequency with the box. This is based on Helmholtz’ work (about 1900!). The total will also have a frequency. Done correctly, the combination will have an audibly deeper and flatter response curve than the individual holes would have had.

Raising the resonance frequency improves miking and recording characteristics, but makes the guitar seem thinner to the player. Opening the access panel or having many openings can do this. Sometimes this less “bassy” voice is advantageous. Here’s what is happening, without all the math. When air is compressed in a cavity, the pressure inside increases. Once this force of air in the cavity disappears, the higher-pressure air inside will flow out. However, this surge of air flowing out will tend to over-compensate, due to the inertia of the air in the body, and the cavity will be left at a pressure slightly lower than the outside, causing air to be drawn back in. This process repeats with the magnitude of the pressure changes decreasing each time. This is what effects the sustain of low notes. What slows down this in and out flow of air is the friction of the column of air as it tries to go through the soundhole. Although it seems counter intuitive, a larger hole raises the inherent resonance of the cavity; a smaller one lowers it. People often cite Clarence White’s Martin, with its enlarged soundhole, as

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an example of a larger hole having more bass. In fact, it is the ability to play closer to the mic because of less bass that gives it that extra punch. All of the above relationships are separate from the effect of the mass of the top and back and the way they affect sustain, and reflection, as well as absorption of energy. By dividing the hole into two smaller, unequal holes, we may use the ideas from Helmholtz to both lower the range of the bass frequencies the guitar is capable of producing, and to flatten out the pronounced peak normally present in acoustic guitars. This is especially valuable in controlling the boominess or evenness of the low register. If the soundhole would normally be 78 square centimetres, we may divide it into two smaller holes, 31 and 47 square centimeters respectively. As discussed earlier the smaller soundhole will provide a lower cavity resonance than the larger, and the two combined will have the resonance of the original, combining to form an overall lower, flatter response than any of the three individually. You can picture three small hills, instead of one big mountain. An additional benefit of multiple smaller holes is the increased freedom to brace without the holes in the way. When I first began to move the hole I simply let the brace jump across the hole. Soon I realized that I could brace in new, more interesting ways, with more ability to control the voice of the guitar. By placing the smaller hole toward the treble side, that area could be relieved or cutaway to provide access to higher frets. By playing around with the soundhole sizes we can adjust the low register of a guitar, emphasize some frequencies, or reduce bass to improve the way the guitar sounds on microphone. There is a lot more to learn about soundholes; I listen forward to hearing what luthiers come up with in the future.

Harry Fleishman is an accomplished luthier based in California. In addition to his work as a custom luthier he spends considerable time teaching in his world famous Luthiers School International. He can be reached at: fleishmaninstruments.com or by clicking on the image above!