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How Does Soundhole Size Affect the Tone and Loudness (dB) of a Guitar?
Abstract: Guitars may have different sized soundholes in their soundboxes. This investigation attempts to reveal how soundhole size affects loudness (dB), and tone frequencies resonated by the guitar. The results of this experiment show that soundhole size does affect loudness and has a minimal affect on tone.
Introduction: The modern acoustic guitar has been a popular instrument for many decades. The steel- string and classical (nylon string) guitars have their roots in 13th^ Century Spain, evolving from what was once called the guitarra latina. The standard tuning (E-A-d-g-b-e’) of the modern six string guitar was adopted by England and France in the 18th^ Century and this design included a soundhole surrounded with a rosette, located just above the waist.^1 Examining many guitars on the market today, one may notice that guitar soundholes come in many different shapes, sizes, and locations. Although guitars with soundholes located on the side of the soundbox (body) or on the sides of the strings, the “standard” six string design includes a soundhole placed in the same location as the first 18th^ Century guitars, beneath the strings, above the waist, and between the neck and bridge. The guitar is a string instrument, based upon the concepts of standing waves and resonance. The steel strings are fixed on both ends, by the headboard/nut and the bridge. Standing waves are produced by either plucking or “strumming” the strings with ones fingers or a type of guitar pick. Sound waves and vibrations are transmitted from the strings to the bridge, which is fixed on the solid top of the soundbox. The neck also serves as a method of sound wave transmission^2 because the strings are fixed to the nut on the headboard. The soundbox, often
compared to a Helmholtz resonator^3 , is made of wood and can have varying air volumes from model to model. Sound energy transferred from the strings to the soundbox causes the air inside to vibrate at the same frequency as the strings, amplifying the sound.^4 This amplified sound then “escapes” (excites air particles outside the soundbox) through the soundhole which is typically 3¼” to 4¼” (8.25 cm -10.8 cm) in radius. Although soundhole size seems to be standard (8.25 cm -10.8 cm in radius) among all center-soundhole guitars, there are a vast array of specialty or custom guitars with large, small, side location, and off-center sound holes. For example, one guitar model, the Ovation Celebrity, has around 10-15 varying size soundholes located on both sides of the strings. These variations of soundhole configuration and size raise a question that many novice musicians may ask when purchasing a guitar: what effect does soundhole size have on the loudness of a guitar? If altering the location of the soundhole will change the tone of the guitar, what will happen to tone if the size of the soundhole is changed?
Experiment Design: To answer this question, a “standard” design steel string guitar was chosen for testing. The guitar tested was a Yamaha CPX-5 acoustic guitar. The guitar has a soundhole with a diameter of 10.25 cm (about 2 inches), a top surface thickness of .32 cm (1/8 in.), and an estimated soundbox volume of 1232 cm^3 (75.2 in^3 ) which was calculated using an online application^5. In order to vary the size of the soundhole, a piece of heavy cardboard (approximately the same thickness as the face of the guitar ~ .3cm) was taped over the soundhole. Concentric circles with radii of 1, 2, 3, and 4 cm were cut out of the board to simulate varying size soundholes. A standard computer microphone was used to record tone and loudness from a distance of 10cm aimed directly at the opening of the soundhole. Measurements were taken and recorded by a shareware software program called Analyzer2000 v5.0 which displays a Fourier
In examining each string tested (e, g, E), the Fourier analysis graphs (top half) and spectrum analysis (bottom half) shows both frequency and decibel level. Figures 2.1 through 2. show the graphs for soundholes of radius 0cm, 1cm, 2cm, 3cm, 4cm, and 5.125cm (standard size) on the #1 “high” e string. In comparing the graphs, one can see that as the soundhole increases in size, the decibel level of the highest peak goes up (from around -64dB at 0cm to -56dB at 5.125cm). Upon analysis of the frequencies measured, the graphs of 0cm radius (2.1) and 1cm (2.2) radius show little or no measurement in the 2000-3300Hz range. From radii of 3cm to 5.125cm, there is a steady increase in presence of higher frequency sound waves in the 2000- 3300Hz range. Figures 3.1 through 3.6 show the graphs for soundholes of radius 0cm, 1cm, 2cm, 3cm, 4cm, and 5.125cm (standard size) on the #6 “low” E string. Viewing the graphs, there is an increase in loudness from soundholes of radii 0cm to 2cm; however, there is a decrease in dB level from 2cm to 4cm (figures 3.3 - 3.5). The frequency analysis shows that in figures 3.2 – 3. there is an increased presence of sound waves in the 1475-1950Hz range. Figures 4.1 through 4.6 show the graphs for soundholes of radius 0cm, 1cm, 2cm, 3cm, 4cm, and 5.125cm (standard size) on the #3 g string. The loudness of the #3 g string steadily decreases from soundhole with a radius of 0cm to the soundhole with a radius of 3cm. Thereafter, the loudness takes a small jump back up (figures 4.4 - 4.6). The Fourier graphs show that as the sound hole increases, the presence of sound waves in the frequency range of 1800-3400Hz range slowly increases as well. The data collected (figures 2.7, 3.7, and 4.7) seems to follow the prediction that the increases in loudness and presence of higher frequency harmonics in the sound waves occurs where the sound frequency comes close to the resonant frequency. The frequency of the #6 “low” E string (329.63 Hz) should come close to the calculated resonant frequency of a soundhole with a radius between 0cm (0Hz) and 1cm (490.5Hz). Data
does show an increase in loudness between these two soundhole sizes, but the loudness continues to increase to the 3cm radius soundhole before falling back down. A small increase in loudness also occurs from the 4cm radius to the actual “standard” radius. The frequency of the #1 “high” e string (1318.51 Hz) should come close to the calculated resonant frequency of a soundhole with a radius around the length of 3cm (1471Hz). The actual increase in loudness and increase in presence of high frequency harmonics occurs from a 3cm radius to a 5cm radius. Finally, the frequency of the #3 g string (783.99Hz) should come close to the calculated resonant frequency of a soundhole with a radius between 1cm (490.5Hz) and 2cm (980.1cm), but loudness actually increases from 0cm to 3cm. These measurements show that the predictions are slightly off.
Conclusion/Suggestions: Although the end results of the experiment may vary slightly from the predictions, they are close enough to support the theory: as sound wave frequency (from the strings) gets closer to the calculated resonant frequency, the loudness will increase. We must remember that the guitar is a complex instrument with numerous variables contributing to the sound waves resonated by the soundbox. Perhaps our error appears because the resonant frequencies have been miscalculated due to a bad soundbox volume measurement. The irregular shape of the guitar soundbox makes it difficult to accurately measure the air volume inside without chance for error (air temperature also is a factor). In the end, we learn that soundhole size affects the calculated resonant frequency and thus, guitar designers and manufacturers must find a soundhole size that will adequately resonate all of the frequencies of the 6 strings. A larger radius soundhole will amplify/resonate high frequency tones, while a smaller radius soundhole will amplify mid-to-low frequency tones.
