19 th INTERNATIONAL CONGRESS ON ACOUSTICS MADRID, 2-7 SEPTEMBER 2007

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1 19 th INTERNATIONAL CONGRESS ON ACOUSTICS MADRID, 2-7 SEPTEMBER 2007 Acoustic Radiation Pattern of the Sanjo Gayageum: A Korean traditional plucked string instrument PACS: z Jung Uk Noh; Hyun-Woo Koh; Koeng-mo Sung Institute of New Media and Communications, School of Electrical Engineering, Seoul National Univ. Shillim-dong, Kwanak-gu, Seoul, South Korea; junoh37@acoustics.snu.ac.kr; kenshin@acoustics.snu.ac.kr; kmsung@acoustics.snu.ac.kr ABSTRACT The Sanjo Gayageum is the most popular traditional plucked string instrument of Korea. It consists of wooden body which is about 140 cm long by 20 cm wide by 10 cm high, twelve silk thread strings and twelve wooden bridges that support each string. In this paper, we measured acoustic radiation pattern of two Sanjo Gayageum samples. To exclude any influences that would affect the vibration of the Gayageum body as much as possible (such as player, floor or room), we designed a duralumin frame to support the Gayageum body on a turn table, and used electromagnetic exciter for non-contact excitation. Also, all the measurements are performed in anechoic chamber at Institute of New Media and Communications, Seoul National University. We used 1/6 octave stepped sine signal which is ranged from 50 Hz to 12.8 khz. And rotation interval of turn table was 15 degree. INTRODUCTION The Gayageum is one of the representative Korean traditional string instruments. It is a plucked string musical instrument that originated in ancient China. We can find many Gayageum-like musical instruments in Asia. The Zheng (China) and the Koto (Japan) are also Gayageum-like musical instruments. Nowadays, the Gayageum is employed in two different types of music. One is Jeong-ak, which is court music in Korea. The other is Sok-ak, which is folk music. The Gayageum used in Jeongak is called Pungryu-Gayageum. It is the original form of the Gayageum. Sanjo-Gayageum, which is used in Sok-ak, has smaller size and narrower gap between the two adjacent strings than the former to suit the music with fast tempo such as Sanjo. These days Sanjo-Gayageum is widely used and preferred. In this paper, Sanjo-Gayageum is measured and presented. The Gayageum is about 160 centimetres in length, 26 centimetres in width and 5 centimetres in height, which is made of paulownia wood. The top plate has a gently curved shape, and the bottom plate has a flat shape with three holes on it. (Figure 1) The Pungryu-Gayageum does not have bottom plate. Figure 1.- Top and bottom view of the Sanjo Gayageum

2 Sanjo Gayageum (also Pungryu-Gayageum) has twelve strings, which are made of twisted silk threads with different gauge. They are generally tuned to 75 Hz ~ 432 Hz (E2b~B4b) (Table I) Table I.- Generally tuned pitch of each string string No. pitch [Hz] - key E 2 b A 2 b B 2 b E 3 b F A 3 b B 3 b C E 4 b F A 4 b B 4 b The bridge of the Gayageum is called Anjok. One Anjok supports only one string. And it can be moved along the string. With its relatively tall height (about 6.5 ~ 7.5 centimetres), the mobility of the Anjok makes easy not only to tune the string immediately but also to play the Nong-hyun technique (which is similar to vibrato technique). The playing technique of the Gayageum is as follows. The left-hand is used for pressing the strings and its finger movements can produce various types of movements such as shaking, bending, vibrating the strings. The right hand is used for plucking the strings. (Figure 2) The Nong-hyun technique of the Gayageum has very slow rate (0.3~0.4 seconds in period) and wide pitch variation (80~130 cents). Figure 2.- A Sanjo Gayageum with a player Generally, a string instrument can be regarded as the combination of string, bridge and body. The timber of a string instrument is determined by the acoustical characteristics of these parts and of the auditory space which the instrument is played [1]. In according to past research works, the body plays a most important role in characterizing the timber. [1] Much work has been done on violin family, but very little on these traditional musical instruments [1], [2], [3], [4]. Including the Anjok, the string and the body of the Gayageum are coupled at many points. This makes it complex to study the body s transfer function. In this paper, we grouped twelve positions of twelve Anjoks into four measurement positions, which are the position of 2nd, 5th, 8th, 11th strings Anjok. We considered each position as representative position among the positions of adjacent three Anjoks, because the differences in position among them are relatively small to the size of the Gayageum. Then, we measured acoustic radiation pattern of two Sanjo Gayageum samples. 2

3 MEASUREMENT When a Gayageum is played by a player and radiates its sound, there are some external conditions that would affect the sound of Gayageum, such as player, floor or room. To obtain acoustic characteristics of the Gayageum only, we need to exclude these external conditions as much as possible. So, we designed a support frame, which is made of duralumin rods, to support the Gayageum body on a turn table. (Figure 3(a)) With this support frame, we can also regularize boundary conditions of our measurement. We made an electromagnetic exciter for non-contact excitation. (Figure 3(b)) And we muted all strings on the Gayageum body with some pieces of felt cloth. Also, all the measurements are performed in anechoic chamber at Institute of New Media and Communications, Seoul National University. (a) (b) Figure 3.- A support frame (a) and an electromagnetic exciter (b) We used 1/6 octave stepped sine signal which is ranged 50 Hz to 12.8 khz. And rotation interval of turn table was 15 degree. We measured radiation pattern on two axes, vertical and horizontal axis. Table II shows measurement equipments we used. Table II.- Measurement equipments Measurement equipments B&K PULSE System type 3560C Liberty Instruments, Praxis B&K type 4133 and 2669 B&K type 2672 B&K Controllable Turntable type 5960 Turntable Controller type 5949 Notes I/O Front-end Microphone & Preamp Signal conditioning amplifier Turn table and controller We measured acoustic radiation pattern of two Sanjo Gayageum smaples. Each one is made by different master craftsman in Korea. ACOUSTIC RADIATION PATTERNS We measured acoustic radiation patterns of two Sanjo Gayageum samples. For each Gayageum, we measured on four positions, which are the position of 2nd, 5th, 8th, 11th strings Anjok. We will show radiation patterns of each string s fundamental frequency (pitch) and around some harmonic frequencies. (Figure 4 ~ 11) 3

4 Figure 4.- Radiation patterns of Sanjo Gayageum 1 on horizontal axis (left) and vertical axis (right), at 2nd string s position Figure 5.- Radiation patterns of Sanjo Gayageum 1 on horizontal axis (left) and vertical axis (right), at 5th string s position Figure 6.- Radiation patterns of Sanjo Gayageum 1 on horizontal axis (left) and vertical axis (right), at 8th string s position Figure 7.- Radiation patterns of Sanjo Gayageum 1 on horizontal axis (left) and vertical axis (right), at 11th string s position 4

5 Figure 8.- Radiation patterns of Sanjo Gayageum 2 on horizontal axis (left) and vertical axis (right), at 2nd string s position Figure 9.- Radiation patterns of Sanjo Gayageum 2 on horizontal axis (left) and vertical axis (right), at 5th string s position Figure 10.- Radiation patterns of Sanjo Gayageum 2 on horizontal axis (left) and vertical axis (right), at 8th string s position Figure 11.- Radiation patterns of Sanjo Gayageum 2 on horizontal axis (left) and vertical axis (right), at 11th string s position 5

6 There are some interesting radiation patterns in above figures. For the first Gayageum sample, at about 473 Hz, the Gayageum body radiates sound mostly forward and backward on horizontal axis, like dipole, while it radiates omni-directionally on vertical axis. (Figure 5, 6, 7) At about 322 Hz, However, it radiates sound mostly form side to side on horizontal axis. (Figure 5) The results of the second Gayageum sample are somewhat similar to that of the first sample. However, in some frequencies, the radiation patterns are much different. Generally in low-frequency range, about from 100 Hz, to 300 Hz, which is the range of low- and mid-pitched strings fundamental frequencies, the radiation efficiency seems inefficient, compared with the mid-frequency range, about from 300 Hz to 1.5 khz, which are both the fundamental frequencies of high-pitched strings and harmonic frequencies of all strings. In high frequency range, about above 1.5 khz, the radiation patterns have complex shape and radiation efficiencies seem to decrease. (Figure 7, 11) CONCLUSIONS In this paper, we studied on the acoustic radiation patterns of the Sanjo Gayageum body. We designed some measurement equipments, which are a support frame and electromagnetic exciter, to exclude any influences that would affect the vibration of the Gayageum body and regularize measurement boundary conditions. With these equipments, we measured acoustic radiation patterns of two Sanjo Gayageum samples on both horizontal and vertical axes. It will be our future works that not only to measure more samples as much as possible but also to study the other type of Gayageum, the Pungryu-Gayageum. ACKNOWLEDGMENTS This work was supported by The National Center for Korean Traditional Performing Arts (NCKTPA). References: [1] Neville H. Flethcer, Thomas D. Rossing: The Physics of Musical Instruments. 2nd edition, Springer (1998) [2] Gabriel Weinreich, Eric B. Arnold: Method for measuring acoustic radiation fields. Research Papers in Violin Acoustics , vol. 1 (1997) [3] Jürgen Meyer: The sound of the Orchestra. Research Papers in Violin Acoustics , vol. 1 (1997) [4] Carleen Maley Hutchins: Research Papers in Violin Acoustics , vol. 1 (1997)