BRONZE ALLOY AS AN ALTERNATIVE MATERIAL FOR
BALINESE MUSICAL INSTRUMENTS
I Made Miasa
Department of Mechanical and Industrial Engineering, Gadjah Mada University, Yogyakarta, Indonesia 55281
e-mail: miasa@ugm.ac.id
I Ketut Gede Sugita
Department of Mechanical Engineering, Udayana University, Bali, Indonesia
This research was carried out to investigate the acoustics feasibility of using silicon-bronze (Cu-5%Si) alloy to replace tin-bronze (Cu-20%Sn) alloy for Balinese traditional musical instruments. This research is the next step of the previous research that has sucessfully showed better mechanical properties of Cu-Si alloy compared to Cu-Sn alloy. The acoustical properties of interest in this study are: speed of sound (c), sound radiation coefficient (R) and frequency resistance (i.e. frequency’s shift). The casted alloys were cut and machined for the test specimens. The acoustical properties of both alloys were measured and compared. The investigations were carried out according to ASTM Standard E 1876-01. For frequency shift investigation, the original fundamental frequency of each alloy is measured and then each al-loy is subjected to impact load by hitting them as they are played in daily use. The total num-ber of cycles (hits) experienced by each alloy is 72,000. Finally the resulting fundamental frequency is measured and compared to the previous (before hitting) conditions. The fre-quencies were measured after a certain interval numbers of hit until total of 72,000 hits. The results showed that Cu-Si alloy has speed of sound (c) and sound radiation coefficient (R) that are almost the same as those of Cu-Sn alloy. For frequency’s shift, both alloy showed that they do not experience any frequency change after 72,000 hits. This is a good sign show-ing the potential of silicon-bronze alloy as an alternative material for Balinese musical instruments replacing tin-bronze alloy currently used.
The high tin-bronze alloy with composition of 18-22 wt. % Sn has good acoustical properties which is capable of producing long-lasting sound due to its low vibration damping 1,2. It is commonly used for music instrument materials such as bell, Javanese and Balinese gamelan. This is a double phase alloy containing brittle particles of Cu31Sn8 intermetallic (δ-phase), which is harder and more brittle than other alloys. However, the main disadvantages of high tin-bronze properties are: it has a low crack resistance (it cracks easily), it is brittle and expensive. Moreover, tin-bronze cannot resist low temperature because it has a low frost resistance. At temperature below minus (20 to 25)oC this alloy becomes brittle and cracks may appear resulted in poor sound quality3. Meanwhile silicon bronze is an alloy that has good cast ability and higher mechanical properties than tin- bronzes. In addition, the silicon bronze has high elastic properties, possesses a higher corrosion resistance than a tin bronze, does not lose its properties at low temperatures and it is cheaper than tin- bronze4,5. Therefore, based on the abovementioned facts, this study was carried out
to investigate the feasibility of silicon bronze alloy as a subtitute of tin-bronze alloy used for music instrument materials.
2. Experimental procedure
2.1 Materials and process
This study was started with the preparation of raw materials that will be casted such as copper (Cu), tin (Sn) and silicon (Si). The composition of copper and tin for tin-bronze alloys and the com-position of copper and silicon for silicon-bronze alloys were designed in accordance with the results of the authors’ previous studies 6,7. The alloys under investigation were made of Cu-5%Si and Cu-20%Sn. The composition of the alloy studied in this research is listed in Table 1. The acoustics properties comparisons were made with respect to Cu-20%Sn as the reference alloy because it is commonly used for making bells or gamelans. The commercial pure copper and commercial pure silicon were melted in crucible furnace at temperature of 1000oC. The resulting casted billets were cut and finished for acoustical test specimens. In this investigation three pieces of elements were prepared for each alloy, so that in total there are 6 specimens.
Table 1. Composition of the alloys.
Figure 1. Set–up of fundamental frequency measurement.
For frequency shift investigation, the original fundamental frequency of each alloy is meas-ured and then each alloy is subjected to impact load by hitting them such as when they are played in daily use. The total number of cycles (hits) experienced by each alloy is 72,000. Finally the result-ing fundamental frequency is measured and compared to the previous (before hittresult-ing) conditions. The frequencies were measured after a certain interval numbers of hit until total of 72,000 hits. Fig-ure 2 shows the experimental set up for frequency resistance tests.
Figure 2. Set–up for frequency resistance measurement.
From the measurements of fundamental frequency based on ASTM Standard E 1876-01, the Young’s modulus (E) can be calculated from
E=0.9465
(
m ff 2 b)
(
L3
t3
)
T1 (Pa) (1)where:
E = Young’s modulus [Pa] m = mass of the bar [gr] b = width of the bar [mm] L= length of the bar [mm] t = thickness of the bar [mm]
ff = fundamental resonant frequency of the bar in flexure [Hz] T1 = correction factor, in this case it has a value of
T1=[1.000+6.585(Lt ) 2
] . (2)
As one of the acoustics properties of interest in this study, the speed of sound traveling through material (c) is then calculated using the following formula
stiffness for a given mass (a static property) . Particularly, a large sound radiation coefficient is pre-ferred if a loud sound is desired. The sound radiation coefficient can be calculated from9
R=
√
Eρ3 (m4.kg/s) . (4)
3. Results and discussions
3.1 Fundamental and shift frequencies
Figure 3a and 3b show the fundamental frequencies of the specimens for both Cu-20%Sn and Cu-5%Si alloys respectively. As can be seen, for both alloys, the first three modes appeared. The fundamental one is used to calculate the Young’s Modulus (E) using Eq. (1) and then the remaining c and R are calculated from Eq. (3) and Eq. (4).
(a) (b)
Figure 3.Fundamental resonant frequency of the bar in flexure
NOTES: Bilah = bar
Bar 1, 2 and 3 are Cu-5%Si alloy Bar 4, 5 and 6 are Cu-20%Sn alloy
Bar 7, 8 and 9 are Cu-20%Sn alloy with forging process
Figure 4. Frequency resistance test results
3.2 Acoustical properties
From the measurements and calculations above, the speed of sound c, the sound radiation coefficient R and the frequency resistance of all alloys are tabulated in Table 2.
Table 2. Acoustical properties of bronze alloys No
. Alloys c (m/s) R (m
4/kg.s) Frequency
Resistance
1. Cu-20Sn 3398.4 0.393 Stable
(no frequency shift)
2. Cu-5Si 3380.4 0.407 Stable (no frequency shift)
It can be seen from Fig. 4 that most of the frequencies of the bars (Bilah # 1- 6) measured after a certain number of cycles remain unchanged (constant). No frequency shift indicates that the bar will not detune (lost its tune) easily so that a good sound quality can be maintained. An interest-ing phenomenon was observed for Cu-Sn alloys that experienced forging process after casting and cutting. These alloys easily change their frequencies, and in this study frequency shifts up to 6 Hz were observed. It seems that the forging process that is currently applied for these alloys tends to deteriorate their acoustics properties as indicated by the frequency shift. Furthermore, Table 2 showed that that Cu-Si alloy has a speed of sound (c) and a sound radiation coefficient (R) that are almost the same as those of Cu-Sn alloy.
4. Conclusion
composi-REFERENCES
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