FEASIBILITY STUDY ON THE USE OF SILICON-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 alloy is subjected to impact load by hitting them as they are played in daily use. The total number 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 frequencies 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 showing the potential of silicon-bronze alloy as an alternative material for Balinese musical
instruments replacing tin-bronze alloy currently used.
1.
Introduction
Bronze is a metal alloy primarily made of copper (Cu), usually with tin (Sn) as the main additive. This alloy is one of the oldest alloys known to human civilization. Bronze is often called tinbronze because its main components are copper and tin. Tin-bronze is widely used for machine elements such as bearings, pump vanes, piston rings, bells, gears, etc. Compared to pure copper and
brass, bronze is an alloy which is easily casted and formed.
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20th International Congress on Sound and Vibration (ICSV20), Bangkok, Thailand, 7-11 July 2013
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 tinbronzes. 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 tinbronze4,5. 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 composition 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 Cu20%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.
Bronze
alloys
Cu-20Sn
Cu-5Si
(wt %.)
Cu
Sn
Si
79.18 19.1
93.55 0.427 4.73
Pb
1.18
0.696
Zn
0.505
0.427
Mn
S
As
0.001 0.014 0.055
0.002 0.046 0.004
2.2 Measurements and calculations
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 acoustical properties of both
alloys were measured and compared. The investigations were carried out according to ASTM
Standard E 1876-018, and the lay out of the measurement is depicted in Fig.1.
Figure 1. Set–up of fundamental frequency measurement.
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20th International Congress on Sound and Vibration (ICSV20), Bangkok, Thailand, 7-11 July 2013
For frequency shift investigation, the original fundamental frequency of each alloy is measured 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 resulting fundamental frequency is measured and compared to the previous (before hitting) conditions.
The frequencies were measured after a certain interval numbers of hit until total of 72,000 hits. Figure 2 shows the experimental set up for frequency resistance tests.
Bar/specimens
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
𝑚𝑓𝑓 2
𝐸 = 0.9465 (
𝑏
𝐿3
) (𝑡 3 ) 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 (𝑡⁄𝐿)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
𝑐 = √𝐸⁄𝜌 (m/s)
(3)
where 𝜌 is the measured density of the specimen.
The sound radiation coefficient R describes how much the vibration of the body is damped
due to sound radiation9. It can also be regarded as an index of the amplitude of vibration that a material will sustain for a given excitation force (a dynamic property) combined with a measure of its
stiffness for a given mass (a static property)10. Particularly, a large sound radiation coefficient is
preferred if a loud sound is desired. The sound radiation coefficient can be calculated from9
𝑅 = √𝐸⁄𝜌3 (m4.kg/s) .
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(4)
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20th International Congress on Sound and Vibration (ICSV20), Bangkok, Thailand, 7-11 July 2013
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).
(b)
(a)
Figure 3.Fundamental resonant frequency of the bar in flexure
The frequency resistance which describes the resistance of the material with respect to the
shift of frequency after being hit for certain numbers of cycles is depicted in Fig. 4 below. In addition to the “untreated” Cu-20%Sn and Cu-5%Si alloys, the results from Cu-20%Sn alloy with prior
forging treatment (bar #7-9) were also presented in Fig. 4.
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.
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20th International Congress on Sound and Vibration (ICSV20), Bangkok, Thailand, 7-11 July 2013
Table 2. Acoustical properties of bronze alloys
Alloys
c (m/s)
R (m4/kg.s)
1.
Cu-20Sn
3398.4
0.393
2.
Cu-5Si
3380.4
0.407
No.
Frequency
Resistance
Stable
(no frequency shift)
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 interesting 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
Based on the results of this study, it can be concluded that Cu-Si alloy with a proper composition can become a potential material to substitute the currently used Cu-Sn alloy while maintaining
its sound quality.
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