sound performance investigation of composite materials for

SOUND PERFORMANCE INVESTIGATION OF COMPOSITE
MATERIALS FOR THE SOUNDBOX OF A MEMBRANE MUSICAL INSTRUMENT
Rusen Can Acet and Ferina Saati Khosroshahi
Mechanical Engineering Faculty, Ozyegin University, Nisantepe Mah., Orman Sok., Cekmekoy Istanbul
34794 Turkey
email: [email protected]
The materials used in the manufacture of musical instruments with wooden sound box is
obtained from trees in general. This fact creates difficulties due to problems with limited
environmental resources. In addition, such materials vary widely in quality as they are
biomaterial, and so it is necessary for musical instrument makers to invest a great deal of
effort in quality control and in the wood drying process. To resolve these issues, we have
begun the development of synthetic structure using glass fiber and carbon fiber
composites with polyester resin for Turkish folk violin (Kemane) traditionally made from
gourd was used in the construction. Three instruments were manufactured exactly the
same using the traditional measures; two kinds of composite materials and the gourd.
Computer modeling was created for composite and sound analysis was performed
according to sound recordings of instruments. The analyzes were compared with the
traditional gourd Kemane.
1. Introduction
Makers of stringed musical instruments (luthiers) have an increasing problem in securing high
quality materials. Over centuries and millennia, our ancestors worldwide found the most appropriate materials for increasingly complex acoustical applications. While man-made materials have been
used in successful instruments, they are overwhelmingly made of wood [1] to take advantage of its
unique property combination and its aesthetic appeal. However, there are other suitable materials
which are economic, can be easily sourced and still have good acoustic properties; composite materials as an example has recently been used extensively in the music industry. Nowadays, many
researchers have focused on other materials, such as carbon fiber composite material, foamed polycarbonate and wood plastic composite (WPC) used for creating soundboards [2]. The main reason for
this is because the naturally available materials such as wood are not always easy to get and are widely
used in engineering fields. The biggest advantage of composite materials compared with nature-based
ingredients is their high strength and temperature resistance [3], easy shapability, electrical properties,
resistance to corrosion and chemical attack, heat and fire resistance, the permanent colouring and vibration damping. The manufacture of stringed instruments has strong elements of traditional practice,
particularly in the choice of materials from which the instruments are made. In this paper, two types
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The 22nd International Congress of Sound and Vibration
of composite materials are used in making the resonance box of a Turkish spikle fiddle called "Kabak
Kemane".
2. Introduction to "Kabak Kemane"
Kabak Kemane, a spike fiddle with a gourd soundbox is a Turkish folk music spike fiddle that was
primarily known in Mediterranean and Aegean regions but is now an inseparable part of Turkish folk
music orchestra. Sound box of the instrument is cut out of approximately one third of sphere of a
gourd. The gourd is acoustically the most important part of the instrument along with the membrane.
The membrane is the heart tissue of a cow and the neck is made from one of beech, maple, hornbeam
or walnut wood for which maple is mostly preferred; also over the fingerboard where fingers actually
touch, ebony, wenge or juniper wood can be mounted. The bridge is made of maple and the tuning
pegs are generally metallic nowadays. The bow is originally made of horsehair, the length of a 1/2
or 3/4 violin bow, but synthetic bows are also becoming popular, made from thin fishing line for
example.
Figure 1. Typical kabak kemane
A typical kabak kemane looks like Figure 1. A model of the instrument was created in Solidworks
as seen in Figure 3.
Figure 2. 3D-rendering of the Kabak kemane
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2.1
The Composite Kemane
When the sound box of kabak kemane, shortly known as kemane, is made from composites (carbon or glass-fibre-reinforced polyester) in order to contribute to the timbre of the instrument, we call
it composite kemane. An example of a 3D-rendered composite kamane is seen in Figure 3. The main
goal for using composites in the current study is to find out about the possible deficiencies of gourd
and an attempt to increase the strength of the soundbox while looking at the changes in timbre as a
result of using different materials.
Figure 3. Kabak kemane as rendered in Solidwroks
3. Making the Composite Kemane
For the resonance box, the inner and outer molds were first prepared (Figure 4). The rigid outer
mold of the sound box was made to prevent the two parts from mixing with each other; the internal
mold was a scale-down of the external mold.
Figure 4. Preparation of inner and outer molds
The sound box geometry is preferably chosen close to spherical shaped gourds. Gourds are therefore ripened while hanging. For this reason, an elastic ball in ideal measures for a kemane was used
as the outer mould as seen in Figure 5 (to prevent sticking, diesel, oil separators, etc. is applied to the
outer surface).
Figure 5. An elastic ball as outer mold
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After being cured, the air inside the inflamed ball was let out and ball was then taken out. Mold
material was plasterboard because of its surface quality and faster curing. Due to the rapid freezing
and good surface quality cornice plaster was used as a mold material (Figure 6).
Figure 6. The semi-completed kemane
After compression, the composite is allowed to set for 45 minutes (air temperature and standby
time vary depending on the freezing properties of the matrix). Mould was then removed easily. Sound
box is re-sanded with 600 to 1200 sandpaper and polished after drying the polish brush and then the
membrane is placed on the sound box as seen in Figure 7.
Figure 7. Placing the membrane on the composite kemane
4. Investigation of membrane vibration
In order to compare membrane vibrations for each fiddle, sound spectrogram in time was derived
when the membrane was vibrated (without bridge and thus, no pre-pressure) for each fiddle. The
sound spectrogram are drawn in the Wavelab analysis program (as seen in Figure 8).
Analysis shows that when strokes are applied to membranes of two composite fiddles, the carbon
fiber vibrates with frequencies closer to the mean frequency range of a gourd, while glass fiber is less
discrete and thus has a smoother spectrogram. None of the composite fiddles produce recognizable
sound below 100 Hz which is unfortunate in terms of full similarity to the gourd but since higher frequency range is of much more importance, this fact may not prevent the practical usage of composite
Kemane. Since membrane is the most important part of the vibrating system [4], the membrane study
as an individual will be focused further in the future studies.
5. Investigation of Strings’ Vibration
A gourd fiddle is tuned either as DADA, BEBE or as CFCF, depending on the piece played or
the polyphony (and the leader’s) requirement. After the bridges were placed on each of the finished
fiddles, tuning was done according to BEBE, having a tuned piano as reference. Vibrating the strings
constantly, sound spectrogram was derived as seen in Figures 9 and 10.
According to analysese, when lower notes are played on the string, the glass fiber fiddle spectrogram shows a close behavior to the gourd than carbon fiber does.
When vibrating the strings using the bow on higher notes (higher B and E), the glass fiber adds
extra frequencies and therefore changes the timbre more than the carbon fiber. Carbon fiber, on the
other hand, has a more discrete behavior (with a clean sound, mostly in one single frequency).
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Figure 8. Sound spectrogram of (a) Gourd fiddle (kabak kemane) ,
(b) Carbon fiber fiddle, (c) Glass fiber fiddle
In order to compare the fiddles better, a single piece of Turkish music was played on each individually (Figure 11). The authors preferred the carbon-fiber in terms of analogy with gourd. The
two composite fiddles each have their advantages and disadvantages in lower and higher notes and
depending on the practical usage, luthiers will suggest either one of them. Results gave the authors
new ideas for further research.
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Figure 9. Sound spectrogram in time when bass B string is constantly vibrated for:
(a) Kabak Kemane,(b) Carbon fiber fiddle, (c) Glass fiber fiddle; E string vibration
spectrogram for: (d) Kabak Kemane,(e) Carbon fiber fiddle and (f) Glass fiber fiddle
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Figure 10. Sound spectrogram in time when B string is constantly vibrated for:
(a) Kabak Kemane,(b) Carbon fiber fiddle, (c) Glass fiber fiddle; E string
vibration spectrogram for: (d) Kabak Kemane,(e) Carbon fiber fiddle and (f)
Glass fiber fiddle
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Figure 11. Sound spectrogram when the same piece of music is played on (a) Kabak Kemane,
(b) Carbon fiber fiddle and (c) Glass fiber fiddle
6. Conclusions
Composite musical instruments have a number of advantages. Among them is increasing the
strength and resistance against humidity and temperature conditions. In this paper, Turkish gourd
fiddle is produced using carbon fiber and glass fiber composite materials. Three fiddles are compared
in terms of membrane and string vibrations and also when played in real conditions. Results revealed
that although having different sound colors, the composite gourd fiddles can actually be used as
replacement. Therefore, composite instrument construction for this membrane spike fiddle is reported
as possible. Characteristics were also analyzed in comparison for each fiddle.
REFERENCES
1. Celik, O., Three bow instruments in Turkish music: Kilkobiz, Kamancha, Kabak Kemane,Ege University
Publications, Izmir, (2010).
2. Sun, Ruting, Smith, MP., Goodier, Chris I, Flint, James A., "‘Rock Guitar, optimising concrete properties
for the manufacture of a concrete guitar"’, The Ceramics Society, The Institute of Materials, Minerals and
Mining (IOM3), (2011).
3. Wegst, Ulrike GK., Bamboo and wood in musical instruments, Materials Research 38.1 (2008): 323.
4. Saati Khosroshahi F., Sepahvand K., Guttler M., Pourtahmasi K., Marburg S., Modal Analysis of the Persian Music Instrument Kamancheh: Finite Element Modeling and Experimental Investigation,ISMA Le
Mans, France (2014).
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