Acoust Aust DOI 10.1007/s40857-015-0008-5
ORIGINAL PAPER
An Overview of Fibre-Reinforced Composites for Musical Instrument Soundboards Ajith Damodaran · Larry Lessard · A. Suresh Babu
Received: 23 December 2014 / Accepted: 7 February 2015 © Australian Acoustical Society 2015
Abstract Traditionally the material of construction of many musical instruments has been limited to wood. The unique mechanical and acoustic properties of wood make it the material of choice for making musical instruments. In recent years, wood for musical instruments is depleting, becoming more expensive and is of less acceptability due to environmental changes. This has resulted in most musical instrument builders searching for alternative materials to traditional musical instruments. This paper presents an important overview of recent research and developments and presents an initiative focusing on fibre reinforced composites as an alternative material for stringed instruments. Fibre composites are emerging as a competitive alternative material. Composite instruments has potential advantages for players concerned with functionality, sound, choreography and cost. Keywords
Carbon fibre · Soundboard · Stringed instruments · Wood · Violin
1 Introduction Musical instruments can be classified into four main categories: (1) idiophones: instruments that produce sound by vibrating themselves, e.g. xylophones (2) membranophones: instruments that use stretched vibrating membrane to produce sound, e.g. drums (3) chordophones: instruments that relay on a stretched vibrating string e.g. violins, guitars (4) aerophones: instruments that use vibrating air column for producing sound e.g., flutes, bagpipes [1]. The major structural part of many musical instruments is made from wood A. Damodaran (B) · A. Suresh Babu Department of Manufacturing Engineering, Anna University, Sardar Patel Road, Chennai 600 025, India e-mail:
[email protected] Present address: A. Damodaran Manufacturing Engineering Department, Central Institute of Plastics Engineering and Technology, Guindy, Chennai 600 032, India L. Lessard Department of Mechanical Engineering, McGill University, Macdonald Engineering Building, 817 Sherbrooke West, Montreal, QC H3A 2K6, Canada
[2,3]. In stringed instruments a soundboard is used to amplify the string vibrations and thus converts string vibration in to sound energy. For example, a violin top plate is made out of spruce wood due to its high specific stiffness (i.e., stiffness relative to density) and lower internal friction in the longitudinal direction [4,5] however the back plate is made with curly maple. Curly maple is used for its specific mechanical properties: it acts as a very elastic spring, especially due its curly texture. The xylophone consists of a wooden vibrating plate hit by a mallet. Tropical hardwoods are generally preferred for making xylophone bars due to their superior hardness, density, stiffness and its peculiar damping [6,7]. African blackwood and Brazilian rosewood are preferred for clarinets and oboes due to their high density and a fine grain to obtain an optimal finish of the tube walls and finger holes. For wind instruments the interior surfaces should be smooth and non-porous to reduce viscous losses in the air coloumn. In drums with membranes the supporting structure should be sufficiently strong and rigid. Jackfruit wood is most preferred for Indian tabala and mridangam and a wide variety of stringed instruments like veena and tampura, due its fine grain structure, aesthetics (golden yellow colour) and termite resistance (protection from insects). In general, the best musi-
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cal instruments are still made from wood species due to their unique range and combination of mechanical and acoustical properties [8]. Recently, the use of man-made materials like fibre composites has found applications in high performance musical instruments [9]. Among the various musical instruments, stringed instruments present the most important potential use of fibre composites.
2 The Need for Wood Alternatives The sound quality of a particular instrument depends on the quality of the raw material used and the skill of the craftsman. It is almost impossible to control the quality of the wood as it varies with the geographical origins, and from sample to sample [10]. Despite the unique mechanical and acoustic properties of wood, it suffers the disadvantages of drying, cracking and inconsistent quality. There are significant variations in both mechanical and acoustical properties of wood internally within one piece of wood and from piece to piece of the same species of wood or even within the same tree. Also, quarter-cut boards are preferred for soundboard applications. However, wood sold for this purpose is not exactly quartercut and thus reduces the quality of the instrument. Further the best quality wood is sourced by the furniture industry and it becomes more difficult for the musical instrument builders to maintain the quality of the instrument. The current supply of wood cannot meet the significant demand for quality musical instruments. In the near future musical instrument manufacturers will likely switch to non-traditional materials like plantation timbers, polymers or composites [11]. In addition, there is intensified pressure to limit the use of endangered wood species. Brazilian rosewood used for guitars, and African blackwood used for making western clarinets have already been added to the protected species list by the Convention for International Trade in Endangered Species (CITES). These problems have resulted in many traditional musical instrument builders searching for alternative materials. Hence alternative material for musical instruments, to reduce manufacturing time and overcome problems encountered using traditional wood, is both desirable and necessary [12]. This paper aims to provide researchers, engineers, musicians and musical instrument builders with information on the recent developments on fibre reinforced composites being used for soundboards and the ongoing research conducted throughout the world focussed on high performance stringed musical instruments. 2.1 Selection Criteria for Stringed Instruments The construction of musical instruments starts with the selection of materials. There has already been vast research in order to characterise the acoustically important properties
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of wood for musical instruments [13–15]. Ono and Norimoto [4] showed that plates with a large variation of in-plane stiffness respond better to a wider range of frequencies. In general, researchers have established that materials with a low density, high specific modulus and low internal friction in the x-direction (along the grain or fibre direction) are best suited for violins and guitars. Soundboard quality criteria can be defined as the ratio of damping (Q−1 ) to that of specific modulus (E/ρ). A material with a low value of Q−1 /(E/ρ) is best suited for soundboards. Material property charts that plot mechanical properties such as Young’s modulus, damping factor and the density against one another will help to analyse material performance and develop selection criteria for soundboards [16]. Yoshikawa [17] presented a classification scheme for non-traditional materials for making stringed instruments. In that study, selection criteria were proposed by plotting transmission parameter cQ to antivibration parameter ρ/c, where c denotes the propagation of speed of the longitudinal wave along the wood grain, the Q value is the reciprocal of the loss factor and ρ is the density of the material. These regression lines, defined by the classical wood materials, establish criteria to select non-traditional materials for musical instrument applications. Based on this criteria [18] figured out that balsa wood can also be used in soundboard applications. It is much lighter than the conventional wood used in making violins which would normally lead to a louder instrument. Based on his results, a balsa violin shows frequency peaks comparable to those of traditional violins, however the balsa violin has much lower internal damping. Recently, [19] investigated the acoustic properties of carbon fibre, glass fibre and hemp fibre reinforced polyester composites for musical instrument soundboards. The results showed that carbon fibre reinforced composites exhibited less damping which could be desirable for soundboards. Further, water absorption studies also revealed that the performance of composites will be less affected by humidity. A combination of elastic constants and their anisotropy together with the aesthetics are the most important factors for selecting alternative materials. Prerequisites for alternative materials are (1) to match the vibrational behaviour of the target wood, and (2) to obtain good mechanical strength and workability to match that of the existing wood [20–23]. 2.2 Wood Alternatives for Stringed Instruments The elastic and damping properties of carbon fibre/balsa sandwich were investigated by [24]. The results showed that the sandwich structure had lower internal friction compared to that of spruce. Ono et al. [25–27] has performed significant work to develop soundboards and resonating cavities of violins and guitars. The anisotropic nature of the sound-
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boards can be mimicked by proper orientation of particular fibre layers in the matrix structure. However, the low density of wood resulting from its complex microstructure is more challenging. In that study, an alternative material was developed by using advanced fibre materials with large elastic modulus and plastic foam with low density. From their study it was also concluded that surface reinforced sandwich panels performed well in all the listening tests. However, the frequency responses of their samples were different because of their smaller shear modulus. Subsequent to the previous studies, to mimic the microstructure of wood, a flax fibre composite sandwich structure for stringed musical instruments was developed by [28–30]. A ukulele (stringed instrument) was manufactured by a novel manufacturing technique. The monocoque soundboard developed from that study does not require bracing and the use of animal glue is not required compared to the wooden instrument. This kind of design offers more uniform stiffness across the entire surface and requires less maintenance. It was concluded that “the sound output level and low frequency response problems need to be addressed before it can match the sound of a wooden soundboard”. An attempt to develop an Indian stringed instrument called the veena from lesser-known plantation timbers was carried out by [31]. Veena, made from locally grown species like artocarpus hirsuts and swintenia mahagony, showed a similar frequency spectrum compared to the traditional instrument. In that study it was observed that dynamic modulus of elasticity (MOE) is the deciding factor for selecting the alternative material. Yano et al. [32] studied a new type of lightweight phenolic resin treated veneer compressed boards to replace Brazilian rosewood for use in guitar back plates. In the low-frequency range tonal quality of guitars made with this novel material is comparable to that of guitars with back plate of Brazilian rosewood. This new material offer a choice for instrument makers that uses no endangered species of wood. 3 Advanced Composite Materials Advanced composite materials account for the majority of the alternative materials investigated for soundboard applications. As discussed above, composite materials are a good replacement for wood due to their inherent orthotropic properties and superior stability. Composite materials typically consist of strong fibres embedded within a light polymer matrix. Fibre composites are appropriate as this material can be engineered based on the acoustically related mechanical properties, namely, stiffness and vibration damping [9,33,34]. The anisotropic nature of the soundboards can be mimicked by correctly orienting fibre layers in the matrix. Matching the dynamic Young’s modulus and internal damping characteristics are achieved by using a suitable core struc-
ture. Fibre reinforced composites are very stiff and the performance will not vary with the humidity, temperature etc. Furthermore, they offer better resistance to environmental changes, less material variability and lower manufacturing time. Most fibre composite materials have higher cost compared to traditional wood. However, the cost of high quality wood, such as that required for high end instruments, are even higher. The premium benefits of fibre composites will make this material a viable alternative to traditional wooden musical instruments. For these reasons, complete a composite acoustic guitar is marketed as early as 1995 under the trade name RainSong [35]. 3.1 Development of Soundboards As discussed above, for the development of fibre reinforced soundboards, the four key properties, namely bending stiffness, areal density, internal friction and degree of anisotropy, have to be compared with that of the traditional wood [25]. Classical laminate theory can be used to predict the in-plane and bending stiffness of the composites. Areal density is proportional to the thickness of the plate, which varies a bit across the soundboard of a musical instrument. In particular, this areal density can be matched by orienting one or two layers of fibers in an epoxy resin matrix [36]. The internal friction can be matched by adopting a sandwich structural concept [28]. A sandwich structure consists of a lightweight core in-between two stiff face sheets. The general procedure for fabricating top plates from composite materials by hand lay-up is discussed below. Carbon fibre is the most preferred material for the top plate applications as it possesses high specific mechanical properties. Pre-impregnated unidirectional (CYCOM 3250) or woven (MTM 451) (prepreg) is widely used to reduce the manufacturing time. Figure 1 illustrates the detailed hand lay-up procedure for a violin top plate. This is a part of an ongoing project on the development of musical instruments from high performance materials carried out at the Structures and Composite Laboratory at McGill University. Templates are used to trace the necessary profile of the top plate onto the backing paper of the material. They are then carefully cut using a sharp blade. The outermost layer is placed into the treated aluminum mould and pushed down, to follow the profile of the mould. Then the smaller inner layers are placed on top of the bottom layer. Sufficient care must be taken to make sure no air pockets form in-between the layers. In order to consolidate the lay-up, a vacuum bagging technique can be used [37]. This process makes use of vacuum pressure to consolidate the lay-up while curing. A detailed procedure for the vacuum bagging technique is given in the reference [38]. Further, the mould was placed inside an oven for about 20 h at 90 ◦ C. The finished part was then de-moulded and fixed to the base of a wooden violin (Fig. 2). As per the luthier, Peter Purich,
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Fig. 1 Development of violin top plate. a Mark profile of the top plate, b cut preform, c treated mould, d placement of the fibres, e release film on top of the fibres, and f vacuum bagging
Fig. 2 Finished violin
“the composite violin sounds lively”. Further, the vibrational characteristics were compared by numerical techniques and modal analysis for both carbon fibre and spruce wood top plates. The results showed that the damping ratio of the composite top plate is comparable to that of spruce wood [39]. 3.2 Recent Developments on Composite Instruments Research on Carbon fibre violins was also carried out in the U.K. A procedure was also developed for testing the composite instrument by [40]. Blackbird guitars in U.S.A., has produced a limited edition Ferrari model of its rider guitar, which features a monocoque structure, hollow neck and carbon fibre construction that is less than two-thirds the size of a standard acoustic guitar, [41]. Several composite musical instruments can now be found in the market place ranging from cellos to flutes. Luis Leguia,
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founder of Luis and Clark String instruments is a pioneer in making a carbon fibre cello, viola etc. These days these instruments are becoming a more common sight in the hands of professional musicians. For example, popular cellist Yo-Yo ma has now become interested in carbon fibre cello for live performances. The carbon fibre cello presents some advantages due to its greater durability and resistance to humidity [42]. A Finish company called Flaxwood has developed hybrid materials for electric guitars. As a result of their research, sustainable hybrid natural fibre composites are developed from recycled materials. Due to advancement in manufacturing techniques, these hybrid materials have large fibre content, are denser than wood and exhibit high tonal quality. A German company called Mezzo-Forte produces a variety of stringed instruments, principally cellos, violins and viola using carbon fibre [43]. In the last two decades the development of natural fibre reinforced composite have gained lot of interest. A prototype ukulele (stringed instrument) made from flax fibre composite is shown in Fig. 3. Natural fibre composites not only mimic the vibrational characteristics but also improve the aesthetics of the instrument. The high availability of these fibres also reduces the cost of the instrument substantially [28]. Recently at the International Conference on Composite Materials (ICCM 19) held at Montreal, Canada in July 2013, a composite band in which at least one part of each instrument was made from a composite material was successfully demonstrated. It is evident that the search for wood substitutes has a tremendous interest worldwide [44]. Apart from stringed instruments, fibre composites could also find interesting applications in traditional Indian percussion instruments [45]. The advantages of fibre composite structure also
Acoust Aust Table 1 Recent developments on fibre composite musical instruments Country Description of material
Type of application
Level of development
References
Canada Carbon fibre and flax fibre reinforced composites
Violin, ukulele,
Trial application
[28–30,39]
U.S.
Carbon fibre composite
Guitar, cello,
Actual application
[41,42]
Japan
Wood powder composite, Carbon fibre composite
Traditional Drum, Guitar
R&D
[27,46]
India
Carbon fibre/balsa core sandwich composite
Chenda Drum
Trial application
[45]
U.K.
Carbon fibre composite
Violin
R&D
[40]
Finland Recycled wood Guitar plastic composite
Actual application
[43]
Germany Carbon fibre composite
Actual application
[43]
Violin, cello
Fig. 3 Flax fibre Ukulele
support the development of high strength, visually appealing and durable drum shells. Table 1 summarizes research and developments in the application of composite materials for the design of musical instruments.
4 Concluding Remarks The widespread decline of most traditional wood combined with intensified pressure to limit its use as a result of environmental reasons are the main drivers for research throughout the world aimed at finding a suitable alternative for traditional wooden instruments. Composites can be engineered based on the required soundboard applications and have excellent durability requiring less maintenance. Several research and development studies on fibre reinforced composites have shown that these alternative materials have physical and acoustic properties comparable to that of the wooden species. Development of a violin top plate is also discussed to illustrate the potential of fibre composites as a wood substitute. However, the performance history of these new materials is relatively short compared to traditional wood. Continuous research and development are essential to develop the science and increase confidence among musicians. It is hoped that composite materials can help to make smarter, stiffer, stronger and high performance musical instruments.
Acknowledgments The authors acknowledge the financial support from the Centre for Interdisciplinary Research in Music Media Technology (CIRMMT) at McGill University and the Department of Foreign affairs and International Trade (DFAIT) Canada. We also thank Mr. Hossein Mansour of CIRMMT and Dr. Iris Bremaud of CNRS, France for stimulating discussion on stringed instruments.
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