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Experimental Investigation on Variability in Properties of Amazonian Wood Species Muiracatiara (Astronium lecointei) and Maçaranduba (Manilkara huberi) Focusing Guitar Fingerboards Manufacturing

  • In face of scarcity in the supply of non-traditional Brazilian woods properly treated for use in high quality musical instruments, pieces of Amazonian wood species muiracatiara (Astronium lecointei) and maçaranduba (Manilkara huberi) purchased in the common internal Brazilian timber market were examined. These species were pre-selected for use in fingerboards of acoustic and electric guitars due to similar properties with ebony (Diospyros crassiflora). Variabilities of elastic modulus parallel to grain and density were investigated inside wooden pieces. In addition, referred parameters were used in calculation of speed of sound. Statistical tests were performed in order to compare both species and revealed inequality for variances of dynamic elastic modulus (Ed) and speed of sound, but equality for density. Equality of means was also examined via unequal variance t-test. Despite color differences, lower variability of M. huberi led to the indication of this species as likely capable to substitute satisfactorily ebony in fingerboards manufacturing.
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  • [1]

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Experimental Investigation on Variability in Properties of Amazonian Wood Species Muiracatiara (Astronium lecointei) and Maçaranduba (Manilkara huberi) Focusing Guitar Fingerboards Manufacturing

    Corresponding author: Roseli Felix da Silva Ribeiro, roseliribeiro@id.uff.br
  • a. Federal Fluminense University, PPGEM, Volta Redonda, RJ, Brazil
  • b. Federal Fluminense University, PGMEC, Volta Redonda, RJ, Brazil

Abstract: In face of scarcity in the supply of non-traditional Brazilian woods properly treated for use in high quality musical instruments, pieces of Amazonian wood species muiracatiara (Astronium lecointei) and maçaranduba (Manilkara huberi) purchased in the common internal Brazilian timber market were examined. These species were pre-selected for use in fingerboards of acoustic and electric guitars due to similar properties with ebony (Diospyros crassiflora). Variabilities of elastic modulus parallel to grain and density were investigated inside wooden pieces. In addition, referred parameters were used in calculation of speed of sound. Statistical tests were performed in order to compare both species and revealed inequality for variances of dynamic elastic modulus (Ed) and speed of sound, but equality for density. Equality of means was also examined via unequal variance t-test. Despite color differences, lower variability of M. huberi led to the indication of this species as likely capable to substitute satisfactorily ebony in fingerboards manufacturing.

1.   Introduction
  • In order to increase the participation of Brazilian wood industry in the musical instruments market, it is verified a significant increase in the research that aims to enlarge knowledge about properties of wood. In this effort, non-traditional species capable to substitute satisfactorily traditional ones like European spruce (Picea abies) and mahogany (Swietenia macrophylla) are analyzed. Portela (2014), for instance, compared an acoustic guitar soundboard made of marupá (Simarouba amara) to others identical soundboards made of traditional woods, and results showed that investigated species had a great potential in the construction of quality instruments. The author performed a numerical-experimental analysis emphasizing the quality criteria of an actual soundboard, developed in a luthier school. Flores (2015) studied some Brazilian forest species for production of good-quality electric guitars. The author compared properties of marupá, araucaria (Araucaria angustifolia), andiroba (Carapa guianensis), jenipapo (Genipa americana) with traditional wood species. Wave propagation velocity measurements and impulse excitation tests were performed, by which modulus of elasticity, shear modulus, sound impedance, sound irradiance coefficient and damping were calculated. Numerical and experimental results were compared, even as moisture content and densities. Teles (2005) provided a list of Amazonian wood species pre-selected for use in construction of percussion, string and wind instruments. Some anatomical, physical and mechanical properties were taken into account, and wave propagation velocity was determined via forced vibration method. Twenty-nine species were pre-selected and classified considering properties similar to those of traditional ones, however, the author emphasized that the selection presented would not be a definitive criterion but an indication of the potentiality of these woods. Costa (2017), from a survey of the most commercialized species at Cruzeiro do Sul-AC, selected and characterized Amazonian species in a similar way to traditional ones. Density, retractability and anisotropy coefficient were evaluated and it was concluded species caroba (Jacaranda copaia) and marupá can be used for soundboards of musical string instruments, and muirapiranga (Brosimum rubescens), tauari (Cariniana decandra) and cedar species for back plate and sides.

    In a significant number of studies, impulse excitation technique (IET) has been adopted. This method is based on the vibratory phenomenon whereby, upon receiving an impact force of short duration, the excited body tends to vibrate at characteristic vibration modes, in particular frequencies. These modes and frequencies are named resonant and are dependent on body geometry, boundary conditions and on the material by which the body is made. For regular geometries (rectangular parallelepipeds, cylinders, and disks) vibrating with no significant restraint, resonant frequencies are determined by elastic modulus, mass, and dimensions of the body (ASTM E1876 2015). The relationship between these parameters has allowed the development of mathematical models by which dynamic elastic modulus (Ed) is evaluated from frequency, mass and dimensions (Pickett, 1945; Spinner et al., 1960; Spinner and Tefft, 1961; Barboni et al., 2018). Following expression was established for the acquisition of Ed using a sample with rectangular cross section subject to an excitation that induces the longitudinal mode of vibration (ASTM E1876 2015):

    where L is sample length (mm); m is sample mass (g); fl is sample fundamental longitudinal resonant frequency (Hz); b is sample width (mm); t is sample thickness (mm); and K is correction factor.

    where µ is poisson coefficient; and De is sample effective diameter (mm).

    In this procedure, the sample is placed upon stressed cables in order to avoid external damping of natural vibrations (ASTM E1876 2015). An acoustic sensor is directed to one of the sample cross sections and a light and short impact is applied at the opposite end (Fig. 1). With the impact, the sample emits a sound signal which is picked up by an acoustic sensor and receives a mathematical treatment (Fast Fourier Transform, Heideman et al., 1985) by which the frequency spectrum is obtained (Otani and Pereira, 2016). Taking anisotropy of wood into account, anatomical elements must be considered in the cutting process, depending on modulus desired (parallel to fibers, perpendicular or tangential to growth rings).

    Figure 1.  Position of sample at the support system, and faces for impact and vibration capture.

    Regardless of efforts to increase knowledge about properties of Brazilian wood, it is known this material can present significant variability in its properties and, depending on the intensity of it, sound response can be affected. Several researches show that variability is not necessarily homogeneous, may vary longitudinally or radially, randomly or not, or do not present significant alterations depending on the species observed (Cruz et al., 2003; de Lima Melo et al., 2013; Valente et al., 2013; de Araújo et al., 2016). It is usually related to anatomical and physical characteristics (Castera and Morlier, 1994; Dünisch, 2019).

    In view of that, in present work, variabilities inside pieces of two Amazonian wood species, maçaranduba (Manilkara huberi) and muiracatiara (Astroniun lecointei), pre-selected for acoustic guitar fingerboards (Teles, 2005) were investigated. Between general properties, elastic modulus and density were chosen due to its highlighted role in dynamic behavior of structures. It was considered one piece from each species, from which the guitar element could be extracted. For fingerboards, hard and heavy timbers are preferred, since the strings cut the wood during playing (Slooten and Souza, 1993). Investigations about influence of this guitar element in tone of the instrument as a whole has been conducted, and distinctions between woods which compose it are pointed out (Paté et al., 2015). Maçaranduba is a high-density wood resistant to the attack by decay fungi and termites, moderately resistant to dry-wood termites and little resistant to marine xylophages (Rosa et al., 2014), considered highly durable in contact with soil (having a useful life of more than eight years). It has a high economic value, being much used for structural purposes. Muiracatiara is considered heavy, presenting a good mechanical resistance. Teles (2005) indicated acoustic variations according to origin, and Susin (2018) defined its dimensional stability as normal, in accordance with criteria of Durlo (1992).

2.   Materials and Methods
  • In face to the scarcity in the supply of non-traditional Brazilian woods properly treated for the use in musical instruments, wooden pieces were purchased in the common internal timber market of Brazilian southeast region. One piece for each species (muiracatiara and maçaranduba) was acquired. Samples were cut for acquisition of longitudinal elastic modulus (parallel to fibers) according to ASTM E1876 2015 for application of impulse excitation technique (Lord and Morrel, 2006); dimensions adopted were 150 mm length, 24 mm thick and 24 mm wide. Surfaces were mechanically flattened and polished by the use of 80 grit Sic to 1000 grit Sic abrasive papers. Dimension measurements were carried out with digital caliper Starret 799A-6 of precision 1 × 10-2 mm, and mass was determined by the use of a digital electronic balance BEL L3102i with precision of 1 × 10-2 g. Samples were kept in air-conditioned room with an average temperature of 22 ℃ and relative humidity of about 56% for a period not less than three months. Since samples for application of impulse excitation technique must be free from defects, those that presented cracks were discarded. A large number of maçaranduba samples were found with cracks, reason by which only eight could be analyzed for this species. Wooden piece of muiracatiara showed fewer number of defects, thus twelve samples suitable for testing were obtained (Fig. 2).

    Figure 2.  Samples of maçaranduba (a) and muiracatiara (b).

    Impulse excitation technique was applied using Sonelastic system for medium-sized samples. The system consists of sample support, acoustic sensor, manual impulse device suitable for application of a light and short impact, and a software designed to calculate dynamic elastic modulus in agreements to ASTM E1876 2015. Five tests were performed for each sample. Figure 3 exemplifies the spectrum of sound signal emitted by samples under impulsive load. Peaks represent fundamental frequencies of longitudinal vibration mode excited.

    Figure 3.  Fundamental resonant peak of maçaranduba sample.

    Densities were calculated by dividing mass by volume, and speed of sound was computed via the expression (Carrasco et al., 2017):

    where Ed is dynamic elastic modulus (GPa); V is speed of sound (m/s); and ρ is density (g/cm3).

    Statistical processing of data was performed aiming the acquisition of values that allow comparison between experimental results; no global parameters estimation is intended owing to features of sample set tested. The F-test was applied to test equality of variances (measurement of the spread between numbers in data set), as cited by Box (1953). Considering results of F-test, unequal variance t-test was also adopted in order to verify equality of means for Ed (Ruxton, 2006). By Shapiro-Wilk test, data set normality was examined (Royston, 1992; Razali and Wah, 2011).

3.   Results and Discussion
  • Testing procedures showed the importance of good flatness and parallelism in sample in order to achieve accurate values for fundamental frequencies, as indicated by Lord and Morrel (2006); adequate polishing is also necessary to avoid production of undesired modes.

    The high value of the said fundamental frequency (Fig. 3) is in agreement with Lord and Morrel (2006) which indicated fundamental longitudinal mode frequency 5-10 times greater than that of the fundamental flexural mode. Fundamental frequencies are also related to material that composes the sample and are used for elastic modulus evaluation. Amplitudes exhibited in graph are relative.

    In Table 1, Ed, density and speed of sound data sets are presented. General average, standard deviation and coefficient of variation are also exposed. Figure 4 presents dot plot graphs of elastic modulus and density evaluated for all samples tested. For maçaranduba, Ed values were found in the range of 17.6-21.1 GPa, and density in the interval 1.07-1.11 g/cm3; for muiracatiara, these properties were maintained in the ranges 10.8-19.6 GPa and 0.87-0.95 g/cm3. Variability of Ed was more expressive than density to both wood species, and led to a similar variation of speed of sound; for this last property, averages were close (Table 1) despite the distinct variability patterns observed. Taking density into account, variation remained below 3%. Averages were found in agreement with the literature (Longui et al., 2010; Eleotério and Silva, 2012; Rosa et al., 2014; Alves et al., 2015). Level of variability in maçaranduba is similar to that presented by Longui et al. (2010) that obtained speed of sound propagation to evaluation of dynamic modulus. Results were compared with ebony (Table 1), which is traditionally used in fingerboards construction.

    Sample code Maçaranduba Muiracatiara
    Ed (GPa) Density (g/cm3) Speed of sound (m/s) Ed (GPa) Density (g/cm3) Speed of sound (m/s)
    TP-01 18.4 1.10 4090 10.8 0.88 3510
    TP-02 20.0 1.11 4245 14.1 0.87 4026
    TP-03 18.6 1.09 4131 14.6 0.91 3992
    TP-04 19.8 1.09 4262 19.6 0.91 4577
    TP-05 18.3 1.08 4116 17.3 0.89 4409
    TP-06 19.2 1.10 4178 10.9 0.88 3519
    TP-07 17.6 1.07 4056 16.4 0.9 4269
    TP-08 21.1 1.09 4389 19.6 0.95 4542
    TP-09 - - - 13.3 0.87 3910
    TP-10 - - - 12.1 0.91 3646
    TP-11 - - - 17.0 0.90 4346
    TP-12 - - - 17.1 0.89 4383
    Mean 19.1 1.09 4183 15.1 0.9 4094
    SD 1.03 0.01 103 2.88 0.02 369
    CV 5% 1% 3% 19% 2% 9%
    Ebony*
    Mean 20.8 1.14 - - - -
    CV 15% 5% - - - -
    Note: Means and CV of Ed and density for Ebony (Sproßmann et al., 2017).

    Table 1.  Dynamic elastic modulus (Ed), density and speed of sound of maçaranduba and muiracatiara samples.

    Figure 4.  Distribution of Ed and density of maçaranduba and muiracatiara samples examined.

    Equality of variances and means were investigated. Normality of data sets was verified via Shapiro-Wilk test for a significant level of 5%, allowing implementation of F-test (Table 2). Equality of variances was observed for density, and inequality for Ed and speed of sound for a significance level of 5%. Unequal variances t-test revealed statistical inequality of means for Ed and equality for speed of sound.

    F-test (5% of significance) Unequal var. t-test (5% of significance)
    Muiracatiara (n = 12) Macaranduba (n = 8) Muiracatiara (n = 12) Macaranduba (n = 8)
    Density Var. 0.00049697 Var. 0.00015536 - - - -
    P = 0.13357 > 0.05 -
    Ed Var. 9.4588 Var. 1.2764 Mean 15.233 Mean 19.125
    P = 0.01387 < 0.05 P = 0.00117 < 0.05
    Speed of sound Var. 148380 Var. 12073.7 Mean 4094.08 Mean 4183.4
    P = 0.002944 < 0.05 P = 0.4614 > 0.05

    Table 2.  Results of F-test and unequal variances t-test for 5% of significance level.

    Relationship between Ed and density was examined by means regression analysis, revealing no satisfactorily correlations. Scattering graph is presented in Fig. 5. In spite of the limited number of samples, analysis of Fig. 5 allows to observe clearly relevant distinctions between both sets. The lesser scattering of maçaranduba is significant and, even though some Ed values are close (between muiracatiara and maçaranduba), no similarity is verified relatively to density.

    Figure 5.  Scattering graphs of Ed · density for maçaranduba and muiracatiara pieces.

    In a visual exam of grain direction patterns of samples, it was possible to observe a large variation in inclination of fibers in muiracatiara, which exhibited pronounced variability in elastic modulus. On the other hand, maçaranduba, with low Ed variability, presented regular grain slope. This phenomenon seems to be in agreement with results provided by Carrasco et al. (2017). In view to determine Ed in function of fibers direction, authors applied acoustic tomography to some species, including Manilkara sp., and identified important correlations. Indeed, these conceptions are also in agreement with theories which investigate mechanical property variation of wood with grain slope, as that presented by Liu and Ross (1998).

    In spite of findings achieved in this work, it is considered that relevance of differences must be examined in the light of numerical vibroacoustic analysis of the guitar. Numerical simulations involving all guitar elements can be conducted aiming to investigate possible alterations in tone derived from wood distinctions. In this context, Paté et al. (2013) studied alterations in sound and damping due to the conductance involved in string-structure coupling for two electric guitars having ebony and rosewood fingerboards, and it was verified that whatever the tuning is, it is likely the rosewood fingerboard grasps more energy of vibration from the string. Sproßmann et al. (2017) affirm that a high hardness value indicates good abrasion resistance on the fingerboard surface due to strings scratching while guitar playing. Taking into account proximity between maçaranduba and ebony relatively to properties like elastic modulus average, density, shrinkage, Janka hardness (Teles, 2005, Pereira et al., 2016, Sproßmann et al., 2017), grain straightness and fine texture, added to the least variability observed in the piece of maçaranduba examined here, it is possible to visualize this species as potentially capable to be applied in fingerboards providing quality results. In regard to inclination of fibers, straight-grained woods are appointed as the most desirable for musical instruments (Patten et al., 2010), which was not observed in muiracatiara. As previously mentioned, this wood exhibited high variability in Ed and large irregularity in fibers inclination. Such characteristics could lead to undesirable results relatively to the sound quality and can be an issue when tension applied by strings is regarded.

4.   Conclusions
  • Faced with the limited amount of material analyzed in this study, the findings are limited to the involved pieces of wood. Diversified variability patterns of Ed and density inside maçaranbuba and muiracatiara wooden pieces (purchased in the common internal Brazilian timber market) were observed and it was verified this was likely related to fiber inclination. Maçaranduba exhibited the lowest degree of variability, while muiracatiara presented the largest. Statistical processing of data was performed aiming the comparison between species. The F-test revealed equality of variances for density, but inequality for Ed; unequal variances t-test was employed to examine equality of means and it was found inequality for Ed and equality for speed of sound. Disregarding color differences, relevant similarities between properties of ebony (traditional endangered species, used for fingerboards composition) and maçaranduba, added to lower variability of this last, indicated this species as a good substitute for ebony. Relevance of results should be examined in the light of numerical vibroacoustic analysis of the guitar.

Conflict of Interest
  • There are no conflicts to declare.

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