关键词: 弦乐器, 指板, 共鸣板, 声学振动性能

Abstract:

Objective: The acoustic vibration properties of commonly used string musical fretboard wood were comprehensive analyzed and summarized, which could be the basis for the search of alternative wood or modification of fast-growing wood. Method: The X-ray profile densitometer was used to characterize the density differences between the earlywood and latewood in a growing ring and their uniformities along the radial density distribution. The ultrasonic microsecond meter was used to test the sound propagation speed of the wood. The vibration frequency, torsional frequency and damping ratio of wood were tested by modal analysis method. The acoustic vibration performance parameters of wood were also calculated according to the Euler-Bernoulli equation of rectangular section. Result: The oven dry density of fretboard wood was 1 180 kg·m^-3 for ebony(Diospyros crassiflora), 810 kg·m^-3 for Indian rosewood(Dalbergia latifolia), 1 320 kg·m^-3 for African blackwood(Dalbergia melanoxylon), 660 kg·m^-3 for the hardwood hard maple(Acer saccharum) string instrument soundboard and 480 kg·m^-3 for non-instrumental wood radiata pine, respectively. The density differences of fretboard wood between the earlywood and latewood in the growth ring and between the adjacent growth ring were not significant, and had a uniform texture. The sound propagation velocity of the fretboard wood was lower than that of hard maple and radiata pine in axial and radial direction. The sound propagation speed ratio of parallel grain to vertical grain of hard maple was 3.2, which was the maximum value. The acoustic anisotropy of hard maple was much better. The dynamic modulus of elasticity(E')of ebony, African blackwood and Indian rosewood were 18.2, 16.8 and 14.8 GPa, respectively, which indicated that the dynamic modulus of elasticity of fretboard wood was greater than 14.0 GPa. The specific dynamic modulus of elasticity (E_sp) of fretboard wood was less than 18.0 GPa, those of hard maple and radiata pine were 24.5 and 26.8 GPa, respectively, which were all higher than the E_sp of fretboard wood. The acoustic radiation quality constants(R)of ebony, Indian rosewood, African blackwood, hard maple and radiata pine were 3.21, 5.08, 2.58, 7.17 and 9.41 m³·Pa^-1s^-3, respectively. The acoustic impedance(ω), logarithmic attenuation coefficient(λ)and loss tangent(tanδ)of ebony, Indian rosewood and African blackwood were all higher than those of hard maple and radiata pine. The ratio of acoustic conversion(ACE)to E'/G' of fretboard wood was lower than that of hard maple. The dynamic shear modulus(G')of ebony, Indian rosewood, African blackwood, hard maple and radiata pine were 1.97, 1.72 and 2.58, 1.21 and 1.09 GPa, respectively, which were higher than those of hard maple and radiata pine. Conclusion: The oven dry density of fretboard woods are larger than 800 kg·m^-3. There might be a functional relationship between wood density and acoustic vibration performance parameters of wood. When selecting materials, density and acoustic vibration performance should be considered simultaneously. In terms of acoustic vibration performance, E' and G' could be the main evaluation indicators of wood acoustic vibration when selecting materials, and it should be required to select the wood with high E' and G' values as much as possible. The higher the E' value, the stronger the resistance to bending deformation caused by different string tensions. The greater the G' value, the stronger the ability of the fretboard to suppress torsional deformation under the action of different string tensions. The acoustic vibration parameters of wood such as E_sp, R, ACE, E'/G', λ, tanδ and ω might be not the important evaluation indicators for fretboard selection.

Key words: string musical instruments, fretboard, soundboard, acoustic vibration performance