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= The Effects of Seasoning and Aging on Wood Sound Qualities = Methods have been used to explore the changes that occur to the vibrational properties of the wood used to create wooden and stringed instruments due to seasoning, an aging and drying process of less than five years, and aging, a drying process over the course of greater than five years. Specifically, they look at two parameters. The first is the Young’s Modulus of the wood, which measures the tensile stiffness of the wood. The second is the internal friction, a measure of internal force restricting the movement of the internal molecular properties of the wood. The results were concurrent with conventional artisan knowledge, implying that seasoning is significantly more impactful towards acoustic properties and sound quality than extended aging of the wood instruments.

Background
Wood is often used in the creation of soundboards of various instruments because of its lightness, elasticity, and durability. The effects of seasoning and aging of wood have been studied by musicians and artisans, and have been used to determine good acoustic quality for instruments, such as violins, harps and guitars. Conventional knowledge among artisans has stated that the seasoning effect, drying the wood for less than five years, is significantly more important for sound quality than the conventional aging process, which can extend to upwards of hundreds of years. Studies have been made to determine what specific properties of the wood changes throughout these processes. One such method developed by Eiichi Obatayaa, Nanami Zeniya, and Kaoru Endo-Ujiie in 2020 was developed because the effects of aging and seasoning on wood has had remarkably little scientific research. Despite the fact that the aging effect on polymers has been extensively studied, these studies have focused primarily on synthetic polymers as opposed to natural polymers such as wood. Previous studies have concluded that aging for 200 years has resulted in a slight increase in the Young's Modulus and a slight decrease in the Internal Friction of the wood, something that is determined to correlate positively with good acoustical sound.

Cultural Differences
Worth noting is that the studies presented here focus on acoustic properties consistent with Western ideology in regards to what qualifies as "good acoustic quality". This qualitatively changes with different instruments as well. For example, with instruments such as xylophones, what qualified as good acoustic sound varies wildly in comparison to something such as violins. In addition, woods used in traditional Asian music relied heavily on having high density as opposed to the several factors of importance in western instruments. This could possibly attributed to native flora in these areas, as woods found in Asia tend to have higher average densities than Western flora.

Short Term Seasoning of Green Wood
The study by Obatayaa et al. began by gathering samples of Green Spruce, which were dried in a conditioning room at 20°C and seasoned for six months while having their acoustic properties examined throughout. These properties included mass, density, Young’s Modulus, internal friction, and sound velocity. After this process, the wood samples were moistened at 20°C and 100% relative humidity for one month before being dried for another two days. Lastly, the wood specimens were oven dried to determine their absolute dry mass.

Vibration Measurement of Seasoned Wood
In order to characterize the acoustic behavior of the wooden instruments, the viscoelastic constants of the wood is needed to be known in all three directions, because wood is an anisotropic material. However, the study in question did not have the dimensions required to precisely measure the properties along the width and height, and thus focused exclusively on the length direction. To broaden the reach of the study, seasoned and aged spruce lumber was acquired by artisans and companies who make violins, harps, pianos and guitars. The lumber was then cut into strips and used for the acoustic measurements.

The vibrational properties of wood are heavily dependent on its moisture sorption history and its current moisture content. In order to make conditions the same for all samples of wood, all samples were dried completely at 20 °C and then conditioned for one month prior to measurement.

To investigate the reversibility of the seasoning effect, all measurements were repeated after a moistening treatment on the wood, where the samples were moistened at 100% relative humidity for more than one month, before being dried and measured again. This process was repeated twice.

Free Flexural Vibration Method
The viscoelastic properties of the wood were determined using what is known as a Free Flexural Vibration Method, which is widely used to measure these properties in wood. The samples were hunk horizontally by silk strings, and its resonant vibration was excited by a magnetic driver or an audio speaker, with the amplitude of vibration being measured using an eddy-current sensor, a laser displacement sensor, or a high precision microphone. Different methods were used depending on the physical properties of the wood. For larger samples, mall iron pieces were fastened to the ends of the samples with one end excited by the magnetic driver, while the deflection of the other end was measured using the eddy-current sensor. When the frequency was higher than 500 Hz, deflection was observed using the microphone. For particularly light samples, displacement was measured using the laser displacement sensor.

A similar study had been made previously by the same group, with a few key differences. Firstly, the Free Flexural Virational Method used only a microphone to record data and did not take into consideration the shortcomings and nuances explained previously. Secondly, this research focused on a few more parameters of the wood, such as the loss tangent and shear modulus of the wood.

Calculations
The EL ' and VL values were calculated from the dimension and resonance frequency (fr) using the following equation, as established by Hearmon (1958).

$$V_L=\sqrt{E_L^,/\rho}=4\sqrt{3}\pi f_r/m_nh$$

where the h and l are height and length of the sample, respectively, and mn is a constant dependent on the mode of vibration.

Problems With Measuring the Effects of Aging
Historically, the effects of aging have been discussed by comparing recently cut wood with aged wood. However, wood, being a natural material, has density-dependent properties that can vary wildly, even within a single tree. In addition, even if the density is similar between two samples of wood, the vibrational properties themselves can vary depending on the microfibrils within the cell wall of the wood itself. Thus, a superficial method of aging wood, such that the density and vibrational properties themselves remain constant, was needed.

Superficial Aging Process
A study by Zeniya et al. in 2019 proposed the use of an artificial aging process, artificially accelerating the aging process of samples. The method used is known as Hygrothermal Treatment, which is the use of intense heat, usually through oven heating, to artificially age the wood samples used. The reason Hygrothermal Treatment is used is because the effects due to treatment; such as an increase in brittleness and darkened color, are qualitatively similar to natural aging. Worth noting is that the actual temperature of heating changes the chemical reactions within the wood when above 150°C, thus, while this process may be qualitatively similar to prolonged aging, it is not perfectly accurate and quantitatively.

This method focused on the properties of spruce wood due to its prolific use in the soundboards of instruments and its vibrational properties and color are important qualitative factors to conventional musicians.

Changes in Wood Sound Properties
The moisture content is a key factor is determining the overall vibrational qualities within wood. The moisture content is defined to be the percent ratio of the mass of moisture in the wood to the absolute dry mass of the wood. In tests measuring over a six month conditioning, the moisture content of green wood was reduced to 25% in just one day before equilibrating at 11.7% within two days of drying. While some artisans have suggested that aging increases the dimensional stability of the wood, this implies that this is not the case, as no meaningful dimensional stabilization occurs beyond day two of seasoning.

The test results did, however, show that the vibrational properties of the wood continued to change beyond the second day of seasoning. In particular, the sound velocity continued to increase, while the internal friction continued to decrease, even after moisture content equilibrized. This is concurrent with conventional knowledge that violin makers have that wood with higher sound velocities and lower internal friction tends to positively correlate with better sounding instruments. Therefore, the seasoning effects are ideal for sound radiation from the wooden soundboards of the instruments.

Changes to Wood Chemical Properties
Because wood is chemically stable in dry conditions, small amounts of seasoning is not enough time to induce significant changes in the wood. Therefore, the physical aging of the wood is the most likely cause behind seasoning and aging effects, as opposed to chemical changes over time. Therefore, once the wood is subjected to a moistening and drying, the Young's Modulus and internal friction of the wood can change with little changes to the actual chemical properties of the wood. The viscoelastic stress relaxation of wood polymers is the main explanation behind their destabilization and stabilization during drying and seasoning. In green wood, the polymers have completely absorbed the water, resulting in them being swollen. In subsequent drying, the polymers shrink with the removal of this water, and are thus unnaturally distorted, likely resulting in greater mobility and lower rigidity. During subsequent seasoning, these polymers continue to relax, resulting in an increase in sound velocity and a decrease in internal friction. These effects were originally used to explain the effects of heat treatment of wood, and reasonably explain the effects shown in the study.

Empirical evidence supports this argument, as if it were true, the effects of aging could be reversed by moistening the wood, something that was concluded to be accurate in practice, with the sound velocity and internal friction values returning to their original values upon moistening.

Aging
While the effects of seasoning are shown to be reversible, the effects of long term aging include irreversible effects due to chemical changes within wood polymers. The results of artificial aging have shown only a 2% increase in sound velocity and a 2% decrease in internal friction over a synthetic aging of 2000 years. The results are noted to be under relatively dry Hygrothermal conditions, as these qualities are shown to seriously degrade when aged in humid conditions.