Tại Sao Đàn Guitar Thường Được Làm Từ Gỗ Vân Sam và Tuyết Tùng

The best acoustic guitars in the world in history are made from premium woods, such as spruce, cedar and rosewood. Is this by tradition or proven by science?

Part of the answer lies in the properties of wood. A comprehensive study conducted in the late 1970s evaluated 25 species of wood (over 87 wood samples) that were used to make high-quality musical instruments including guitars, violins and pianos. According to the article by Daniel W Haines, On Musical Instrument Wood.

“Maple is suitable for violin backs just as Rosewood is for guitar backs. Tradition enriches the way the guitar is made by adding: the way the wood is cut from the tree and the personal preference for the woods. Why? We believe the answer lies in some of the music-related mechanical properties of these woods, namely hardness, density, and vibration damping." Haines incorporated these wood properties. into the Loudness Index (L), which indicates the potential to generate more loudness.

“Low density, high Young’s modulus [wood stiffness] and low damping contribute to the high L-value.” The L-value effectively measures the characteristics of the wood at a given level of resonance. Dr. Bernard Richardson of Cardiff University continues to study the properties of wood today and says the measure has some limitations.

He said the Radiant Ratio - another measure of sound power minus the impact of damping - is a better measure of wood's properties. This ratio (technically a measure of the ratio of hardness and density) better describes the combination of modes in the acoustic radiation above their resonant frequency.

The chart below shows the Radiance Rates of several samples of wood used to make guitars (and violins) tested by Haines. It shows that some common woods, such as European Spruce and Western Red Cedar, rank particularly high, although there is great variation between samples.

Spruce and cedar: strong radiation ratio

There are different varieties of spruce, but Haines calls European spruce the first choice of musical instrument makers (which he calls Picea excelsa - a now obsolete scientific term, replaced by Picea abies).

"This wood is generally fine-grained, light-colored, and favored by instrument makers over other spruce species," Haines reports.

The irradiance rate for a single European Spruce sample is as high as any of the 87 samples tested by Haines, which is also consistent with the Sitka Spruce and Western Red Cedar samples.

“Those who like Cedar understand how important it is to sound quality, as it is easy to split and concave. Compared to spruce, this wood is less dense, less hard, and most significantly less susceptible to moisture.” The back of the guitar is traditionally made of Rosewood, unlike the violin, which is made of Maple.

Haines attributes this choice to the lower damping qualities of Rosewood compared to Maple. This improves the durability of the guitar, which is the opposite of a violin.

Haines may have exaggerated the case. The transfer of energy from the strings to the body of the guitar is the limiting factor in energy loss, and it is the loss of the strings that is the main factor, especially at higher frequencies.

Sound quality is not just a function of wood species

The wood used to make the instrument does not go hand in hand with the great sound, it will even have a melody of its own. There are so many factors at play, even within the same wood species, and in guitar making.

Luthier Trevor Gore has estimated that the material properties of wood from the same species can vary by a factor of two. “There is therefore considerable overlap in the material properties of one species with others, implying that wood species can be replaced with little acoustic impact if the composition is designed and engineered. to negative tolerances rather than dimensional tolerances,” he wrote in an article, Wood for Guitars , published in the Journal of the Acoustical Society of America. This is partially reflected in Haines' tests. For example, one sample of European Spruce has a Radiance Ratio of 15 while another (used to make violin tips) stops at only 10.

Haines speculates that the wood's poorer performance is due to the veins in the sample (ripples in the wood fibers), which may increase the density of the wood (and internal damping, reflected in the lower Noise value). ). Dr. Richardson is currently testing light-weight cedar and spruce, for similar characteristics. “I suspect the difference then becomes quite small,” he said. "What matters is the real physical features - not the species."

While the average cedar patch at the cellular level has thinner walls, resulting in less density and stiffness (density is a key factor), these general differences can easily change. “I've had well-cut spruce samples at a lower density than cedar - it's just a weird thing about how they've grown."

Other differences found between the same species of wood include grain size, which can vary depending on the area where the wood is grown and the density. According to Haines, the density of dark summer wood or the growth ring of late wood is much higher than that of early morning wood in early spring.

The way wood is cut also has a significant effect on its acoustic properties. According to Dr. Richardson, the traditional way to cut wood is a quarter cut, keeping the grain and fibers as straight as possible to maximize stiffness. This maximizes the radiation ration for the selected wood (regardless of species).

“It is very rare to find spruce trees that do not grow in a spiral, so it is very difficult to get a full width plank that is sectioned across its entire width,” he said. or have threads running parallel to the sides over the full width. There are always compromises in the best treated wood – and even bigger compromises in most woods cut to maximize commercial benefit, rather than acoustic criteria.”