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Wood exhibits a low thermal conductivity (high heat-insulating capacity) compared with materials such as metals, marble, glass, and concrete. Thermal conductivity is highest in the axial direction and increases with density and moisture content; thus, light, dry woods are better insulators.
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Thermal properties Although wood expands and contracts with varying temperature, these dimensional changes are small compared with shrinkage and swelling caused by varying moisture content. In most cases, such temperature-related expansion and contraction are negligible and without practical importance. Only temperatures below 0 °C (32 °F) have the potential to cause surface checks; in living trees, unequal contraction of outer and inner layers may result in frost cracks. Wood exhibits a low thermal conductivity (high heat-insulating capacity) compared with materials such as metals, marble, glass, and concrete. Thermal conductivity is highest in the axial direction and increases with density and moisture content; thus, light, dry woods are better insulators. When exposed to sufficiently high temperatures, wood burns. This property makes wood suitable for heating purposes but is disadvantageous for its technical utilization. The maximum heating value of one kilogram of oven-dry wood averages about 4,500 kilocalories (with a range of 4,100–6,800 kilocalories). In general, softwoods possess a higher heating value than hardwoods, and extractives have an important influence; for example, a kilogram of the oleoresin in pines has a heating value of about 8,500 kilocalories. Moisture reduces the heating value; air-dry wood has about 15 percent less heating value than oven-dry wood. Wood must be raised to a temperature of about 250 °C (about 480 °F) for a spark or flame to ignite it, but at a temperature of about 500 °C (about 930 °F) ignition is spontaneous. The flammability of wood can be reduced by chemical treatment (see the section Preservation). Electric properties Oven-dry wood is electrically insulating. As moisture content increases, however, electric conductivity increases such that the behaviour of saturated wood (wood with maximum moisture content) approaches that of water. Noteworthy is the spectacular decrease of electric resistance as moisture content increases from zero to the fibre saturation point. Within this range, electric resistance decreases more than a billion times, whereas from the fibre saturation point to maximum moisture content, it decreases no more than about 50 times. Other factors, such as species and density, have little effect on the electric resistance of wood; differences among species are attributed to the chemistry of the extractives. Axial resistance is about half that of the transverse. Resistance increases with decreasing temperature; in oven-dry wood it doubles over a temperature drop of 12.5 °C (22.5 °F). Practical use of the relationship of wood’s moisture content to its electric resistance is made in electric moisture meters. Important also are the dielectric, or poor-conductor, properties of wood. These properties—dielectric constant and power factor—assume a practical importance in drying wood with electric current (a theoretical possibility, although not presently a reality), gluing wood with high-frequency electric current, or making electric meters (capacity and radio-frequency power-loss type) for measuring its moisture content. Wood exhibits the piezoelectric effect—that is, electric polarization (the appearance of opposite electric charges on opposite sides of a piece) occurs under mechanical stress. Conversely, when subjected to an electric field, wood exhibits mechanical deformation (changes in size). Acoustic properties Wood can produce sound (by direct striking) and can amplify or absorb sound waves originating from other bodies. For these reasons, it is a unique material for musical instruments and other acoustic applications. The pitch of sound produced depends on the frequency of vibration, which is affected by the dimensions, density, moisture content, and modulus of elasticity of the wood. Smaller dimensions, lower moisture content, and higher density and elasticity produce sounds of higher pitch. When sound waves of extrinsic origin strike wood, they are partly absorbed and partly reflected, and the wood is set in vibration. The sound can be amplified, as in violins, guitars, organ pipes, and other musical instruments, or it can be absorbed, as in wooden partitions. Normally, wood absorbs a very small portion of acoustic energy (3–5 percent), but special constructions incorporating empty spaces and porous insulation boards can increase absorption to as high as 90 percent. The speed of sound in wood is about 3,500–5,000 metres (about 11,500–16,400 feet) per second axially and 1,000–1,500 metres (3,300–4,900 feet) per second transversely; the axial value approaches the speed of sound in iron and is 10 times higher than that in air. The velocity of sound in wood is reduced by moisture, which therefore contributes to faster damping of sound. For musical instruments, a preference exists for selected spruce wood, but fir, pine, maple, and tropical woods also are used. Abnormalities such as decay affect acoustic properties; use of this fact is made in nondestructive testing of wood.
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