Post by ChrisB on Oct 7, 2014 8:10:33 GMT
Much discussion is had in audio circles about the differing effects that are attributed to the structure of various different types of materials. There’s plenty of theory, experimentation, conjecture and pseudo-science surrounding materials like aluminium, slate, mdf/ply/chip boards, wires, granite etc, etc but there's not much that considers the physical structure of wood.
When you take a look at the subject, it becomes clear that wood is not 'just wood'.
I have an interest in this subject because I’m a forester and apart from being involved in the planting of rather a lot of trees (I had a spell of a few years when we did a quarter of a million per year) and cutting them down to sell the wood (including logs destined for musical instrument making), I’ve done some other interesting stuff. I had the opportunity to study the structure of various types of wood and its behaviour under all sorts of conditions as part of a quest to find the cheapest way to reduce half a million tonnes of trees a year to particles the size of talc.
There are some subtly different types of cell structures in timber and I often wonder what might be the best type to use in an audio application. I’ll set out the botanical facts and maybe there will be some comment on what might be the best type to use in an actual application.
The Basics
We all know that a tree lays down a growth ring every year but if you look closely at a ring, you’ll see some subtle variations across the width of that, sometimes tiny, band of wood.
The cells that make up the structure of a tree have to basically carry out two different basic functions
1. Provide support and structural integrity
2. Convey liquids up and down, to and from all the living parts.
Plants have developed a brilliant solution to this because the cells form fibres which are essentially tubes & a tube is a good thing to transport liquids in and it’s a shape which is also incredibly strong in compression.
So the first thing to do is to think of wood as being made up of bunches of tubes – the fibres. The walls of these are in turn, made up of smaller tubes with closed ends – the cells. The tubes can be of different diameters and/or their walls can be different thicknesses. If you cut across a log and look at the end (cross section), you’ll see tiny holes – these are the vessels and when viewed this way are referred to as pores.
Just under the bark of a tree is a layer called the ‘cambium’ – the inside of this is where new cells are produced. When you think how a tree grows, a good analogy is that of putting a new coat on every year. The coat has a lining which is laid down in the spring (the earlywood or springwood) and a second sub-layer called the latewood or summerwood. The earlywood consists of faster-growing material, while the latewood is comprised of denser, slower-growing material.
Relatively newly formed wood is called sapwood & this is where all the action is because sapwood is alive. As the sapwood builds up over the years, the inner layers become gradually useless for transport as they are no longer plumbed in to the growing parts of the tree. The tubes get ‘gummed up’ and filled with compounds of chemicals – this is heartwood, and because of the solidified material inside the fibres, it is more dense and stronger than sapwood. The chemicals in the new filling in the fibres fight off decay that might be caused as a result of a fungal or viral attack. The heartwood also provides a kind of backbone – a strong core to support the increasing bulk and weight of the tree as it grows upwards and outwards and is subjected to the stresses of the wind and other forces such as gravity.
This image illustrates the layers quite nicely. The earlywood and latewood can be seen as sub-sections of each growth ring.
Conifers / Softwoods
Coniferous trees are less evolved than the hardwoods and produce only one type of cell which is called a ‘Tracheid’. These are able to fulfil both functions of support and water conduction.
There are two basic types with the conifers but there’s a big grey area in between those two where there is no clear distinction between them.
Gradual Transition Softwoods
The simplest type of conifer timber is called ‘gradual transition’ and it’s distinctive because of a fairly uniform structure when looking at a cross-section. The cells are all of similar diameter and wall thickness, but there is often a gradual increase in the thickness of the wall near the outside of each the growth ring - this is called ‘latewood’ as it is laid down late in the growing season. The astute amongst you will therefore surmise that the early season wood is called ‘earlywood’(!) If you’ve every tried to count the rings on a piece of wood, you may have got mixed up here because the earlywood & latewood represent one year & sometimes they can look like two separate growth rings.
Example of gradual transition softwood
Examples:
Spruces (Picea species)
True Firs (Abies species eg Grand Fir but not Douglas Fir)
Yews (Taxaceae)
Cypresses (Cupressaceae)
Eastern White Pine (Pinus strobus)
Western White Pine (Pinus monticola)
Sugar Pine (Pinus lambertiana)
Chile Pine/Monkey Puzzle, Norfolk Island Pine, Agathis (Araucariaceae)
Abrupt Transition Softwoods
The other category of conifer is ‘abrubt transition’ and they have evolved to partially separate the support and transport functions. Here, the earlywood has large diameter tracheids with thin walls and the latewood is just the opposite.
Photographic example of abrupt transition softwood
Examples:
Douglas Fir (Pseudotsuga)
Monterey Pine, Scots Pine (Pinus)
Western Hemlock (Tsuga)
Broadleaved Trees / Hardwoods
These trees are rather more highly evolved than the conifers and one of the refinements they have developed is that support and transport are carried out by two different types of cells. This is a good strategy because hardwoods have found ways of arranging the two types of cells in different combinations in order to cope with life in a very wide range of conditions and habitats.
Water transport is assigned to relatively large diameter vessels whereas structural support is achieved with thick walled fibres.
Ring-Porous Hardwoods
Ring-Porous trees are usually found in regions where there is a strongly seasonal climate where there are long periods of cold (known in the trade as ‘winter’!) that cause trees to go into dormancy where growth is temporarily stopped. Just to throw a spanner in the works, though, there are some ring-porous species found in the Tropics, generally trees found in areas where there are short growing seasons due to other climatic conditions such as monsoons or seasonal flooding.
The way that ring-porous trees arrange their vessels is that large diameter vessels are formed in clusters within a fixed width at the beginning of the growing season (springwood) followed by smaller diameter ones dispersed throughout the latewood. Some trees may have only slightly larger and/or fewer larger pores at the start of the growth ring, alternatively, they may have a more gradual change in pore size as early wood moves into the latewood. These are called weakly ring porous and the main distinction between strongly and weakly ring porous trees is the effect of fast growth.
Ring-porous trees respond to speed of growth with marked changes in timber strength and strongly ring-porous species are more affected by growth rate than the weakly ring-porous ones. For example a fast grown Ash will have much stronger timber than slow grown Ash – once the pre-defined layer of early wood (weakened by those big vessels is laid down any extra wood that grows will be the stronger latewood. If the proportion of latewood to earlywood is much greater, then the timber will be denser and therefore stronger. The idea that growing fast makes stronger wood seems counter intuitive because for other timbers, fast growth (and therefore wide annual rings) usually implies low density & therefore lower timber strength.
Photographic example of strongly ring porous hardwood (Ash)
Examples:
Chestnut (Castanea)
Hickory (Carya)
Eucalypus
Ash (Fraxinus)
Walnut (Juglans)
Cherry (Prunus)
Oak (Quercus)
Elm (Ulmus)
Diffuse Porous Hardwoods
Diffuse-porous hardwoods show a fairly even spread of similar sized vessels arranged in various patterns throughout the growth ring. Of the four types, diffuse-porous has the widest range of species - they are very common in temperate areas but they tend to be the dominant type of tree in the tropical regions. They’re easy to spot as they are so different from the ring-porous hardwoods - no bands of larger pores on the inside of the growth ring. Some Tropical hardwoods don’t show obvious growth rings because they grow strongly right throughout the year but they are still classed as diffuse-porous.
Examples:
Maples, Sycamore (Acers)
Alders (Alnus)
Poplars, Basswood (Populus)
Beech (Fagus)
Birch (Betula)
The Forester’s Dilemma: Quality versus Quantity
For most people who grow trees commercially the aim is to get a given area of land to be as productive as possible because the investment is a very long term one (and be under no illusion – broadleaved trees are also planted for timber. Commercial forestry is not synonymous with conifers). This is generally at odds with producing a high quality timber product. Slow grown timber is denser and therefore stronger than fast grown (except in the case of ring-porous trees). And to make this more confusing, when you take trees out of their natural range and grow them in plantations, they can subtly change their properties. (Take a species like Monterey pine (Pinus radiata) away from its natural home in California and stick it in an intensive forestry plantation in New Zealand and an abrupt transition tree suddenly starts to become a gradual transition tree and some timber strength is lost. Timber strength per se isn’t an important parameter in building hifi components, but once you realise it is strongly related to density (and therefore mass) it becomes rather more interesting. Now if you start to think about the different ways that the fibres can be laid down, you can see that there is a lot of potential for variation in the ‘sound’ of timber – not just differences in species but also in the way that a particular tree is grown.
Rays: Another Direction.
There is another structural element of timber – the ‘Medullary Ray’ which is a sort of sheet-like structure running from the centre to the edge (ray : radius). It’s strongly visible in woods like Oak.
As lines in cross section:
And as ‘fleck’ or silver grain when viewed along the grain (especially visible in quarter sawn wood):
So that's the botany angle covered. How do you reckon all this relate to differing sound qualities?
When you take a look at the subject, it becomes clear that wood is not 'just wood'.
I have an interest in this subject because I’m a forester and apart from being involved in the planting of rather a lot of trees (I had a spell of a few years when we did a quarter of a million per year) and cutting them down to sell the wood (including logs destined for musical instrument making), I’ve done some other interesting stuff. I had the opportunity to study the structure of various types of wood and its behaviour under all sorts of conditions as part of a quest to find the cheapest way to reduce half a million tonnes of trees a year to particles the size of talc.
There are some subtly different types of cell structures in timber and I often wonder what might be the best type to use in an audio application. I’ll set out the botanical facts and maybe there will be some comment on what might be the best type to use in an actual application.
The Basics
We all know that a tree lays down a growth ring every year but if you look closely at a ring, you’ll see some subtle variations across the width of that, sometimes tiny, band of wood.
The cells that make up the structure of a tree have to basically carry out two different basic functions
1. Provide support and structural integrity
2. Convey liquids up and down, to and from all the living parts.
Plants have developed a brilliant solution to this because the cells form fibres which are essentially tubes & a tube is a good thing to transport liquids in and it’s a shape which is also incredibly strong in compression.
So the first thing to do is to think of wood as being made up of bunches of tubes – the fibres. The walls of these are in turn, made up of smaller tubes with closed ends – the cells. The tubes can be of different diameters and/or their walls can be different thicknesses. If you cut across a log and look at the end (cross section), you’ll see tiny holes – these are the vessels and when viewed this way are referred to as pores.
Just under the bark of a tree is a layer called the ‘cambium’ – the inside of this is where new cells are produced. When you think how a tree grows, a good analogy is that of putting a new coat on every year. The coat has a lining which is laid down in the spring (the earlywood or springwood) and a second sub-layer called the latewood or summerwood. The earlywood consists of faster-growing material, while the latewood is comprised of denser, slower-growing material.
Relatively newly formed wood is called sapwood & this is where all the action is because sapwood is alive. As the sapwood builds up over the years, the inner layers become gradually useless for transport as they are no longer plumbed in to the growing parts of the tree. The tubes get ‘gummed up’ and filled with compounds of chemicals – this is heartwood, and because of the solidified material inside the fibres, it is more dense and stronger than sapwood. The chemicals in the new filling in the fibres fight off decay that might be caused as a result of a fungal or viral attack. The heartwood also provides a kind of backbone – a strong core to support the increasing bulk and weight of the tree as it grows upwards and outwards and is subjected to the stresses of the wind and other forces such as gravity.
This image illustrates the layers quite nicely. The earlywood and latewood can be seen as sub-sections of each growth ring.
Conifers / Softwoods
Coniferous trees are less evolved than the hardwoods and produce only one type of cell which is called a ‘Tracheid’. These are able to fulfil both functions of support and water conduction.
There are two basic types with the conifers but there’s a big grey area in between those two where there is no clear distinction between them.
Gradual Transition Softwoods
The simplest type of conifer timber is called ‘gradual transition’ and it’s distinctive because of a fairly uniform structure when looking at a cross-section. The cells are all of similar diameter and wall thickness, but there is often a gradual increase in the thickness of the wall near the outside of each the growth ring - this is called ‘latewood’ as it is laid down late in the growing season. The astute amongst you will therefore surmise that the early season wood is called ‘earlywood’(!) If you’ve every tried to count the rings on a piece of wood, you may have got mixed up here because the earlywood & latewood represent one year & sometimes they can look like two separate growth rings.
Example of gradual transition softwood
Examples:
Spruces (Picea species)
True Firs (Abies species eg Grand Fir but not Douglas Fir)
Yews (Taxaceae)
Cypresses (Cupressaceae)
Eastern White Pine (Pinus strobus)
Western White Pine (Pinus monticola)
Sugar Pine (Pinus lambertiana)
Chile Pine/Monkey Puzzle, Norfolk Island Pine, Agathis (Araucariaceae)
Abrupt Transition Softwoods
The other category of conifer is ‘abrubt transition’ and they have evolved to partially separate the support and transport functions. Here, the earlywood has large diameter tracheids with thin walls and the latewood is just the opposite.
Photographic example of abrupt transition softwood
Examples:
Douglas Fir (Pseudotsuga)
Monterey Pine, Scots Pine (Pinus)
Western Hemlock (Tsuga)
Broadleaved Trees / Hardwoods
These trees are rather more highly evolved than the conifers and one of the refinements they have developed is that support and transport are carried out by two different types of cells. This is a good strategy because hardwoods have found ways of arranging the two types of cells in different combinations in order to cope with life in a very wide range of conditions and habitats.
Water transport is assigned to relatively large diameter vessels whereas structural support is achieved with thick walled fibres.
Ring-Porous Hardwoods
Ring-Porous trees are usually found in regions where there is a strongly seasonal climate where there are long periods of cold (known in the trade as ‘winter’!) that cause trees to go into dormancy where growth is temporarily stopped. Just to throw a spanner in the works, though, there are some ring-porous species found in the Tropics, generally trees found in areas where there are short growing seasons due to other climatic conditions such as monsoons or seasonal flooding.
The way that ring-porous trees arrange their vessels is that large diameter vessels are formed in clusters within a fixed width at the beginning of the growing season (springwood) followed by smaller diameter ones dispersed throughout the latewood. Some trees may have only slightly larger and/or fewer larger pores at the start of the growth ring, alternatively, they may have a more gradual change in pore size as early wood moves into the latewood. These are called weakly ring porous and the main distinction between strongly and weakly ring porous trees is the effect of fast growth.
Ring-porous trees respond to speed of growth with marked changes in timber strength and strongly ring-porous species are more affected by growth rate than the weakly ring-porous ones. For example a fast grown Ash will have much stronger timber than slow grown Ash – once the pre-defined layer of early wood (weakened by those big vessels is laid down any extra wood that grows will be the stronger latewood. If the proportion of latewood to earlywood is much greater, then the timber will be denser and therefore stronger. The idea that growing fast makes stronger wood seems counter intuitive because for other timbers, fast growth (and therefore wide annual rings) usually implies low density & therefore lower timber strength.
Photographic example of strongly ring porous hardwood (Ash)
Examples:
Chestnut (Castanea)
Hickory (Carya)
Eucalypus
Ash (Fraxinus)
Walnut (Juglans)
Cherry (Prunus)
Oak (Quercus)
Elm (Ulmus)
Diffuse Porous Hardwoods
Diffuse-porous hardwoods show a fairly even spread of similar sized vessels arranged in various patterns throughout the growth ring. Of the four types, diffuse-porous has the widest range of species - they are very common in temperate areas but they tend to be the dominant type of tree in the tropical regions. They’re easy to spot as they are so different from the ring-porous hardwoods - no bands of larger pores on the inside of the growth ring. Some Tropical hardwoods don’t show obvious growth rings because they grow strongly right throughout the year but they are still classed as diffuse-porous.
Examples:
Maples, Sycamore (Acers)
Alders (Alnus)
Poplars, Basswood (Populus)
Beech (Fagus)
Birch (Betula)
The Forester’s Dilemma: Quality versus Quantity
For most people who grow trees commercially the aim is to get a given area of land to be as productive as possible because the investment is a very long term one (and be under no illusion – broadleaved trees are also planted for timber. Commercial forestry is not synonymous with conifers). This is generally at odds with producing a high quality timber product. Slow grown timber is denser and therefore stronger than fast grown (except in the case of ring-porous trees). And to make this more confusing, when you take trees out of their natural range and grow them in plantations, they can subtly change their properties. (Take a species like Monterey pine (Pinus radiata) away from its natural home in California and stick it in an intensive forestry plantation in New Zealand and an abrupt transition tree suddenly starts to become a gradual transition tree and some timber strength is lost. Timber strength per se isn’t an important parameter in building hifi components, but once you realise it is strongly related to density (and therefore mass) it becomes rather more interesting. Now if you start to think about the different ways that the fibres can be laid down, you can see that there is a lot of potential for variation in the ‘sound’ of timber – not just differences in species but also in the way that a particular tree is grown.
Rays: Another Direction.
There is another structural element of timber – the ‘Medullary Ray’ which is a sort of sheet-like structure running from the centre to the edge (ray : radius). It’s strongly visible in woods like Oak.
As lines in cross section:
And as ‘fleck’ or silver grain when viewed along the grain (especially visible in quarter sawn wood):
So that's the botany angle covered. How do you reckon all this relate to differing sound qualities?