What Every Musician Should Know About Temperature and Humidity

 

 

Understanding Wood:

Wood Cells
There are four types of cell structures, serving specialized purposes. in softwoods, thin- walled tracheids, elongated and tubular in shape and often 3mm long (and up to 8mm), serve the function of sap conduction. The thick walled tracheids with pointed ends  and fewer of the pits for sap conduction, are the main support cells. In hardwoods, needle-shaped thick walled fibers, shorter (about 1mm) than the softwood tracheids, function as support. The density of a hardwood is determined more by the thickness of the fiber walls more than any other feature. Vessels, short pipe-like cells with open ends, form the arteries through which sap flows. The vessels appear as pores on traverse (cross-sectional or end grain) surfaces.

Earlywood or Latewood
Woods growing in temperate latitudes, with distinct seasons of growth, often display variations within the annual growth ring visible on traverse sections. Earlywood ( or springwood) occurs in the first part of the growth season and consists of cells with relatively large cavities and thin walls. Latewood (summerwood) cells have smaller cavities and thicker walls. Earlywood is lighter in weight, softer and weaker than latewood. It shrinks less across the grain and more lengthwise along the grain than latewood. Density in softwoods is determined mainly by the proportions of earlywood to latewood.

Sapwood and Heartwood
Growth in a tree is accomplished by a single layer of cells called the cambium layer. During periods of growth the cambial cells separate into two different types with different functions.Some cambial cells produce sapwood cells on the inside and some produce bark on the outside. The next layer moving to the outside of the cambium layer is called the inner bark. It's function is the movement of food from the leaves to all the living parts of the tree. Sapwood,  functions in the storage of food, and in the transport of sap from the roots to the top of the tree. The sapwood, carries the nutrients up and down the tree suspended in a water solution, this process is called turgor. As the tree grows in circumference, the cambial cells grow in progressive layers,and the sapwood becomes further removed from the active growth region where nutrients are supplied to the cells. This causes the sapwood cells to die. The heartwood of the tree consists of the dead sapwood that ceases to conduct sap, but instead serves the sole function of structural support. Heartwood cells differ slightly in chemical and physical composition from sapwood cells. Extractives of mineral and waste deposits form in the cell walls, and it is these substances that contribute toward color, oder, density, and specific characteristics found in each species of tree. Sapwood in maple can be up to 6" wide, although in most wood it is 1 to 2 inches wide. Water is present even in the heartwood while it is still alive.

Relative humidity
Relative humidity refers to the amount of moisture the air can hold at a given temperature. Warm air holds more moisture than cold air. Above 60%, molds can form and glue joints fail. Below 40%, you can expect cracking of fine instruments caused by shrinking as the wood dries out. The percentage of moisture in the air compared to how much moisture the air can hold at that any given temperature is it's Relative Humidity. Warm air holds more moisture than cold air. In the winter if the cold air is heated artificially without adding moisture the relative humidity will go down because the air has the potential to hold much more moisture when warm. Therefore using a heater in the winter without a humidifier will lower the relative humidity of the room and the guitar's wood can dry out further.

A Practical Methodology
There is a practical methodology for determining when a guitar's wood is drying out due to lack of humidity. Check the action height. If the strings feel lower or closer to the frets, then the top could be shrinking. If strings that were the correct height are now buzzing, it can mean that the top braces are drying out and becoming stiffer. Less flexibility in the braces does not allow the string pressure to pull on the top so it is slightly bowed upward. Some Braces try to warp the top in an irregular fashion, thereby causing the top to warp or crack in various locations. Additionally the edge of the neck, which can have edge binding, feels bumpy or sharp at each fret juncture. The metal frets are not effected by shrinkage but the wood is. This is one of the arguments for instruments to have either no edge binding, or edge binding with the fret inset a little. This way the fret does not make contact with the edge binding, and therefore will not put pressure on it to make it bulge or crack.

In like manner to much moisture in the air, such as happens during the summer can cause a guitar neck to bow which will raise the action height. The top of an acoustic guitar may bulge out at the bridge area or the bridge, and braces can start to loosen or pull off. Additionally, the use of air conditioners can lower the temperature, and thus the humidity, to quickly for a piece of wood to get acclimated. The end result being once again the stretching and shrinking of the top and braces, causing severe damage. A change of 3% moisture content within the wood can change the dimension of a guitar top approximately .125". A general rule for the care of a wooden stringed instrument is if your body is comfortable at the rooms temperature, then your guitars body will be too.

Construction Concerns
The main objective when building a guitar is to highlite the correct harmonics which will create the desired sound. Various species of trees differ in the density of the wood they produce because of differences of cellular arrangement Each type of wood has its own set of resonate frequency bandwidths that it highlights. How the particular species internal structure reacts to the external physical forces of climate and the geographically available nutrients help to further shape frequency response.

The ideal condition in which to build a guitar is an atmosphere with 50% relative humidity. It's the midpoint in the range of 40% to 60% which is considered safe. The wood of a guitar will take about 12 years to acclimate to the atmosphere of it surroundings once the tree is cut down. The wood must age until it has an average of between 5% to 14% water content left in the heartwood cells. This difference of 5% to 14 % is due to the different climates on earth. Any piece of wood with less or more percentage of water content is not considered to be an acceptable material to use for a guitar.

The following chart shows what happens to various types of wood when it water content changes only by 3%.

Dimentional Changes due to water content:
 

The following is an analysis of the effect humidity can have on an acoustic guitar made of
White Spruce, Honduras Mahogany, and Brazilian rosewood.

This formula is setup to calculate wood shrinkage in the tangial direction:
  DI  [ CT  (MF - MI )] = *D

 White Spruce

 5"   [ .00263(7% - 10%)] = - .03945= shrinkage in tangial direction
 5"   [ .00148(7% - 10%)] = - .0222 = shrinkage in radial direction
 .02% shrinkage in longitudal direction = .0010"

 Honduras Mahogany

 5" [ .00238(7% - 10%)] = -.0357" = shrinkage in tangial direction
 5" [ .00172(7% - 10%)] = -.0258 = shrinkage in radial direction
 .01% shrinkage in longitudal direction = .0005"

 Brazilian Rosewood

 5" [ .00205(7% - 10%)] = -.0372" = shrinkage in tangial direction
 5" [ .00120(7% - 10%)] = -.018" = shrinkage in radial direction
 .01% shrinkage in longitudal direction = .0005"

Reaction wood:
 Whether compression wood  in softwoods or tension wood  in hardwoods,
Reaction wood tends to shrink excessively parallel to the grains. Reaction wood exhibits
excessive longitudal shrinkage and may occur with in the same board of normal wood.