If you consider that the air is able to absorb or contain moisture, and that the amount of moisture the air currently contains, relative to its saturation point of 100%, then you know what relative humidity is. It is the quantity of moisture present in the air at a given temperature. For example if the RH is 50% then the air is currently holding 50% of the maximum it possibly can contain at that temperature.
Now, if the temperature were to rise at this point, the RH would fall because the air is capable of holding more moisture at a higher temperature before it reaches saturation point. This is important because now the air will start to remove moisture from surrounding objects in an attempt to arrive at what it considers to be equilibrium. It will absorb moisture from all available sources and one of these could be your guitar. Do not misinterpret this to mean that any rise in temperature will cause a drop in RH. That drop in RH will only happen if all other conditions remains the same when the temperature rises.
Conversely, a drop in temperature does not always mean a rise in RH. A rise in RH may be less harmful to your guitar as the timber will then start to absorb moisture, but each condition, be it high humidity or dry can be bad for your instrument. Artificial environments. When you enter an air conditioned atmosphere, the moisture has been removed as part of the cooling process and the RH is much lower than the outside air. This may all be a bit confusing, so the best thing to remember is that an artificial environment (heated or cooled) will usually have a low RH. This could be bad for your guitar. Humidifiers are sometimes used in these environments to maintain the RH.
| Relative Humidty | ||
| Temp variation from outside air | RH variation | |
| Air-conditioned building | Down | Down |
| Heated building | Up | Down |
So what do you do to combat this problem of low RH. Firstly get yourself an Hygrometer, a device that will measure the RH, and if you live in an artificial environment or one that normally has a low RH, like the desert regions, then invest in a humidifier for your instrument. This is a device that will maintain the RH for you instrument while it is inside the case. These are available in many forms, the most common one is called a "Dampit". If you cannot get one and need one in a hurry, just get a sponge (clean one) from the kitchen and wet it and wring it out. Put this is a plastic bag with little holes in it and put the lot in the guitar case , not touching the guitar preferably. Check this sponge every few days and re-wet it as required. Do not leave there permanently.
I do know of drastic measures taken by touring players hanging their guitar in the motel bathroom with a hot shower running to generate steam in the room. Now this is pretty drastic and should not be necessary if the proper care is taken. Most good instruments are built in humidity controlled conditions, usually around 45% - 50%. When the humidity falls below this level the timber will dry out. Most guitars are built with domes in the top and back and this dome will firstly flatten out. This can influence the action and in the worst case the drop in RH can cause the top or back to crack. These cracks are usually found on the top of the guitar running from the ends of the bridge to the base of the guitar.
The newer the guitar the worse it will be effected by RH changes. As instruments age they are less susceptible to these changes. A well made guitar should not be as susceptible to cracks as a guitar made in poor conditions or without the necessary constraints. A rise in RH is not likely to cause permanent damage to your guitar like a drop in RH can. Mostly guitar just swell up and this can affect the sound and the action again. A guitar will always sound it's best in an environment similar to what it was created in.
What is worse, excessively dry conditions or excessively humid conditions?
It is important to remember that wood will loose moisture far quicker than it will absorb it. This is very important if you are travelling with your guitar. When wood dries, it shrinks, spruce more so that Cedar. A piece of spruce approximately the width of the lower bout of your guitar (about 360mm) can vary by up to 10mm in width from one extreme of RH to the other. What this means that is that the chances of a guitar developing cracks will happen much faster than the problems that occur due to excessive humidity.
What to do about these problems.
Firstly
get an Hygrometer. They are cheap insurance. Most electronic shops carry small inexpensive
ones like the one pictured here. It cost about €25 in Australia. It can be kept in
the case with the guitar. It folds up and measures approximately 55mm x 55mm x 20mm.
Electronic hygrometers are more reliable than the "hair" type for general use. Hair
hygrometers need to be calibrated and are generally too large to fit in guitar cases.
Secondly
get something like the "Dampit" for use in Stringed instruments. Most instruments
are best stored at around 45 - 50% RH.. Try to keep the guitar in an environment
that will suit it best and it will last longer and perform better. Dry or humid conditions
cause tension in the timbers and this will have a negative affect on the sound of
the guitar as well as the possibility of structural damage.
