Wood is still expanding and contracting even when its part of your guitar.
Wood absorbs moisture and releases moisture into the air. As a hygroscopic material wood absorbs moisture from the air when it’s humid, and releases moisture when it’s dry. This causes the wood to swell and shrink. And if it shrinks too much, guess what… It cracks.
Lack of humidity may also have other implications, affecting playability, as well as tone. This is why it is so important to humidify! Don’t let low humidity ruin your pride and joy…
About wood
After harvesting, wood is typically dried in open air (although modern guitars are kiln dried which makes them much more susceptible to humidity levels or the lack of) – the dry time is nominally 1 year per inch of board thickness. During this time it loses all of its FREE WATER content, which can make up to two-thirds of its “green” weight. It then achieves an equilibrated moisture content (EMC) to the environment, leaving only BOUND WATER – water absorbed by the cell walls.
EMC can vary a lot with humidity. At 10% humidity it’s typically 2% or less, whereas at 90% it can be 20% or more. That’s an order of magnitude change that essentially changes the weight of the wood by 20%, which is significant in terms of tone and shape.
In terms of strength – wood is generally stronger along its length than it is tangentially or across the grain. Moreover, wood tends to expand and contract more tangentially than it does along the grain.
Also, each species have different properties in terms of it tensile and compressive strengths, hardness, yield modulus, etc.
Anyway, as wood equilibrates to a low RH it shrinks – stressing the wood. It’s not a foregone conclusion that it will crack, but it certainly has a higher propensity to do so.
Beyond cracking, there are other less visible effects. Shrinkage can affect playability by causing bad ‘action’ and making the guitar hard to fret. It can cause intonation issues, where it is in tune at the first fret but out of tune an octave above at the twelfth. And it can cause a loss of the wholeness of sound. Its hard to explain, but that wholeness is the full-bodied sound one would like to hear from their guitar as opposed to a more objectionable, hollow sound.
About cracking guitars
Guitars can and do crack when there is a lack of humidity. Not all guitars will, but trust us they absolutely do. Take a look online… Taylor Guitars – one of the greats – has a video series on repairing an excessively dry guitar with cracks… One would like to think Taylor know what they are talking about…
A luthier is familiar with the properties of wood and they know that wood is anisotropic, meaning that its material properties are different with the grain than orthogonal to the grain.
Wood expands and contracts at a greater rate across the grain than it does with the grain.
Anyway, if you allow your guitar to “equilibrate” to a dry environment, you really won’t be doing it any favors at all. You will distort the shape of the body. You will start to lose the arch in the top of the guitar, which will affect the action.
As the body shrinks the location of the saddle will move relative to the neck which will affect the intonation. And finally, the density of the wood will change which effects its resonance and hence affecting fullness of the sound. And if you are really unlucky you will end up with cracks.
That said, avoid over humidifying, just maintain the guitar at a reasonable humidity. And if you do have a cracked or overly dry guitar, use a Prolix Music humidifier – it’s fast & effective relief from Fret Buzz & Cracks. Ideally you want your guitar to be in a 50% RH environment. You want the wood to maintain the moisture content that it was built with, which it will do if you keep it between 45-55% RH.
If you live where humidity is 50% all year round… then you have no worries, but for those of us that live with extreme dryness, or big seasonal changes, we really need to be aware of the affects of humidity on our instruments.
#1 – What is humidity?
Humidity is water in the air, held in a gaseous state. Humidity is typically specified as Relative Humidity (RH). It is a percentage, and is temperature dependent, ranging from 0%, meaning that it has no moisture content, to 100% meaning that it is completely saturated with water vapor at the current temperature.
For the same Relative Humidity, cold air has a lot less moisture content than warm air. Heating cold air without adding moisture, causes the Relative Humidity to drop, which is why it always seems dry in the winter.
#2 – What is moisture content?
Moisture content (MC) refers to the amount of moisture that is held by the wood, relative to its dry weight. This amount can vary from 0% (Dry Wood) meaning that the wood is completely dry and brittle, to as much as 200% (Green Wood) meaning that the moisture held by the wood weighs twice that of “Dry” wood.
Before use, wood is typically air or kiln dried until only Bound Moisture remains. Bound moisture is water that has been absorbed by the cell walls. Moisture that has been absorbed by the cell walls then equilibrates slowly to the relative humidity of the surroundings. Changes in Bound Moisture Content result in dimensional changes in the wood. Wood swells and shrinks across and through the grain but is stable with the grain. For quarter sawn lumber (the best cut that is made specifically for high quality guitars and other instruments) expansion is linear.
However, when two pieces are glued together with opposing grain directions (as in the ribs on the underside of the sound board of a guitar), opposing forces cause bowing or cupping with changes in Moisture Content.
#3 – How does relative humidity affect the moisture content of wood?
MC and RH are related. The relationship, however, is complicated. It has to do with vapor pressures and equilibration.
But in simple terms, it’s all about balance.
This means that a change in RH will drive a change in the moisture content of the wood. That said, RH can change rapidly whereas wood is very slow.
This is why we humidify, but also why measuring humidity can be a little misleading.
#4 – What does this mean for you and your guitar?
Changes in MC affect the shape of the sound board, which in turn affects the action. In severe cases you can see it just by looking at the top, but long before you see it, you will feel it when you play. Adding moisture will cause the top to bow, raising the action – too much and you will find those barre chords harder to play. Whereas losing moisture will cause the top to sag, lowering the action – too much and you will start to get that dreaded fret buzz. The sound of the guitar is also another little tell-tale sign, albeit less obvious. A guitar with the right moisture content will have a fuller sound than one that is overly dry.
In the extreme, a lack of moisture can also result in the separation of the braces from the sound board, and separation of the saddle. Moreover, it can even result in cracks appearing in the sound board and/or the back of the guitar. As such, it is not something that should be overlooked.
What about the neck of the guitar?
Both the neck and the fret board have the same grain direction. They will not change length with changes in moisture but may change subtly in width. The good news is that intonation is not affected. However, as the neck dries out use of a fret board conditioner is recommended. And, if you’ve left it a little late you may begin to feel sharp fret ends. Fortunately, this can be simply fixed by filing them smooth. File them once in the dead of winter and chances are, you’ll never have to file them again!
How do you know your moisture content is right?
Feel it in your fingers, hear it in the sound, and stop worrying about the perfect RH – You don’t actually need to know that, a hygrometer is just a guide, your feel is the truth – it’s that simple!
In Summary
Wood and humidity are important.
Many musical instruments are made from wood. Wood is a naturally hygroscopic material, meaning it has the capacity to absorb water. It does so by equilibrating with the relative humidity of the local environment. Increased humidity causes wood to absorb, raising its moisture, causing it to swell.
Similarly, decreased humidity causes wood to desorb, lowering its moisture content, causing it to contract. This can result in many adverse effects, such as bowing, warping, cupping and splitting of the wood. For musical instruments, in addition to structural issues, changes in moisture content can also affect playability as well as tone.
The moisture equilibration process for wood is slow, taking days to weeks, hence short-term changes in humidity have little effect. However, the effect of long-term, seasonal changes can be profound. For musical instruments, ideally the moisture content of the wood should be maintained at around 8%, equating to a humidity of around 50%. Hence, when the ambient humidity drops below that level for an extended period, humidity in proximity to the instrument should be supplemented.
Typically, this would be achieved through the use of a humidifier.
For a musical instrument, ideally a humidifier would have the following features: It would be passive, meaning that there are no batteries, fans, heaters, etc.; it would have a high capacity for water allowing it to supplement humidity for multiple days between refills; it would be easily refillable; it would sequester liquid water in a wholly absorbed form, only releasing it through evaporation; it would have a large surface area that has a high porosity to water, hence allowing water to evaporate efficiently; it would not need to be attached to the instrument; it would have an outer shell that is soft and non-marring; it would have an outer shell that prevents direct contact of the absorbent with the instrument; it would be simple to manufacture in a variety of sizes and colors to suit a variety of instruments; it would be simple to use; and it would have a built-in, unmistakable indication of its state of hydration.
That’s why we developed the Prolix Music Acoustic guitar humidifier range – they’re simply the best guitar humidifiers money can buy. MADE in the USA, and backed by our warranties.
Having the right moisture levels in your tone wood means you can feel it in your fingers, and if you’re a fan you might even feel it your toes.
Only humidify when your guitar needs it. Not all the time. Too much moisture can be equally damaging.
Only humidify when you need to
Drop in the humidifier when the action gets too low, and remove it when that sweet action returns.
If you keep your guitar in a case, monitor the humidity inside the sound hole. When it drops below 45% RH you will know it is time to humidify, and when it exceeds 55% RH you will know it’s time to stop.
Finally, don’t be fooled by the outdoor humidity. Relative Humidity is temperature dependent. As such, heated indoor air is often much drier than the air outside.
Feel the Action
In severe cases you can see a dry or swollen guitar just by looking at the profile. But long before you see it, you will feel it when you play.
Too much Moisture Content will cause the bridge to lift, resulting in high action making it much harder to play, while too little, will cause the bridge to sink, lowering the action and causing fret buzz.
The sound of the guitar is also another little tell-tale sign, albeit less obvious. A guitar with the right moisture content will have a fuller sound than one that is overly dry.
