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. This is why you need to humidify!
Laminates are nearly impossible to crack since the laminate seals the wood and makes it super stable. But you don’t get the rich sounds of a natural tone wood guitar.
Lack of humidity may also have other implications, affecting playability, as well as tone.
About your tonewood 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, which is why you need to humidify!
Fig. 1 – How moisture content affects the bowing or sinking on your acoustic guitar.
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, which is why we wrote a completely different article Zen and the art of guitar humidity about it here…, 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 (makes us shiver down the spine) 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…
And here’s what happened when put the PET-1 inside this guitar after just 10 days. The amazing healing power of the PET-1 brought the wood back together for an easy glue and cleat repair.
So if you ever get a crack – Just remember you can get back!
A luthier is familiar with the physical properties of wood. As such, 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. If wood was isotropic (in that its properties were the same in every direction), you wouldn’t see hocky stick shaped 2 x 4’s at the local big box hardware store.
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, which is why you need to humidify, humidify, humidify.
This film from HVAC school gives some basic information on humidity…
That said, be warned against over humidification. Avoid over humidifying, all we ask is you just maintain the guitar at a reasonable humidity. And if you do have a cracked or overly dry guitar, use a humidifier. Ideally you want your guitar to live 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 at 50%.
If you live in a region where humidity is consistently 50% all year round… then you have no worries, but for those of us that live in areas with extreme dryness, or in areas with seasonal changes, we really need to be aware of the affects of humidity on our instruments.
If you want to learn more about Humidity and Moisture Content… Just click here
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.
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.