Zen and the art of Guitar humidity. The feel for sweet action – instead of using a hygrometer.

Well OK it’s not exactly Zen, but we think it’s close enough.

Your guitar is your hygrometer. Read on and we’ll show you how feeling your inner hygrometer can guide you to humidity perfection… Note from the Author – this is a guide to help you feel when your guitar is experiencing fractionally larger or reduced moisture content in the wood, so that you won’t need a hygrometer to measure RH in the air, you’ll be able to sense how well humidified your guitar is from its action before anything bad happens to your instrument, anyway enjoy the read…

Throw away your hygrometer because, believe it or not, your guitar has a built-in moisture content meter… you can feel it in your fingers!
As musicians, we’re often told that we need to maintain the humidity of our instruments. The nominally suggested value for this is between 40% and 60%, but in fact it’s not the humidity we should be concerned with, it’s actually the moisture content of the wood.

So, why specify humidity values if we are really interested in moisture content?
Well, for two reasons: (1) Hygrometers are cheap whereas non-invasive moisture content meters are expensive; and (2) humidity and moisture content are loosely related.

The wood of your guitar can have the ideal Moisture Content, without having to pick up a hygrometer. We’re going to show you how to feel it in your fingers, hear it in the sound, and stop worrying about ideal Relative Humidity.

But first we need to make sure you know why, which means we have to do the science bit. We’re sorry, but we’ve tried to keep it as brief as possible, without being patronizing.

Contact [email protected] if you want to know even more.

#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.

  • Hygrometer-RH

#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.

  • Hygrometer-MC

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.

  • Hygrometer-MC-RH

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.

So, 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!

So 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, you’re feel is the truth – it’s that simple!

What are the take aways?

  • 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.
  • Long before you see it, you will feel it when you play.
  • Too much moisture will cause the top to bow, raising the action making Barre Chords harder to play.
  • Whereas losing moisture will cause the top to sag, lowering the action – producing dreaded fret buzz.
  • 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. In turn it can even result in cracks appearing in the sound board and/or the back of the guitar. See article why you need to humidify.
  • Hygrometers measure humidity in the air not in the wood.
  • High RH causes wood to absorb, raising its moisture, causing it to swell.
  • Low RH causes wood to desorb, lowering its moisture content, causing it to contract.
  • Instruments are made with cross grain support struts that cause bowing and contraction due to the moisture content of the wood.
  • RH can change rapidly.
  • MC is slow.
  • 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 constant RH of around 50%.
  • When the ambient RH drops below that level for an extended period, RH in proximity to the instrument should be supplemented.
  • Typically, this would be achieved through the use of a passive humidifier.

For a wood based musical instrument, a humidifier would have the following features – 

  • Passive, meaning that there are no batteries, fans, heaters, etc.
  • High capacity for water allowing it to supplement humidity for multiple days between refills.
  • Easily refillable with tap water.
  • Sequester liquid water in a wholly absorbed form, only releasing it through evaporation.
  • Large surface area that has a high porosity to water, hence allowing water to evaporate efficiently.
  • Not attached to the instrument, stopping you play.
  • Non-marring outer shell that is soft and prevents direct contact of the absorbent with the instrument.
  • Use High-Tech materials that allow a variety of sizes and colors to suit a variety of instruments.
  • Simple to use with 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.

That’s why we developed the Prolix Music PET-1 Acoustic Guitar Humidifier – it’s simply the best guitar humidifier money can buy. Better still, it’s born in the USA, and backed by a lifetime guarantee.

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