Humidity is defined as the relative percentage of water moisture in the air, and variations running from low to high can have a detrimental effect on guitars, especially acoustic models. This is because most guitars are made of wood, which is sensitive to changes in relative humidity. It's useful to know the effects of variations in humidity on guitars so that you can take suitable steps to protect your instrument from the damage that might occur.
In high humidity environments the cured timber that guitars are made from can absorb moisture from the atmosphere. This moisture swells the empty cell structures of the cured timber, causing the wood to expand slightly. The effects can be most dramatic on acoustic guitars, where you may see a distinct arching of the top. This arching causes the bridge to rise, increasing the action of the strings and therefore affecting the playability of the neck. The movements associated with arching of the top can also affect the guitar's intonation.
At the opposite end of the spectrum, dry environments can cause the wooden bodies of guitars to shrink, which leads to potentially more serious long-term issues. As the wood dries it shrinks further, which opens up cracks in the guitar's construction; frets can come loose, joints can separate and in some cases the timber can split. These cracks close naturally as humidity returns to a normal level, but each time the wood shrinks and swells again a little residual shrinkage will remain. Over time this will weaken any affected joints and ultimately can render a guitar unplayable.
Humidity has an interesting effect on the sound of a guitar. As the wood dries, the body of a guitar becomes stiffer and more resonant. This gives the overall tone a brighter and louder texture. Conversely, as humidity rises and the wood expands and softens slightly the tone will have greater warmth.
Protective Construction Techniques
Guitar manufacturers take many steps during the construction process to minimize the long-term effects of changes in humidity. The construction of most guitars minimizes the amount of exposed end-grain on the timber, which is more prone to absorbing moisture; this choice of wood cut imparts a great degree of stability. The wax, varnish or lacquer finish on guitars also offers a significant degree of protection by acting as a physical barrier between the wood and the atmosphere. Additionally, manufacturing facilities benefit from maintaining a stable humidity level of 40 to 45 percent to help standardize the output.
Protection Against High Humidity
Musicians, especially those who tour regularly and play in a wide variety of venues, don't have the luxury of being able to control humidity. It is important therefore to take some sensible precautions to minimize, if not eradicate, the effects of humidity on your guitar. Be sure first of all to buy a digital humidity gauge; these are widely available in guitar stores or on the Internet. Wherever you are playing, check the humidity levels. If you record humidity of 70 percent or more, then leave your guitar in its case for as long as possible before taking it out to play. The short period it is out as you play won't have much of an effect, but leaving it out all day could lead to problems. It is worth investing in a good quality case with proper seals to help further protect your guitar.
Protection Against Low Humidity
Low humidity is the most serious hazard to the structure of your guitar. As previously noted, if you are playing in a very dry environment for a short period of time then this limited exposure will not have much of an effect on your guitar. If the humidity levels remain low, however, then even the air inside your case will begin to dry your guitar. In such circumstances you will need to purchase a humidifier. A humidifier releases small quantities of moisture slowly to maintain a more beneficial atmosphere. Fill the device with purified water in accordance with the manufacturer's instructions and place it inside the body of your guitar while it is in the case until the humidity levels have come up to 45 to 50 percent.
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