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เมื่อมันเป็นปัญหาของฝรั่งเมืองหนาว ผมเลยไปขอให้ตา Bob Taylor แกมาอธิบายถึงสาเหตุและวิธีแก้ไขให้น้าเว้บอ่านเองดีกว่า

SYMPTOMS OF A DRY GUITAR

"Solving The Mystery of The Sunken Top"

No environmental condition can do more damage to your instrument
than low relative humidity (RH). Many areas of the US experience
naturally low humidity all year long. The entire Rocky Mountain
region, for example, has notoriously low RH levels, as do Phoenix, Reno, Las Vegas, Tucson, and a number of other cities. In most regions of the country, however, low RH levels accompany the
onslaught of winter, mostly due to the effects of artificially heating our homes and buildings. If you?ve read the various references to humidity-related problems published in our quarterly newsletter, Wood&Steel,
you know that the colder months wreak havoc on acoustic guitars made of solid wood. As the weather turns wet and temperatures free fall, there is a natural tendency to crank up indoor heating units. Unfortunately, what many don?t realize is that heating a room forces the relative humidity down to a level that poses a real threat to quality acoustic
guitars by literally drying them out. The following information deals with
the adverse effects that ?drying? has on guitars, the characteristic indicators of severe drying, and the steps one can take to counteract those effects. Occasionally, we get a call from a customer who is convinced that his guitar has a bad neck. He will cite specific
problems, which frequently include a huge hump at the 14th fret and strings that are extremely close to the frets. The strings ?buzz? when played in the high registers, but it would seem that raising the saddle enough to rectify the problem would result in an uncommonly
high saddle.

In such cases, we immediately recognize that the guitar has ?dried out,?
thus causing a number of related problems: The arch in the top sinks, taking the bridge with it; the neck bows forward, necessitating a truss rod adjustment; and the fingerboard shrinks in width, causing the
sharp ends of the frets to extend beyond the edge of the neck.

Combined, these events can make a guitar look as though it has a badly
made neck. Actually, the neck is fine, the problem is with the body, and that can be restored to good condition with a truss rod adjustment and the use of a soundhole humidifier (e.g. a ?Dampit?).

However, many customers still find it hard to believe that so simple a solution as humidifying the body will fix what appears
to be a grave problem with the neck. Let us convince you.
When we make a guitar, the wood first is dried, ?seasoned,? and acclimated to a certain moisture content. We follow the same, painstaking procedure on all of our guitars. As a result, all guitars leave
the factory in the same condition, and all will react more or less the same when exposed to changes in humidity. Our factory is climate-controlled to maintain a temperature of 75 degrees and a relative humidity of 47 percent. This consistency causes the wood to equalize at a specified moisture content. As the wood?s moisture content changes, so does the size of the wood. Spruce, in particular, shrinks and expands a tremendous amount as it gains and loses moisture. For example, let?s say we condition a spruce top in a room that is 47 percent RH (relative humidity), and then cut that spruce to a width of
16 inches. If we then were to lower the room?s RH to 30 percent, that same piece of spruce would shrink to 15.9 inches in width ? shrinkage of almost 1/8 of an inch! If, instead, we were to raise the room?s RH to 60 percent, the spruce would swell to 16.06 inches, an expansion of almost 1/16 of an inch.

Relative humidity is not an arbitrary matter. We prefer 47 percent RH because it is a very ?normal? or ?median? humidity. When built at that RH level, a guitar can be exposed to more or less humidity and still perform well. The more extreme the temperature and/or humidity fluctuations, the sooner the guitar will be adversely affected. The good news is, your guitar can be protected from many changes simply by storing it in its case. If it does become too dry, both its moisture content and its shape can be restored by exposing it to humidity. A soundhole humidifier accomplishes this very well.

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http://www.taylorguitars.com/guitars/fea.../humidity/

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