The low humidity is less harmful than sudden changes
in either humidity or temperature.
I wouldn't recommend a sound-hole humidifier, for two reasons. The moisture released into the interior of the guitar will try to migrate out through the wood. That can't be good fot the finish. Then when you take the humidifier out of the sound hole, the dry air in the room starts to draw moisture out of the wood cells at the surface, faster than it can be replaced from the cells further in. That can't be good either!
Its much better to provide a steady environment, and keep the guitar tuned and set up. An acoustic guitar will 'breathe'. Once it has stabilized in a given set of conditions, it will be perfectly happy until it has to adapt to a change. Obviously, ambient conditions that are too dry or too damp aren't good, but if you
are comfortable, your guitar
probably is, too.
An effective way to keep a room humidified is to set up a 10 or 15 gallon aquarium, with some tropical fish. This will not only keep the humidity in the room at a healthier level for your guitar - it looks a lot better than any humidfier on the market.
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