What happens when a guitar gets dried out?
Here's one example.
Aside from cracks like the one shown here, the critical geometry is affected. This makes any setup adjustments difficult. There are many different pieces of wood glued together inside a guitar, braces and parts with grain lines pointing in all different directions. When these components dry out they contract and pull in different directions. This creates tension throughout the structure of the body and it doesn't play or sound the same. The insides become stressed under abnormal tension due to the dryness.
Not all guitars or string instruments need the be humidified to prevent cracks or "visible" damage. However, when someone brings me a dried out guitar, even if they are happy enough with it, the effects are visible to me. Some people say, "shouldn't a dry piece of wood sound better than a wet piece of wood?" The simplest answer to this is frankly NO! Not when it's so dry that the wood is cracking and causing the guitar to change shape. Guitars sound best when kept stable. The sweet spot is 40% - 50% relative humidity. This allows the guitar to be at its most comfortable and resonant state.
I've also seen many older instrument that have been dry for a long time. Some say the guitar has been "acclimated." But rarely can a guitar actually be "acclimated," if at all in my opinion. Some of these older instruments don't show many signs of extreme trauma, but none are in perfectly great condition. Most have had crack repairs and other work done to remedy the effects of being turned into guitar-jerky.
Yes, too much humidity makes a guitar play and sound bad as well. However, dryness is more detrimental to the structural integrity of the instrument.
Try to make a habit of monitoring the daily weather. Specifically the RH humidity. If you want to have your guitar out, dedicate a room or area in your home to maintain stable humidity using a room humidifier and hygrometer.
We offer a few different humidifiers.. One of our favorites is the Music Nomad humidifier. There are many different ways to humidify your instrument. Schedule a consultation to discuss the best option for you and your instrument.
Here in Flagstaff, the humidity can go into single digits. This is absurdly low for humans and especially for delicate instruments made of wood. During certain parts of the year, the next day can bring monsoon rains and weather creating high humidity. These changes can be extremely rough on guitars. Keeping your instrument in a case will help insulate and stabilize it from these fluctuations.
For more information on humidity and how climate can effect guitars, check out the Instrument Wellness page.