Guitar less resonant than when new; humidity problem?

Question

Several months ago I purchased a folk-size acoustic guitar. It's not top-of-the-line but it had quite a nice sound up until a few weeks ago.

Previously, it had a rich, resonant tone and was capable of substantial volume when played with a pick. At the present, there's noticeably less "depth" to its sound, particularly for the fifth and sixth strings. Higher notes on the sixth string sound particularly lifeless (fewer overtones?), although all strings sound worse (to varying degrees) than they did a month ago.

Could this change be attributable to low humidity? In the last month, it's been fairly dry in my home; I've also been transporting my guitar outside (in a case) in sub-zero temperatures at least once a week for the past three weeks. If low humidity is the cause, would using a guitar humidifier restore the instrument's original sound? I should note that there's no visible damage (e.g., cracks) on the guitar.

Answer 1

It's quite likely to be the strings themselves. They are sacrificial, in that they need changing to retain that brightness. Try a new set of strings, and wipe them down after each session, and you will hopefully get the 'zing' back. 'Several months' is as long as a lot of strings hold their brilliance for. Some players change each week, or less.

Answer 2

A lack of humidity can cause this. There is a test for dryness to do on a guitar. Run your hand down the neck, not touching the surface of the fret board but on the edge or side of the fret board. If you can feel the frets, your guitar is most likely needing to be humidified.

The purpose of this test is to determine if the frets extend past the width of the fret board. Often when they do, they will feel a little sharp. This test works because when the wood loses water, it shrinks. The metal may change size with temperature, but the metal change is negligible in this case.

@Tim’s answer is most likely the correct answer, btw. This is merely also possible.

It is simple to humidify a guitar. You can use a store bought device, or make one using a sponge, plastic 'ziplock' bag, and a hole puncher (Punch holes in the plastic bag. The plastic bag is to protect the guitar and to control the release of moisture. The holes are important though, because they let the moisture out. Place the sponge in the bag.). All such guitar humidifiers made for individual guitars have a sponge, or something like a sponge. Wet the sponge on a regular basis (typically once a day), squeezing out any excess water you can (you don’t want to actually wet your guitar you see). Enclose the device and your guitar in its case when not in use, or the device will do nothing (too much air). Watch out for store bought humidifiers with holes that are small and or few, they don’t always let out enough moisture.

It is unlikely to over humidify your guitar, but beware of mold. It is best not to humidify unless you know there is an issue.

If your guitar is dry, humidifying it usually improves the sound. Note: This test does not prove that the dryness caused your issue, it is only proof of dryness.