Humidity

The amount of water vapour in the air, which decides how much moisture an instrument's wood holds and therefore how much it moves.

Wood is hygroscopic, so it swells in damp conditions and shrinks when the air is dry, changing the neck's relief, the action and, in extremes, the shape of the top. Dry conditions bring fret sprout, sharp fret ends, sinking tops and cracks; very damp ones make a neck bow forward and raise the action. The usual practical guidance is a stable indoor environment and a soundhole humidifier for acoustics in winter heating.

Example: a guitar bought in a humid climate and moved to a heated flat in winter often needs a truss rod tweak and a humidifier within weeks.

Humidity

See also