Bass humidifiers can be inserted into bass instrument to assist with the regulation of moisture in the air (or within the instrument). As wood interacts with moisture constantly, if the air inside a double bass is too dry, it may suck out the moisture from the wood of the double bass altering the instrument's playing capabilities. The opposite may happen when too much moisture is present and the wood may absorb the excess moisture, causing swelling. When a bass is exposed to extreme variations for long periods of time, open seams or even cracking may result.
A good humidifier will protect the instrument in cases of both high and low moisture in the air and is a "must" especially in areas where humidity varies greatly.