The Ukulele Humidifier Pro is D’Addario’s first ukulele humidifier designed to fit in the soundhole of the instrument. Based on the design of the Guitar Humidifier Pro, this version contains an easy-to-remove rubber cap and a hydrophilic sponge in a much smaller size that is perfect for a ukulele. The hydrophilic sponge holds up to 12x its own weight in water, which means it will hydrate the instrument over a longer period of time.