The Ukulele Humidifier Pro is D'Addario's first ukulele humidifier designed to fit in the Soundhole of the instrument. Based on 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.