The external use of rubber,internal use of absorbent sponge,water absorption capacity is strong. Suitable for heating and air conditioning rooms in winter,with open floors,low air humidity,and dry conditions. The rubber is very soft and does not harm the musical instrument body.Suitable for use with 4/4 models. Cello humidifiers protect your cello from excessive drying and damage. The head is sealed to water leakage,and the bottom is designed to the humidifier from falling into the cello box.