Planet Waves GH-RS Acoustic Guitar Humidifier Replacement Sponges, 3 Pack are suitable for use with the Planet Waves Acoustic Guitar Humidifier. Designed for use with the Planet Waves Acoustic Guitar Humidifier, simply moisten a sponge, install and that's it. The humidifier releases the moisture slowly and evenly keeping your guitar perfectly humidified. When the sponge becomes completely dry, moisten it again and insert back into the soundhole. Sold as a pack of three sponges.