These are the only humidifiers that work on my Harp. They're so easy to install and work great. I buy 3 of them to put in each sound hole. What I love about these is that they're clip on and the humidifier itself has a magnet to stick onto the clip. The only downside is that because the Clip is a metal material it May scratch the wood. I usually just wrap it with a cloth so it won't damage the wood.