"I have 2 of them, and they both work very well. I have one in my main cello case, and one in a cupboard where I keep my teaching cello. The cupboard is tightly closed and takes the place of a case. I stuck the sensor to the inside of the cupboard. I monitor both cellos with the app on my iPhone. I'd like to put the app also on my windows computer but can't get that. That is the only downfall, but hopefully, Planet Waves will fix that app availability. (Please!?)"