I have placed one in the kitchen between the stove and the sink cabinet. The same day, a mouse crossed the room while I never saw them. Since then I can not hear trotter or nibble ... .So suddenly, I bought a I put in the hall closet (because if I understand the written instructions in English (!), ultrasound does not penetrate walls). I must check (droppings) if there is any more mice but a priori I hear nothing. Of course, I mean gallop from time to time in the partitions and the ceiling (although much less than before anyway) but at least they are no longer in pieces around on the dishes or snack packages of food !! I find this very well and with a good value. I read the other mixed reviews and I told myself that I would try and then finally, to me it seems to work.