I find the sonic devices are very effective and although I have taken other precautions have found no sign of rodents and last year we were besieged by voles( on a five year cycle!!) However I know the blighters are lurking as they are settling down in the outhouses where there are no deterrents.It is important with the sonics that the waves brush along the floor and that the device has a clear field.Any change of direction, such as an L shaped room will require an additional unit as they only work in a straight line and as long as the indicator indicates you know it is working.