I have been using a Green shield one for over two years, the manufacturer says that if you have hamsters, gerbils, rabbits or other small rodents as pets do not get it, but it is safe with dogs, birds, cats and fish, I have cats and goldfish.
I got this one because I could hear running around in my loft and being a total chicken wanted an easy way out.
Here is their statement of what it does:
Green Shield creates an pulsating electromagnetic field in and around the building - it uses the electric cables hidden in your walls to spread the field further. This is good, because vermin like rats and mice will avoid human contact and hide themselves away in the walls, or the cavities under floors.
The problem for them is that they don't like the pulsing electromagnetic field. It bothers them a bit like the fingernails scraping on a blackboard bothers you.
So much that after a few hours, maybe a day or so, they start to move out.
They'll put up with the discomfort for a short time, if they have to - especially if they think there's food to be found somewhere. But they won't stay, and if they're newcomers they certainly won't hang around to build a nest.
With a Green Shield box or two plugged into your walls, the building becomes a place that rats and mice don't like to be in.
The result? They go elsewhere. And your problem is solved
Quite a lot of sonic pest thingys don't got into your cavity walls that's why I picked this one.
Since I have been using it I have only heard a noise once or twice and then I couldn't be sure where it was something with legs lol.
The product's life span is between 3 and 5 years, I'm well pleased as being creepy crawlie phobic!
piper