I bought Riddex plus a month ago, very expensive- for 42 euros in Carrefour, Barcelona, Spain Before that I tried Raid Max, Boric Acid and then, while still seeing dead roaches on the floor and nearly fainting from screaming, I plugged Riddex plus in on top of that, just in case. It does make an IRRITATING ultrasound noise EVERY 30 SECONDS. It is in fact it can be heard by humans, and is enough to make you crazy. I don't know if Riddex plus keeps roaches away from my place - I haven't seen any for more than 1 month, but this noise every 30 seconds doesn't make any sense: Whoever I speak on Skype, or telephone they also can hear it and ask me: What is that irritating noise....? So I don't get it, Why does it say that it is only irritating roaches and rats? That is not true, it is irritating me and my friends too, very much.