Jaby....
http://www.deteracat.co.uk/catwatch.htm
These devices are popping up all over. They are "inaudible" to humans.
Oh no they aren't....... because I was walking around one day wondering why I had a high pitched and pulsating sound in my head, and followed the sound to its origin in somebodies garden
I thought I was going mental until I pinpointed what I was hearing. So much for "inaudible".
This most certainly is an explanation for anybody that hears a barely audible but high pitched, pulsating and very, very annoying noise from time to time. We CAN hear these devices. Well I can anyway, so I'm assuming others can.