What to do about a nightly constant 8100 hz sound

Since last year, a neighbor has installed some sort of sound device that plays a constant headache inducing 8100hz sound. It doesn't violate the decibel limit but makes it absolutely impossible to do anything outside without making me come down with a migrance and feel ill. Due to this, I have not been able to open my windows on the side the neighbor is on or use my porch or screenhouse. It recently has started again and I feel like I am going to go insane. I prefer to be outside later at night so I cannot avoid the sound around my house.

Does anyone know what I can do other than living with wax ear plugs? Please help.