[mothership]

Does anybody have any knowledge of sonic deterrents please?

[New shoot]

If you have fox problems I'm afraid you can't rely on a sonic deterrent as not all animals react to them 

I tried one to get cats out of my garden and some stopped coming and others not.

[alancane]

I tried one to get rid of moles in my garden and it didn't work at all, but that was just my personal experience and others might have found it works.