Sand pit for horses

[autumn7]

We're constructing a large sand pit for our horses within a large barn. Inevitably our two cats are already delighted with their new toilet facility - however, we are less so! Considering buying an ultrasonic cat repellent device. Found one online that may work with both cats and equines. Appears hearing range of a cat is 45 - 64 000Hz and hearing range of a horse is 55 - 33 500Hz. The device I've found emits a frequency of 40 000Hz so should be above that of a horse so in theory this should scare off cats but leave the horses oblivious, shouldn't it?
Anything obvious I haven't thought of here? Has anyone tried one of these cat repellents, not necessarily in a horse sand pit setting, and would it be worth a try do you think? Or any other bright ideas to stop the sand area becoming a gigantic cat litter tray? Thanks in advance.
This one:
https://www.bestpestcontrol.co.uk/u...el-repeller--deterrent---45ft-range-122-p.asp

 

[Flowerofthefen]

I regularly ride past a cat repellent device. It sounds horrible. The horses never react but their ears are certainly tuned to it.

 

[autumn7]

Oh dear! Can't recall ever hearing one. Thanks for commenting Fern007. I'd definitely need it to be out of the horses' frequency range but wondered if anyone had tried this before. Seemingly not if responses are to go by.