Have you seen him setting anything up? Police have sonic weapons they use against demonstrators. I've seen pictures of them, but thankfully have never had one used on me, as far as I know.
Whatever it is, since directed audio frequencies or RF may be involved, there must be a meter which could pick it up and document whatever it is. What I would do
if you are 100% certain he is doing this is figure out some way to catch him in the act.
Ultrasonic sound and RF are two different things that would be measured by completely different kinds of equipment. I wouldn't invest a lot of money, I would only do that if a first attempt reveals that there is something being aimed at you.
If he is aiming something at you with the intent to do you harm, that must be a crime.
Ears ringing. This deserves serious looking into, because ears ring when blood brain barrier is disrupted, (at least thats my theory) and I suspect that means you're likely to experience permanent cumulative
hearing loss (if it is audio frequencies being directed against you for extended periods of time) or possibly worse if its powerful RF. (I think that's very very unlikely.) Hardware that's intended for crowd control if misused could likely cause permanent hearing damage. That's no joke.
If you can prove it's there.
Either way its not good. *Both audio and RF likely could cause a ringing in your ears* The ringing may be a secondary effect of ultrasonic sound, or some ototoxic, but non-auditory process, which is modulated by it but which also may be preventable by improving your glutathione status. (See below for more on that)
Did you try to settle the dispute with your neighbour, before building the ugly fence? If that didn't work, you should have left the fence as it was and set up some kill traps designed to catch foxes and coyotes.
I have no idea what it could be, perhaps some sort of microwave or ultrasonic weapon? If so, it's certainly illegal to use it in this manner, as you're not posing any threat to him.
Do ear protectors help? Other than that, there's not much you can do, other than contact the authorities, in the hope they can do something about it. I doubt you can legally do anything to retaliate.
My gut feeling is that the authorities will just write you off as a nut unless they have some reason to believe you are credible. At the end of the day, among other things, you'd get credibility by hiring an environmental lab. But that will be expensive unless you know what you're looking for.
Do you know any local police on a personal level? A good operative theory might be that your neighbor has somehow gotten a hold of one of these crowd control devices normally sold to cops, and is aiming it at you. You have every right to put a fence around your property, unless its prohibited in some way by your local laws. Some laws have height limits on fences.
Maybe he is concerned about his pets?
If I were you I would consider CCTV also, and a DVR.
To measure ultrasonic sound you likely need a computer with an audio input whose sound card can sample above the standard 44-48 KHz. (92 KHz is fairly common) and you need a transducer with range that high.
A piezo transducer may be able to help pick up and document ultrasound.
A program called baudline does a good job of displaying the audio frequencies visually.
For RF you likely need a field strength meter or spectrum analyzer with range in whatever putative frequency he is using. Find out what frequencies these
crowd control "nonlethal" devices use.
I think the audio frequency device is much more likely.
They are almost certainly illegal to use by non law enforcement people unless they are literally in the middle of nowhere.
---
Which makes me want to ask the question, is there any way you can resolve this - even at this point?
Especially if you live in a rural area, its better to have friends for neighbors than enemies.
He sounds like a creepy person, but these days a lot of people are just stressed beyond belief by extraneous stuff.
The more slack we try to give one another (within limits) the better.
Keep also in mind that getting a credible official workup on the situation by a environmental lab that does forensic work could easily run you thousands of dollars. So make sure you have evidence (for yourself) that points in that direction before you hire somebody. Collect your first line evidence perhaps on video with a time date stamping, and perhaps a friend there, preferably somebody who has some heightened credibility in your community, who is unconnected to the situation who can act as a witness.
-----
Here is something I am personally curious about, that may be useful to know.
Just for future reference, the evidence is pretty strong that NAC (see below) will help delay the hearing loss that comes with aging fairly well. The improvement in GSH (redox) status is good for all sorts of things. This might also prevent the ear ringing you describe which I know well. All sorts of things cause that ear ringing in me. Ive found NAC helps reduce or eliminate it more often than not.
Often my ear ringing is caused by various household chemicals, VOCs, even some strong perfumes in lotions middle aged ladies seem to slather on. Big box stores often make me ill because of these low level smells. There is clearly a common mechanism involved which NAC helps fix.
What is NAC?
N-acetyl cysteine (NAC) - the most affordable amino acid precursor of glutathione in the body.
Its possible, maybe even probable, that taking NAC when this is happening makes the ringing get better a few minutes later for a few hours? (but then it would get worse again when it wears off) You can spoon around a level teaspoon into some water and drink it, to get it into your bloodstream where it needs to be to prevent whatever damage may be occurring. As we get older, its a very good thing to take.
(see
https://academic.oup.com/jn/article/134/3/489/4688681?login=true for a good overview of its function)
However if you have "silver" (mercury amalgam) fillings. (dental restorations) that's NOT a good idea because it will pull mercury out of them. If you have silver fillings, fill a bunch of 000 capsules with it and take that instead.