Usually they're tolerable, just annoying.
The screenshot is from one I found installed outside Lidl (supermarket) now, above the entrance. Probably loud enough to "cover" the entire parking lot. Loud enough to get painful after a few seconds. Sweeping between 15.5kHz to 18.7kHz. For comparison, a CRT TV is around 15.7kHz.
I wonder if not caring about loudness of these higher frequencies contributes to hearing damage among people.
Lower frequency, but also awfully loud buzzers can be found in trains near doors, working as a warning when the doors are closing. Thankfully, this probably annoyed some of the staff enough for them to start taping these over.
But those are loud enough to sound distorted.
Anyway, I just hid behind a corner where I couldn't hear it that much while waiting for the manager to call us in.
I can hear them, too. The upper limit for what humans may be able to hear is around 20 kHz, so 15.5 kHz is well within range.
Of course, not everyone can hear them. Some folks just don't have the same range, but in particular also the older you get, the less flexible the eardrum becomes, which zaps that range, too.
And yes, they sound fucking awful and give me instant headache, which is perhaps unsurprising as that's literally how they deter other animals. I wish they were illegal or at least regulated to actually be out of hearing range.