I'd also be suspicious of this. You can also get a lot of beating artifacts from having many amplifiers in close proximity. If the speakers were still larger "m

I'd also be suspicious of this. You can also get a lot of beating artifacts from having many amplifiers in close proximity. If the speakers were still larger "mid-range" horns as were seen in decades past, this also might be less of an issue (low efficiency above 10kHz), but for cost/aesthetic reasons the speakers have shrunk quite a bit so they no longer mechanically filter much. If the problem was something everyone could hear it would likely be fixed, but since it's a minority, it's likely to persist. One other possibility for active US is crowd tracking. I've seen solutions that monitor # people and walking speed at doorways using ultrasonics. A lot of "one way" doors also use simple US detectors.