I came across more bird scare devices [0] lately and most are at ~19-23kHz at -20~30dB (personal measures, device ranges differ). I can hear them and it hurts. Fortunarely they are mainly only deployed near entrances and do not have a broad field of effect. Would be interesting to know how many people do not hear this and feel kinda weird/sick while standing at those places for a longer period of time.

[0] https://en.m.wikipedia.org/wiki/Bird_scarer#Ultrasonic_scare...