I can hear the device in ultra and I'm in my 50s, its definitely not 23000hz. Other noises trigger the device, not just dog barking. Even on the lowest sensitivity.
The target dogs experience a lot of distress when the sound is emitted. They don't bark but they a really unhappy. It feels cruel.
So it works to stop the barking, but its annoying to hear and is distressing to the target animals and sometimes goes off when they aren't barking.