I don't think people are using true ultrasound, e.g. 40kHz. In most cases, it's more like "poor man's ultrasound" at 19 kHz, Just outside the hearing range of m

I don't think people are using true ultrasound, e.g. 40kHz. In most cases, it's more like "poor man's ultrasound" at 19 kHz, Just outside the hearing range of most people. They have to stay as close as possible to the human hearing range to avoid audio low-pass filters and simple capability envelope of the speakers and microphones.