They map each letter (A-Z) to a single frequency. The range is around 18-19.5 kHz. As far as I know, this range is inaudible to most adults. I couldn't hear it

They map each letter (A-Z) to a single frequency. The range is around 18-19.5 kHz. As far as I know, this range is inaudible to most adults. I couldn't hear it for example. Of course, I may be using the term ultrasonic incorrectly. Some preliminary work was done last year to reverse SilverPush[1], but my group and I are planning to take it further. [1]: https://github.com/MAVProxyUser/SilverPushUnmasked https://github.com/MAVProxyUser/SilverPushUnmasked