I did a lab testing based on the open source code on Github for UltraSound Data Transfer and Receiver project. By transmitting data from iOS device to desktop Chrome using Ultra Sound without WIFI network. It just works.
To test this, please open your Desktop browser and a mobile phone, either iPhone or Android to the below URL. Click button on mobile phone, then see response on Desktop browser.
It is really amazing. This is not new, but amazing, it works.
Currently I found it may affected by noisy environment, for example, fan. And it is only capable of transmitting small amount of data, and within short distance due to the nature of UltraSound. But it is enough to transfer PIN securely from one device to another without using networks, which avoids to be hacked by someone.
I found if I encode a six digit pin. Due to noisy environment, sometimes it may transfer six digit, sometimes lesser, but there is an important characteristics, that if transferred digit is six, the number ordered always correct. The orders always correct but some numbers in the middle may be missing due in transmission.