I'm not sure why you'd prefer ultrasonic over bluetooth - the hardware already has bluetooth (the consumer Xiaomis link to an app for various statistics and con

I'm not sure why you'd prefer ultrasonic over bluetooth - the hardware already has bluetooth (the consumer Xiaomis link to an app for various statistics and control functions), you can explicitly target the signal (it's typical for a bunch of these to be sitting near each other), you don't need to mess with the user's volume settings and you get bidirectional communication.