Chirpr is a radio-free acoustic-link simple messenger app for Android devices. Send messages to your friends nearby using sound instead of radio.
Android devices have a variety of audio hardware components and available sampling rates and formats. The important part here is making sure that the sound that travels through the air is identical (or close enough) regardless of the specifics of each device. You can see what chirp format is used on the app screen: 1 kHz to 5 kHz linear frequency sweep over 40 milliseconds.
If your device doesn’t allow you to send the message or to start the receiver then do let me know. If you can let me know what make and model of device you are using, I should be able to alter the software to work on your device.
This is still a work in progress app, and at the moment is more of a demonstrator to show simplistically how underwater modems send data. If you have any suggestions then get in touch: WhimsicalOtter@gmail.com