https://www.theregister.co.uk/2018/05/25/sonic_firewall/ > Boffins in Austria have developed a defense against acoustic cookies, a form of ad tracking by which smartphones can send and receive data using sounds people can't hear. > App developers can implement ultrasound data transfer on their own or using various SDKs, such as XT Audio Beacons or Lisnr, or the Google Nearby API.