Dial-up internet involves converting bits into sound (both in audible and ultrasonic spectrums) and then back on the other end of a phone line.<p>The Google Tone extension (<a href="http:&#x2F;&#x2F;googleresearch.blogspot.com&#x2F;2015&#x2F;05&#x2F;tone-experimental-chrome-extension-for.html" rel="nofollow">http:&#x2F;&#x2F;googleresearch.blogspot.com&#x2F;2015&#x2F;05&#x2F;tone-experimental...</a>) uses both ultrasonic sound and an audible DTMF-based codec.<p>Audio tones are also used for setting up Google OnHub routers; the router emits a tone and the Google On setup app listens for it. (<a href="https:&#x2F;&#x2F;support.google.com&#x2F;onhub&#x2F;answer&#x2F;6246481?hl=en" rel="nofollow">https:&#x2F;&#x2F;support.google.com&#x2F;onhub&#x2F;answer&#x2F;6246481?hl=en</a>)