the author of this app says:
Test your hearing! Find out the real limits of your phone audio! Annoy your friends (at your own risk of course)! Learn and teach about how different tones sound like! Generate exact references tones! All this and more in this app.
-can cover a frequency range from 10Hz to 20kHz (actual range dependent on your audio hardware),
-multiple wave types! Choose from sine, square, sawtooth and triangle,
-exact tone generation, no messy approximation,
-setting the frequency down to a single Hertz (and not 10 Hz, like in some other applications),
-connect external speakers to your device for even better sound and frequency range!
Note: you can set the volume simply by using the buttons on your phone.
Internet permissions are needed for:
-ads (this is an ad-supported ad),
-feedback (you can report a bug or suggest a feature directly from the app),
-ANONYMOUS usage statistics.
Feature enhancements coming this January!