Featuring four waveforms and dynamic frequency and volume control,
Modulo is a real-time audio tone generator. Enter a frequency or
simply drag the bar to adjust the pitch of the signal. Featuring
sine, sawtooth, triangle, and square waves, Modulo also functions
as a single oscillator synthesizer.

Modulo is available on the Google Play Store for Android 4.0 (ICS) and up:
https://play.google.com/store/apps/details?id=com.modulo