SINE Isochronic Entrainer is a Free and Open Source Brainwave Entrainment application for PC and Android devices.
Brainwave Entrainment is the practice of using sounds, visual stimuli or even electromagnetic fields to alter the frequency of brainwaves.
It can be done for several reasons: people with insomnia can take great benefit from it, but it can also be used to improve your attention span, and even simulate the effects of some recreational drugs.
The effect is not permanent, and fades away in less than a minute after the stimuli is removed.
Isochronic tones are a commonly used aural stimuli for Brainwave Entrainment, consisting of short pulses of a sine wave, varying in frequency.
Unlike binaural beats, Isochronic Tones can be played on speakers.
This app comes with lots of free presets.
Source code: https://github.com/adolfintel/sine-android