This app generates a pure sine wave tone at the frequency it is set at. You can hook it up to your subwoofer, car audio or home system and test the bass frequency response of the system. It won't sound like much if you listen to it on your phone, so use a mini plug or bluetooth and connect to your sub woofer to feel the bass. The app also includes some small samples of music designed to produce very low tones. Feel the bass all the way down to 20 Hz and below, if your system can handle it.