Every device has a built-in Android eQualizer. Volume Booster Pro simply adjusts the frequencies to achieve louder effect. We use the loudness module which runs on devices with Android 4.4, you simply need to tap the Boost button.
Use the demo song to spot the difference and adjust the desired level of the effect.
Older version should use the slider only.
For maximum results use external speakers or headsets.
Ringtone: https://goo.gl/U3re2h

Listening music too loud is bad for your ears.