Android : Equalizer does not work

I am now using Visualizer to capture fft data of mediaplayer. However, I find that data captured changes a lot when volume of the device changes, and I want it independent of system volume. I searched for a while and people said the Equalizer should be used. But after I added Equalizer, it did not seem to work. My code is attached as below:

    mEqualizer = new Equalizer(0, mp.getAudioSessionId());
    mEqualizer.setEnabled(true);
    mVisualizer = new Visualizer(mp.getAudioSessionId());
    mVisualizer.setCaptureSize(Visualizer.getCaptureSizeRange()[1]);