Measuring a bullet speed android app by using the microphone

Question

I get stumbled at the part when i need to measure the time between the first and second "bang" (first sound being the bullet fired and the second the sound made by hiting the target). Because after i get the time took the bullet to reach the target i can calculate the speed by the well known formula : speed = distance/time. Please, some help ! Thanks !

This is the part of code where i lost all hope: 

    private long startT, endT, diffT;
    
    public void updateTv() {
    int dB = (int) soundDb(0.1);
    mStatusView.setText(Integer.toString((dB)) + " dB");
    
    //double elapsedTime = mTimer.getElapsedTime();
    //Log.d("soundrecorder", "Elapsed time is : " + elapsedTime);
    
    try {
    if(dB > 100 ) {
    
    startT = System.currentTimeMillis();
    //Thread.sleep(1);
    }
    
    if(dB > 100) {
    
    endT = System.currentTimeMillis();
    //Thread.sleep(1);
    
    }
    
    diffT = endT - startT;
    
    } catch (Exception e) {
    System.err.println("error ..");
    }
    
    Log.d("soundrecorder", "Difference is : " + diffT);
    //diffT = 0;
    
    }
    
    public double getAmplitude() {
    if (mRecorder != null) {
    return (mRecorder.getMaxAmplitude());
    } else return 0;
    }
    
    public double soundDb(double ampl) {
    return 20 * Math.log10(getAmplitude() / ampl);
    }
    
    public void threadStart() {
    if (runner == null) {
    runner = new Thread() {
    public void run() {
    while (runner != null) {
    try {
    Thread.sleep(300);
    } catch (InterruptedException e) {
    System.out.println(e);
    }
    mHandler.post(updater);
    }
    }
    };
    runner.start();
    Log.d("soundrecorder", "start runner()");
    } else {
    if (!interrupted()) {
    runner.interrupt();
    }
    }
    }