Vibrate at loud sounds android applicaion

I'm new to android application development and am trying to develop an application on android studio which provides deaf people with the ability to recognise loud sounds above a specified threshold. the application should listen to the environment in real time and vibrate if the sound is higher than a specified threshold.

I have used the AudioRecord class to record surrounding sound into the buffer at a 16BIT PCM, then read the data into an array(Byte[]). I'm not sure how to convert the 16BIT, 2 Byte, sample into a decimal value so i can compare it to the threshold. I am also getting a runtime error and have searched the internet for an answer but couldn't find any.

This is my MainActivity:

    import android.content.Context;
import android.media.AudioFormat;
import android.media.AudioRecord;
import android.media.MediaRecorder;
import android.os.Vibrator;
import android.support.v7.app.ActionBarActivity;
import android.os.Bundle;
import android.view.Menu;
import android.view.MenuItem;
import android.view.View;
import android.widget.Button;
import android.widget.Toast;

import java.io.IOException;


public class MainActivity extends ActionBarActivity {

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
        Button vib = (Button) findViewById(R.id.button);
        LoudSoundDetector action = new LoudSoundDetector();
        vib.setOnClickListener(action);
    }


    @Override
    public boolean onCreateOptionsMenu(Menu menu) {
        // Inflate the menu; this adds items to the action bar if it is present.
        getMenuInflater().inflate(R.menu.menu_main, menu);
        return true;
    }

    @Override
    public boolean onOptionsItemSelected(MenuItem item) {
        // Handle action bar item clicks here. The action bar will
        // automatically handle clicks on the Home/Up button, so long
        // as you specify a parent activity in AndroidManifest.xml.
        int id = item.getItemId();

        //noinspection SimplifiableIfStatement
        if (id == R.id.action_settings) {
            return true;
        }

        return super.onOptionsItemSelected(item);
    }


    public class MyVibrator {

        public Vibrator vibrator;

        public void vibrate1() {
            Toast.makeText(getBaseContext(), "Vibration test", Toast.LENGTH_LONG).show();
            long pattern[] = {0,100,200,300,400};
            vibrator = (Vibrator) getSystemService(Context.VIBRATOR_SERVICE);
            vibrator.vibrate(pattern,0);
        }
    }


    public class LoudSoundDetector implements View.OnClickListener{

        private int amplitudeThreshold = 65;

        int checkThreshold(){
            return amplitudeThreshold;
        }

        @Override
        public void onClick(View v) {
            // TODO Auto-generated method stub
            Listener lsn = new Listener();
            try {
                lsn.listen();
            } catch (IOException e) {
                // TODO Auto-generated catch block
                e.printStackTrace();
            }
        }


    }

    public class Listener{
        int audioSource = MediaRecorder.AudioSource.MIC;
        int samplingRate = 44100; /* in Hz*/
        @SuppressWarnings("deprecation")
        int channelConfig = AudioFormat.CHANNEL_CONFIGURATION_MONO;
        int audioFormat = AudioFormat.ENCODING_PCM_16BIT;
        int bufferSize = AudioRecord.getMinBufferSize(samplingRate, channelConfig, audioFormat);
        boolean isRecording = false;


        public void listen() throws IOException{

            AudioRecord recorder = new AudioRecord(audioSource, samplingRate, channelConfig, audioFormat, bufferSize);
            recorder.startRecording();
            isRecording = true;


            //capture data and compare threshold
            int readBytes=0;
            do{
                byte[] data = null;
                readBytes = recorder.read(data, 0, bufferSize);

                if(AudioRecord.ERROR_INVALID_OPERATION != readBytes){

                    int sampleVal = 0;
                    for(int i=0;i<data.length; i+=2)
                        sampleVal = data[i] + data[i+1];
                    float db = (float) (20 * Math.log10(Math.abs(sampleVal)/32768));
                    LoudSoundDetector lsd = new LoudSoundDetector();

                    if(db>=lsd.checkThreshold()){

                        MyVibrator mVib = new MyVibrator();
                        mVib.vibrate1();
                    }

                }


            }while(isRecording);


        }


    }
}

and the log cat:

12-28 14:23:51.192  12962-12962/com.rayan.graduationtest E/AndroidRuntime﹕ FATAL EXCEPTION: main
java.lang.NullPointerException
        at com.rayan.graduationtest.MainActivity$Listener.listen(MainActivity.java:122)
        at com.rayan.graduationtest.MainActivity$LoudSoundDetector.onClick(MainActivity.java:80)
        at android.view.View.performClick(View.java:4432)
        at android.view.View$PerformClick.run(View.java:18339)
        at android.os.Handler.handleCallback(Handler.java:725)
        at android.os.Handler.dispatchMessage(Handler.java:92)
        at android.os.Looper.loop(Looper.java:137)
        at android.app.ActivityThread.main(ActivityThread.java:5283)
        at java.lang.reflect.Method.invokeNative(Native Method)
        at java.lang.reflect.Method.invoke(Method.java:511)
        at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:1102)
        at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:869)
        at dalvik.system.NativeStart.main(Native Method)