setAudioSource failed.

Question

java.lang.RuntimeException: setAudioSource failed.이라는 에러가 뜹니다

main소스코드는 

public class MainActivity extends AppCompatActivity {
    SoundMeter sb;
    TextView result;
    double decibel;
    Button start;
    String str;


    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);

        sb = new SoundMeter();

        result = (TextView)findViewById(R.id.check_deci);
        start = (Button)findViewById(R.id.button1);

        start.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View v) {

                sb.start();
                str = "현재 데시벨 : " + sb.getAmplitude();
                decibel = sb.getAmplitude();
                result.setText(str);
            }
        });

    }



    public class SoundMeter {
        static final private double EMA_FILTER = 0.6;

        private MediaRecorder mRecorder = null;
        private double mEMA = 0.0;

        public void start() {
            if (mRecorder == null) {
                mRecorder = new MediaRecorder();
                mRecorder.reset();
                mRecorder.setAudioSource(MediaRecorder.AudioSource.MIC);
                mRecorder.setOutputFormat(MediaRecorder.OutputFormat.THREE_GPP);
                mRecorder.setAudioEncoder(MediaRecorder.AudioEncoder.AMR_NB);
                mRecorder.setOutputFile("/dev/null");
                mRecorder.start();
                mEMA = 0.0;
            }
        }

        public void stop() {
            if (mRecorder != null) {
                mRecorder.stop();
                mRecorder.release();
                mRecorder = null;
            }
        }

        public double getAmplitude() {
            if (mRecorder != null)
                return  (mRecorder.getMaxAmplitude()/2700.0);
            else
                return 0;

        }

        public double getAmplitudeEMA() {
            double amp = getAmplitude();
            mEMA = EMA_FILTER * amp + (1.0 - EMA_FILTER) * mEMA;
            return mEMA;
        }
    }

}

이고

매니페스트에는 

<uses-permission android:name="android.permission.RECORD_AUDIO"></uses-permission>

추가가 되어있는 상황입니다.