问当使用加速度计时，当没有传感器值被改变时，为什么onSensorChanged()方法会被定期调用？

EN

public class AlarmActivity extends Activity implements SensorEventListener {
    ...
    private SensorManager sensorManager;
    private Sensor mAccelerometer;
    private AudioPlayer mPlayer = new AudioPlayer();

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_alarm);
        sensorManager = (SensorManager) getSystemService(Context.SENSOR_SERVICE);
        mAccelerometer = sensorManager.getDefaultSensor(Sensor.TYPE_ACCELEROMETER);
    }

    @Override
    public void onResume() {
        super.onResume();
        sensorManager.registerListener(this, mAccelerometer, SensorManager.SENSOR_DELAY_NORMAL);
    }

    @Override
    public void onPause() {
        super.onPause();
        mPlayer.stop();
        sensorManager.unregisterListener(this);
    }

    @Override
    public void onStop() {
        super.onStop();
        mPlayer.stop();
    }

    @Override
    public void onDestroy() {
        super.onDestroy();
        mPlayer.stop();
    }

    @Override
    public void onSensorChanged(SensorEvent event) {
        Log.d(TAG, "onSensor() called");

        class StartGuardingTask extends AsyncTask<Void, Void, Void> {
            @Override
            protected Void doInBackground(Void...params) {
                mPlayer.play(AlarmActivity.this);
                return null;
            }
        }

        if(mCurrentXVal != mOldXVal || mCurrentYVal != mOldYVal || mCurrentZVal != mOldZVal) {
            mOldXVal = mCurrentXVal;
            mOldYVal = mCurrentYVal;
            mOldZVal = mCurrentZVal;

            mXValue = (EditText)findViewById(R.id.editText1);
            mXValue.setText("x:" + mCurrentXVal);
            mYValue = (EditText)findViewById(R.id.editText2);
            mYValue.setText("y:" + mCurrentYVal);
            mZValue = (EditText)findViewById(R.id.editText3);
            mZValue.setText("z:" + mCurrentZVal);

            new StartGuardingTask().execute();
        }
    }
}

public class AudioPlayer {
    private MediaPlayer mPlayer;

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

    public void play(Context c) {
        if(mPlayer == null) {
            mPlayer = MediaPlayer.create(c, R.raw.police_siren);

            mPlayer.setOnCompletionListener(new MediaPlayer.OnCompletionListener() {
                public void onCompletion(MediaPlayer mp) {
                    stop();
                }
            });
            mPlayer.start();
        }
    }

    public MediaPlayer getPlayer() {
        return mPlayer;
    }
}