AudioRecord在启动后返回一些空数据

我用AudioRecord编写了一个用于从麦克风接收数据的标准代码。这里是我的代码：AudioRecord在启动后返回一些空数据

AudioReceiver() { 
    int minHardwareBufferSize = AudioRecord.getMinBufferSize(SAMPLE_RATE, 
      CHANNEL_CONFIG, AUDIO_FORMAT); 
    Log.d(TAG, "minHardwareBufferSize = " + minHardwareBufferSize); 
    int bufferSizeBytes = (minHardwareBufferSize > MIN_BUFFER_SIZE_BYTES) ? 
      minHardwareBufferSize : MIN_BUFFER_SIZE_BYTES; 
    bufferSizeShorts = bufferSizeBytes/2; 

    //резервируем буфер с запасом в 2 раза 
    audioRecorder = new AudioRecord(MediaRecorder.AudioSource.MIC, 
      SAMPLE_RATE, CHANNEL_CONFIG, AUDIO_FORMAT, bufferSizeBytes * 2); 

    testStack = new short[bufferSizeShorts * 4]; 
    Arrays.fill(testStack, (short) 2000); 
} 


boolean startReceive() { 
    audioRecorder.startRecording(); 
    isReceiving = true; 
    int recordingState = audioRecorder.getRecordingState(); 
    Log.d(TAG, "recordingState = " + recordingState); 
    new Thread(receivingRunnable).start(); 
    return (recordingState == AudioRecord.RECORDSTATE_RECORDING); 
} 


boolean stopReceive() { 
    isReceiving = false; 
    audioRecorder.stop(); 
    int recordingState = audioRecorder.getRecordingState(); 
    Log.d(TAG, "recordingState = " + recordingState); 
    return (recordingState == AudioRecord.RECORDSTATE_STOPPED); 
} 


private Runnable receivingRunnable = new Runnable() { 
    @Override 
    public void run() { 
     int readCount = 0; 
     short[] dataBuffer = new short[bufferSizeShorts]; 
     while (isReceiving) { 
      testBusy = true; 
      for (int j = 0; j < 4; j++) { 
       readCount = audioRecorder.read(dataBuffer, 0, dataBuffer.length); 
       Log.d(TAG, "receive " + readCount + " bytes"); 
       System.arraycopy(dataBuffer, 0, testStack, bufferSizeShorts * j, readCount); 
      } 
      isReceiving = false; 
      testBusy = false; 
     } 
    } 
};

但我注意到，startReceive功能的第一次启动后，在testStack缓冲区的开始，还有空的数据（约1000个样本上的Nexus 4，看资料图）。