title: Noise while altering audio data
body: <p>I am playing audio with python and i don't understand why  i hear  noise on the ouptut when executing code like this: </p>
  
  <pre><code>import pyaudio  
  import wave 
  import numpy as np
  
  f = wave.open('blabla.wav',"r")  
  p = pyaudio.PyAudio()  
  # open stream  
  stream = p.open(format = p.get_format_from_width(f.getsampwidth()),  
                  channels = f.getnchannels(),  
                  rate = f.getframerate(),  
                  output = True)  
  float_array = np.fromstring(f.readframes(10000000), dtype=np.uint16).astype('float32')
  output = 0.9 *  float_array
  stream.write(output.astype('uint16').tostring())
  </code></pre>
  
  <p>When i multiply by <code>0.9</code> i expect weakening a signal a little bit.  But where from this ouput noise came from ? 
  I don't even add anything to initial data!</p>
  
  <p>Basically i want to add two signals : </p>
  
  <pre><code>   output signal =  0.5 * the origin one  + 0.5 * shifted origin one
  </code></pre>
  
  <p>But i get mess out of this process, because even a multiplying  an original array make the signal near entirely  sound like a mess. </p>
  
  <p>Can you point me out what am i doing wrong and how to make the formula </p>
  
  <pre><code>   output signal =  0.5 * the origin one  + 0.5 * shifted origin one
  </code></pre>
  
  <p>work right ? </p>