title: Return value of librosa.effect.Split is strange
body: <p>As titled, the result of this function is not logical and I don't understand what the function is doing.</p>
  
  <p>For example, here is some reproducible code:</p>
  
  <pre><code>#load sample audio
  filename = librosa.util.example_audio_file()
  audio, sr = librosa.load(filename)
  
  #get intervals which are non-silent
  inter_20 = librosa.effects.split(audio, top_db=20)
  inter_5 = librosa.effects.split(audio, top_db=5)
  
  #create audio
  above_20 = np.zeros(audio.shape)
  above_5 = np.zeros(audio.shape)
  
  for i in inter_20:
      start,end = i
      above_20[start:end]=audio[start:end]
  
  for j in inter_5:
      start,end = j
      above_5[start:end]=audio[start:end]
  
  #plot them out:
  plt.figure(figsize=[15,3]) #figure 1
  plt.plot(audio)
  plt.plot(above_5,color='red')
  plt.title('Audio above 5 dB')
  
  plt.figure(figsize=[15,3]) #figure 2
  plt.plot(audio)
  plt.plot(above_20,color='red')
  plt.title('Audio above 20 dB')
  </code></pre>
  
  <p>you can see from here:
  for figure 1, which is audio above 5dB:</p>
  
  <p><img src="https://i.stack.imgur.com/cfqc7.png" alt="audio above 5db"></p>
  
  <p>for figure 2, which is audio above 20dB:</p>
  
  <p><img src="https://i.stack.imgur.com/2VHUl.png" alt="audio above 20db"></p>
  
  <p><strong>How can it be that audio above 20dB is more than audio above 5dB?</strong>
  To me this doesn't make sense.</p>