什么是“ iPhone音频工具”框中的音频电平表？

audioPlayer.meteringEnabled  = TRUE;
[audioPlayer prepareToPlay];
[audioPlayer play];
[NSTimer scheduledTimerWithTimeInterval:1.0 
      target:self 
      selector:@selector(getGain:) 
      userInfo:nil 
      repeats:YES];

-(void)getGain:(NSTimer*)sender{
   [audioPlayer updateMeters];
   float avgPower = [audioPlayer averagePowerForChannel:0];
   NSLog(@\"avgPower : %f\",avgPower);
   float peakPower = [audioPlayer peakPowerForChannel:0];
   NSLog(@\"peakPower : %f\",peakPower);
}