AVAudioPlayer - Metering - Want to build a waveform (graph)

I need to build a visual graph that represents voice levels (dB) in a recorded file. I tried to do it this way:

NSError *error = nil;
AVAudioPlayer *meterPlayer = [[AVAudioPlayer alloc]initWithContentsOfURL:[NSURL fileURLWithPath:self.recording.fileName] error:&error];

if (error) {
    _lcl_logger(lcl_cEditRecording, lcl_vError, @"Cannot initialize AVAudioPlayer with file %@ due to: %@ (%@)", self.recording.fileName, error, error.userInfo);
} else {
    [meterPlayer prepareToPlay];
    meterPlayer.meteringEnabled = YES;
    
    for (NSTimeInterval i = 0; i &lt;= meterPlayer.duration; ++i) {
        meterPlayer.currentTime = i;
        [meterPlayer updateMeters];
        float averagePower = [meterPlayer averagePowerForChannel:0];
        _lcl_logger(lcl_cEditRecording, lcl_vTrace, @"Second: %f, Level: %f dB", i, averagePower);
    }
}
[meterPlayer release];

It would be cool if it worked out however it didn't. I always get -160 dB. Any other ideas on how to implement that?

UPD: Here is what I got finally:

alt text http://img22.imageshack.us/img22/5778/waveform.png

Solution

Ok guys, seems I'm going to answer my own question again: http://www.supermegaultragroovy.com/blog/2009/10/06/drawing-waveforms/ No a lot of concretics, but at least you will know what Apple docs to read.