如何在ios 7中获得麦克风音量？

NSURL *url = [NSURL fileURLWithPath:@"/dev/null"];

NSDictionary *setTings = [NSDictionary DictionaryWithObjectsAndKeys:
                          [NSnumber numberWithFloat: 44100.0],AVSampleRateKey,[NSnumber numberWithInt: kAudioFormatAppleLossless],AVFormatIDKey,[NSnumber numberWithInt: 0],AVnumberOfChAnnelsKey,[NSnumber numberWithInt: AVAudioQualitymax],AVEncoderAudioQualityKey,nil];

NSError *error;

recorder = [[AVAudioRecorder alloc] initWithURL:url setTings:setTings error:&error];

if (recorder) {
    //recorder.delegate = self;
    [recorder prepareToRecord];
    recorder.meteringEnabled = TRUE;
    [recorder record];
    levelTimer = [NSTimer scheduledTimerWithTimeInterval: 0.1 target: self SELEctor: @SELEctor(levelTimerCallBACk:) userInfo: nil repeats: YES];
} else {
    NSLog(@"");// mic error message
}

- (void)levelTimerCallBACk:(NSTimer *)timer {
    [recorder updateMeters];

    const double ALPHA = 0.05;
    double peakPowerForChAnnel = pow(10,(ALPHA * [recorder averagePowerForChAnnel:0]));
    db = 20* log10(peakPowerForChAnnel);
    db += taraturaDb;
    db = db < 0 ? 0 : db;
    lowPassResults = ALPHA * peakPowerForChAnnel + (1.0 - ALPHA) * lowPassResults;

    if (lowPassResults < 0.95) {
        NSLog(@"%f",db);
}

这在iOS6中有效,在iOS 7中不起作用

谢谢