IOS 通过麦克风检测声音分贝

在iphone真机上测的数据始始终没有变化， 在模拟器上是可以的，折腾了半天，终于搜索找到解决方法，在代码中添加

 [[AVAudioSession sharedInstance]
         setCategory: AVAudioSessionCategoryPlayAndRecord error: nil nil];

1. 开发环境xocde5.1.1 ， IOS sdk7.1

2.  引入框架：AVFoundation.framework

3.  部分源码如下：

 #import "MicBlowViewController.h"

 @implementation MicBlowViewController

 - (void)viewDidLoad
 {
     [super viewDidLoad];

      /* 必须添加这句话，否则在模拟器可以，在真机上获取始终是0  */
     [[AVAudioSession sharedInstance]
         setCategory: AVAudioSessionCategoryPlayAndRecord error: nil nil];

     /* 不需要保存录音文件 */
     NSURL *url = [NSURL fileURLWithPath:@"/dev/null"];

     NSDictionary *settings = [NSDictionary dictionaryWithObjectsAndKeys:
                 [NSNumber numberWithFloat: 44100.0], AVSampleRateKey,
                 [NSNumber numberWithInt: kAudioFormatAppleLossless], AVFormatIDKey,
                 [NSNumber numberWithInt: 2], AVNumberOfChannelsKey,
                 [NSNumber numberWithInt: AVAudioQualityMax], AVEncoderAudioQualityKey,
                 nil nil];

     NSError *error;
     recorder = [[AVAudioRecorder alloc] initWithURL:url settings:settings error:&error];
     if (recorder)
     {
         [recorder prepareToRecord];
         recorder.meteringEnabled = YES;
         [recorder record];
         levelTimer = [NSTimer scheduledTimerWithTimeInterval: 1 target: self selector: @selector(levelTimerCallback:) userInfo: nil repeats: YES];
     }
     else
     {
         NSLog(@"%@", [error description]);
     }

 }


 /* 该方法确实会随环境音量变化而变化，但具体分贝值是否准确暂时没有研究 */
 - (void)levelTimerCallback:(NSTimer *)timer {
     [recorder updateMeters];

     float   level;                // The linear 0.0 .. 1.0 value we need.
     float   minDecibels = -80.0f; // Or use -60dB, which I measured in a silent room.
     float   decibels    = [recorder averagePowerForChannel:0];

     if (decibels < minDecibels)
     {
         level = 0.0f;
     }
     else if (decibels >= 0.0f)
     {
         level = 1.0f;
     }
     else
     {
         float   root            = 2.0f;
         float   minAmp          = powf(10.0f, 0.05f * minDecibels);
         float   inverseAmpRange = 1.0f / (1.0f - minAmp);
         float   amp             = powf(10.0f, 0.05f * decibels);
         float   adjAmp          = (amp - minAmp) * inverseAmpRange;

         level = powf(adjAmp, 1.0f / root);
     }

     /* level 范围[0 ~ 1], 转为[0 ~120] 之间 */
     dispatch_async(dispatch_get_main_queue(), ^{
         [_textLabel setText:[NSString stringWithFormat:@"%f", level*120]];
     });
 }


 - (void)dealloc {
     [levelTimer release];
     [recorder release];
     [_textLabel release];
     [_cLabel release];
     [super dealloc];
 }

 @end