ios怎么对声音信号进行放大

最近在用audioUnit录音和播放模式进行录音开发，但是由于获取的声音比较小，我想将声音放大一点在播放输出，求各位大神指点

参考下：
http://stackoverflow.com/questions/11573796/audio-unit-recording-increased-volume-of-buffers
http://stackoverflow.com/questions/27661574/how-to-increase-the-volume-of-ios-app-playing-generated-pcm-audio

http://stackoverflow.com/questions/17044084/how-to-increase-volume-of-a-sound-and-remove-background-noise-in-objective-c