How to get mute raw audio from iPhone mic in NSData

Question

I need to record 3 seconds of the original audio data: PCM 11025 samples per second, 16-bit mono, and then send it as a base64 encoded string.

With AVAudioRecorder, I can record the .CAf file and access it at any time, but the question arises: how can I get the silent audio source data from the recorded file into an NSData object?

Thanks!