iOS AVCaptureSession: Reduce callback rate of audio recording

I have an AVCaptureAudioDataOutputSampleBufferDelegate implemented on my ViewController. My goal is to show the average power my audio recording.

The problem is that the callback

func captureOutput(captureOutput: AVCaptureOutput!, let didOutputSampleBuffer sampleBuffer: CMSampleBuffer!, fromConnection connection: AVCaptureConnection!)

is being called too frequently (roughly 45 times per second, 10 would do).

How can I influence this? Do I have to set something in the AVCaptureSession or the AVCaptureAudioDataOutput?