Low pass sound filter computing RMS on iOS

I am working on an iPad App and I want to parse the audio from the video that I play. Everything is going well using MTAudioProcessingTap. I currently have test code for testing / measuring the volume of the left and right channels. Everything is going well:

void process(MTAudioProcessingTapRef tap, CMItemCount numberFrames,
         MTAudioProcessingTapFlags flags, AudioBufferList *bufferListInOut,
         CMItemCount *numberFramesOut, MTAudioProcessingTapFlags *flagsOut)
{
    OSStatus err = MTAudioProcessingTapGetSourceAudio(tap, numberFrames, bufferListInOut,
                                                  flagsOut, NULL, numberFramesOut);

    if (err)
        NSLog(@"Error from GetSourceAudio: %ld", err);

    float leftVolume, rightVolume;

    for (CMItemCount i = 0; i < bufferListInOut->mNumberBuffers; i++)
    {
        AudioBuffer *pBuffer = &bufferListInOut->mBuffers[i];
        int cSamples = numberFrames * pBuffer->mNumberChannels;

        float *pData = (float *)pBuffer->mData;

        float rms = 0.0f;

        for (int j = 0; j < cSamples; j++)
        {
            rms += pData[j] * pData[j];

        }

        if (cSamples > 0)
        {
            rms = sqrtf(rms / cSamples);
        }

        if (0 == i)
        {
            leftVolume = rms;
        }

        if (1 == i || (0 == i && 1 == bufferListInOut->mNumberBuffers))
        {
            rightVolume = rms;
        }
    }

    NSLog(@"Left / Right Volume: %f / %f", leftVolume, rightVolume);
}

But for this application, I just want to measure the RMS ("intensity") of the 0-80 Hz range (as an example). So I need a low pass filter.

I have been working at Googling for a long time, but my problem is that I can not find any notes, tutorial or solutions that are obvious. Almost every problem that sounds like mine has a random piece of code underneath it with crappy or lack of comments, so I can't figure out what all the magic numbers do there and what they mean.

- ? , do .