float cutoff = 4000.0;

float filter(float cutofFreq){
    float RC = 1.0/(cutofFreq * 2 * M_PI);
    float dt = 1.0/SAMPLE_RATE;
    float alpha = dt/(RC+dt);

    return alpha;
}

float filteredArray[numSamples];
filteredArray[0] = data.recordedSamples[0];

for(i=1; i<numSamples; i++){
    if(i%SAMPLE_RATE == 0){
        cutoff = cutoff - 400;
    }
    data.recordedSamples[i] = data.recordedSamples[i-1] + (filter(cutoff)*(data.recordedSamples[i] - data.recordedSamples[i-1]));
}