A musical highpass filter (two pole recursive, direct form 1). The filter frequency and quality (resonance) can be specified. The filter frequency is expected to be normalized, ie. between 0 and 0.5. The normalized frequency is simply the target frequency divided by the sample rate (eg. 1000Hz / 44100Hz = 0.02267). To let the filter ring out, a number of samples can be specified that will be appended as zeros to the input, and extend the filter response by the same number of samples.
double inputArray, double normalizedFrequency, double gainDb (, int numAppendedSamples)
The input is not modified.