A method, apparatus and computer program product provide an improved filter calibration procedure to reliably equalize the long term spectrum of the audio signals captured by first and second microphones that are at different locations relative to a sound source and/or are of different types. In the context of a method, the signals captured by the first and second microphones are analyzed. The method also determines one or more quality measures based on the analysis. In an instance in which one or more quality measure satisfy a predefined condition, the method determines a frequency response of the signals captured by the first and second microphones. The method also determines a difference between the frequency response of the signals captured by the first and second microphones and processes the signals captured by the first microphone for filtering relative to the signals captured by the second microphone based upon the difference.