A spreading function which takes into account both in-band masking and inter-band masking is (taken from )
where z is the frequency in Bark. This equation is modified to take into account the decreasing slope at higher masker amplitudes i in the following manner:
where , f is the frequency of the masker and BW(f) is the critical bandwidth at f. A higher value of i gives a flatter SF'(z), and the formula for i is an experimentally found heuristic which compensates a frequency bin if it is only a small part of a wide critical band. Setting the max of i to 2.0 was necessary for some test sounds, where e.g a base drum could make a large part of the high frequency spectra vanish. See figure 2 for an illustration of the spreading function.
Figure 2: The masking spreading function for a single masker. The scaled amplitude i varies from 0 to 2, where 2 corresponds to the wider spread. Note the flatness of the curve within critical bands. This is from the intra-band-masking.