A spreading function which takes into account both in-band masking and inter-band masking is (taken from [6])
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.