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.

Sat Mar 7 16:27:43 PST 1998