Spreading Function

In experiments [1][2], masking experiments has been made with two narrowband noise maskers, with a frequency difference $\Delta f$, masking a tone in between, and the other way around. It shows that the masking from the two maskers is approximately constant as long as the $\Delta f$ is less than the critical bandwidth at that frequency. But masking does not only occur within the critical band, but also spreads to neighboring bands. A spreading function $SF(f,a)$ can be defined, where $f$ is the frequency and $a$ the amplitude of a masker. This function would give a masking threshold for neighboring frequencies of a single tone. The simplest function would be a triangular function with slopes of +25 and -10 dB / bark, but a more sophisticated one is highly nonlinear and depends on both frequency and amplitude of masker.