In  a special masking experiment is presented; Two narrowband noise maskers, with a frequency difference , are masking a tone in between if the tone has low enough intensity. The other way around is also tested, where two tones mask noise. The experiment shows that the masking from the two maskers is approximately constant as long as the is less than the critical bandwidth at that frequency. Thus, a masker can mask almost uniformly within the critical band.
Masking does not only occur within the critical band, but also spreads to neighboring bands. A spreading function SF(z,a) can be defined, where z is the frequency and a the amplitude of a masker. This function would give a masking threshold produced by a sinle masker for neighboring frequencies. 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.