Adding tones lowers threshold only until about nine tones span the critical band

Evidence

For eight tones, the threshold is found at -6 dB SPL of each tone. This means that threshold expressed in level per tone decreases with the increasing number of tones, as expected. However, beyond a certain number of tones, no further decrease occurs, as Fig. 6.1 indicates for the sets of 16 and 32 tones used. With a certain number of individual tones, in our case about nine, the decrement in level produced by adding tones stops. The transition point is marked in the figure by an arrow and is used as a measure of the critical bandwidth. — Professor Dr.-Ing. Hugo Fastl, Professor Dr.-Ing. Eberhard, Psychoacoustics- Facts and Models, 877