This demonstration illustrates Fletcher's critical band experiment (explanation below).
The purpose of this experiment was to measure the bandwidth of the auditory filters. The principle is that the masked tone level threshold is determined by the power of the signal that passes through the auditory filter (of unknown shape and bandwidth). If the bandwidth of the noise is narrow it has little masking effect (low threshold). As the bandwidth of the noise is increased, more noise energy passes through the filter and so there is more masking and thus a raised threshold. But once the masker bandwidth exceeds the bandwidth of the filter no further masking can occur and so the tone threshold reaches a plateau. The knee-point of at which this change in tone-threshold occurs represents what Fletcher termed the critical bandwidth. The right-hand plot shows the change in tone-threshold with masker band-width. The Left-hand plot illustrates the masker and the underlying filter. The rectangular filter is the underlying shape assumed by Fletcher.
The rounded exponential (roex) filter (blue-dashed line) is a slightly more realistic filter shape. The threshold vs bandwidth curve for this (shown on the right) is determined (according to this simple power-spectrum model of masking) by the shaded area under the filter curve on the left hand plot.