Comodulation Masking Release (CMR) is the decrease in masked thresholds that occurs when the masker is amplitude-modulated. Remember that masked threshold is the lowest tone level at which it can just be heard over the noise. A decrease in masked threshold means therefore that it becomes easier to hear the tone. As a rule, masked thresholds increase with increased bandwidth of the masker, simply because more energy is available for masking. The peculiar property of CMR is that masked thresholds actually decrease with increased bandwidth of the masker when maskers are amplitude-modulated, as illustrated in the following figure.
Here is a sound example illustrating the difference in thresholds of a tone in modulated and unmodulated maskers. Start by setting the threshold with the modulation button in the 'off' position. At that sound level, turn the modulation on. Can you hear the tone now?
While this example illustrates the drop in threshold due to the introduction of the modulation, it is rather the decrease in threshold due to the increase in bandwidth that is the most interesting feature of CMR. However, this dependence is much more difficult to illustrate with the often rather lousy loudspeakers that many computers are fitted with.