In evaluating a speech‐processing device, the result of applying a suitable fidelity criterion to its output and the tolerance within which the reduced signals must be represented to satisfy this criterion, as well as the corresponding band width, are important. These factors establish a measure of the efficiency of the particular representation achieved by the device. This quantity can be utilized to compare it with systems having known characteristics. An important constraint is that the systems being compared have equal fidelity according to the criterion being used. The quantitative result of this comparison depends markedly upon the criterion selected. It should be chosen with the contemplated application in mind and can be either objective or subjective in nature. Since speech‐processing devices operate on the basis of an objective criterion, it appears that as future research establishes relations between subjective impressions and physical measurements, better and perhaps even the ultimate speech‐processing devices will become realities.