A new definition of S/N is proposed in order to evaluate systems whose noise is a function of the signal, such as compressors, expanders, and dynamic noise filters. This new definition defines S/N as the ratio of short-term average signal-to-noise, instead of the usual definition of the maximum signal to the noise in the absence of signal. A pilot psychophysical investigation shows that a system need have only 52 dB S/N in order for the noise to remain undetectable. For wide dynamic-range classical music this translates into a ratio of 79 dB using the classical definition. The data also predict the optimum compander characteristics for noise reduction systems.
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