AbstractMeasures exclusively reflecting phase distortion are examined and a new phase distortion ‘index’ is proposed. This index, based on the mean‐squared degradation in the impulse response of a filter, is shown to be equivalent to a phase distortion ‘spot‐measure’ using the variation in the peak value of the impulse response, due to phase distortion, of the cascade combination of two identical filters. This equivalence allows the spot‐measure to be employed for rapid phase distortion evaluation without loss of accuracy in reflecting phase distortion.