Sound level conversion [SLC], a single-number measure describing the noise reduction afforded by ear protectors, was calculated for 30 ear protectors in 615 industrial noise spectra. The results vindicate the utility of this measure for situations in which the noise hazard is characterized only by C- and A-weighted sound levels. A simple procedure for calculating SLC, accurate within .+-. 1 dB, is proposed. Minor adjustments of the calculation procedure enable wearer protection rates in the range 50-97% to be achieved.