Objective To investigate hearing conservation theory for the protection of the naval ship crew exposed to occupa -tional noise , so as to provide scientific evidence for the development of noise protection measures .Methods Occupational noise expo-sure level was studied in the light of sound theory and equivalent principle of sound energy .Results A mathematical formula for the de-termination of occupational noise exposure level was thus brought about .The following countermeasures were practical approaches for the control of noise exposure levels:the decrease of environmental noise , the selection of better ear attenuators , the persistent wearing of ear attenuators or mufflers , the decrease of daily noise exposure time , and the perfection of noise protection regulations .Conclusion The mathematical formula for the determination of occupational noise exposure level forms a close coverage of the noise protection methods .