The sound-exclusion properties (acoustic attenuation) of various earphones and ear defenders and combinations thereof were determined by the binaural, free-field, absolute-threshold-shift technique with the psychophysical method of adjustment. The earphones included dynamic earphones in two different sockets and hearing-aid-type receivers with individually molded inserts. The ear defenders included certain conventional and newly developed earplugs and muffs, and a hard-shelled noise-exclusion helmet. In a preliminary evaluation of a limited number of devices, both tones and noise bands were used as stimuli, and comparative results are shown. In evaluating the effectiveness of some of the newer devices, noise bands alone were used. For the exclusion of external sound, particularly that below 1200 cps, the order of merit for earphone combinations was: (l) large volume muffs with insert phones; (2) helmet with insert phones; (3) semiplastic sockets with dynamic phones; and (4) helmet with dynamic phones. The best combination provided the following amounts of attenuation in the seven octave bands from 75 to 9600 cps: 14, 22, 28, 33, 38, 44, and 40.