Valuable recommendations for the choice, utilization, care, and maintenance, and for the measurement of sound attenuation of hearing-protective devices have been laid down in international standards. Yet, by considering the wearing time of a hearing protector, the standard DIN EN 458 assumes a scarcely understandable drastic reduction in the effective attenuation even when the device is not used for only a short time in a noise-filled area. A 30-dB sound attenuation of such a protective device would, e.g., decrease to 12 dB if it were unused for only 30 min of an 8-hour shift. Thus, the actual influence of a shortened wearing time on the protection of ear muffs was tested in a laboratory study using audiometric measurements of the temporary threshold shift (TTS₂) and its recovery after exposure to noise. For that purpose, the effectiveness of a hearing-protective device depending on the amount of time worn as prognosticated by DIN EN 458 was compared with the actual physiological effect of the ear muffs. 10 test subjects (Ss) participated in 3 test series (TS), each. In the first of the TS, the Ss were exposed to a sound pressure of 106 dB(A) for 1 h, during which the Ss wore noise-insulating ear muffs with an attenuation of 30 dB. The Ss were exposed to the same sound pressure in TS II; however, after 30 min, the ear muffs were removed for a duration of 3¾ min. Mathematically, this reduced the sound attenuation of the ear muffs to 12 dB; i.e., the average noise level over 1 h should be 94 dB, which would be equivalent to 85 dB(A) over 8 h. In order to evaluate the actual additional physiological cost of TS II, the Ss were exposed to 94 dB(A) / 1 h without ear muffs in a third TS. This acoustic load, which is energy equivalent to the load in TS II, is also equivalent to 85 dB(A) / 8 h. The results show that the continuous wearing of the ear muffs offers secure protection. However, the energetic approach and the levelling of differently structured noise loads according to the principle of energy equivalence leads to misconceiving results. The drastic reduction of the sound attenuation of the ear muffs predicted from the energetic point of view must be regarded as exaggerated. The TTS values show that TS II – which, according to the equal energy concept, should result in the same effects as TS III – represents significantly less auditory fatigue. Thus, if the ear muffs are taken off briefly, a drastic reduction in the protection – as predicted in DIN EN 458 – does not result.