The effects on hearing thresholds, sound attenuation, and consonant discrimination of wearing a balaclava under hearing-protecting earmuffs were studied. This combination is commonly worn during cold weather military operations. One group of 20 normal-hearing adults (10 male and 10 female subjects) was tested. Within-subject measurements were made of diffuse-field hearing thresholds from 0.25 kHz to 8 kHz and consonant discrimination in quiet with the ears unoccluded and protected with the earmuffs alone and with the balaclava worn full face or rolled. Attenuation was derived from the protected and unoccluded thresholds at each frequency. When the balaclava was worn full face, attenuation decreased by 16 to 18 dB, relative to the muff alone, below 6.3 kHz. With the balaclava worn as a cap, there was an inverted U-shaped decrement in attenuation of 18 to 27 dB from 0.25 Hz to 4 kHz. Consonant discrimination decreased by 7% with the muffs alone. These findings underscore the importance of assessing protective equipment under the conditions in which it will be worn.

Standards for personal protective equipment typically provide guidelines for the evaluation, classification, selection, and use of classes of devices, such as respirators, eyeglasses, or hearing protectors.1-3 In practice, however, safety devices are generally worn in combination in both civilian and military operational environments. The Canadian standard on hearing protection, the subject of the present investigation, cautions that the performance of these devices may be compromised by other equipment worn around the head.4 To date, however, only a few scientific studies have been conducted to determine the extent to which the combining of personal protectors may interfere with the components.5-7 Chung et al.,6 for example, investigated the effect on earmuff attenuation of long hair over the ears, either alone or with glasses or a thin or thick cap. The stimulus was 103-dBA pink noise (equal energy per octave). Decrements in insertion loss, the difference in the physical measurements of sound levels inside and outside the ear cup,8 relative to short hair alone, were highly variable across subjects. They ranged, on average, from 2 dB for long hair to 13 dB for long hair with a thick cap. More recently, Abel et al.7 measured the effects on earmuff attenuation of safety glasses and/or an air-purifying half-mask respirator worn in combination, using the real-ear attenuation at threshold procedure.2 The sound attenuation observed for earmuffs worn alone decreased by 5 dB with either the glasses or respirator and by as much as 9 dB with both devices. Maximal decrements were observed below 1 kHz. The effect diminished with increases in the frequency of the test stimulus.

Decrements in attenuation observed in the studies cited above were attributed to leakage of sound under the ear cup of the hearing protector. Leakage results from any obstruction that precludes an airtight seal of the earmuff to the circumaural region surrounding the pinna of the ear.9 Crabtree10 modeled this scenario by using a passive earmuff and a flat plate coupler containing a measurement microphone. Measurements were confined to frequencies below 200 Hz. A 1.6-mm tube was inserted between the ear cushion and the coupler, to model the leakage that would be caused by the metal side frame of military-issue sunglasses worn under the muff. With an airtight seal, insertion loss increased from ~5 dB to ~30 dB with increases in frequency from 6 Hz to 137 Hz. With the tubing in place, there was virtually no attenuation below 100 Hz. In fact, negative insertion loss was observed in the region of 50 to 100 Hz, i.e., sound energy was amplified by 5 dB in the enclosed air space.

The decrement in sound attenuation attributable to sound leakage under the hearing protector may affect speech understanding. When hearing protectors are worn, subjects typically show a decrease in the percentage of words correctiy recognized, which is proportional to their difficulty in hearing the speech stimulus. The attenuation provided by the protector adds to unoccluded hearing thresholds. Improvements in speech understanding in noise with protectors worn were reported for normal-hearing subjects. This effect may result from reductions in cochlear overload and concomitant signal distortion with the overall decrease in the level of signal plus noise.9 In contrast, individuals with even mild hearing loss show decrements in speech understanding in noise.11 Although one might predict that sound leakage would improve performance, at least one study showed the reverse. Wagstaff et al.12 assessed the ability of normal-hearing male subjects to understand digits and words in 97-dBA helicopter noise while wearing a communication headset, with or without glasses. Performance was significantly poorer when the glasses were worn. The authors argued that breaking the seal of the earmuff resulted in greater low-frequency masking of the speech. In the study by Abel et al.,7 no differences in consonant discrimination were observed across ear conditions, either in quiet or in noise. The difference in outcomes for the two studies may be attributable to differences in the type of background noise (low-frequency vs. speech spectrum) and/or differences in the noise level (97 dBA vs. 80 dB sound pressure level [SPL]).