Attenuation of low-frequency noise by hearing protectors.

The ability of earmuffs and helmets to reduce low-frequency (4-250 Hz) noise was tested in an anechoic chamber, in a paint factory, outside and in a low-frequency noise chamber. Prototypes of a new noise helmet were also tested. Steady noise and low-frequency noise impulses were used as the sound sources. The testing was sometimes difficult because physiological noise raised the sound level inside the hearing protector at some frequencies after the protector was put on. Therefore the low-frequency sound level had to be over 100 dB at frequencies of 2-250 Hz during the tests. The earmuffs attenuated noise levels of 1-20 dB in the 4-250 Hz frequency range. Commercial noise helmets tested were not as good as earmuffs in attenuating low-frequency noise. New noise helmet prototypes attenuated low-frequency noise better than did earmuffs or commercial helmets (attenuation 5-39 dB).