The EPA has established a standard — the Noise Reduction Rating or NRR — for hearing protection devices. The NRR value is determined by determining the mean attenuation values (in dB) of the hearing protection device and then subtracting two standard deviations. Thus, the NRR should represent the minimum noise reduction achieved by 98% of the population.
The Aearo E-A-R Classic earplugs used at BIAC have an EPA Noise Reduction Rate (NRR) of 29 dB. However, the rated NRR of an earplug may not be met if they do not fit properly or if they have been improperly inserted. Indeed, for meeting compliance, NIOSH derates earplug NRR by 50% if no subject fit data is available. You must instruct the subject on the proper insertion procedure for the earplug and then inspect the fit to make certain that this procedure has been followed. Instructions on properly fitting Aearo earplugs can be found at the E-A-R website. Instructions can also be found in the MRI suite.
Wearing headphones and earplugs together do not produce additive effects in hearing protection, but can increase the NRR by an additional 5-6 dB (equivalent to approximately half the sound intensity). At >2000 Hz, hearing protection is bone conduction limited and the combination of headphones and earplugs is least effective. However, at lower frequencies there is considerable gain in using both devices in combination. As there is considerable low frequency sound in MRI, the use of earplugs and headphones should be considered.
