Human subject investigation of MIRE microphone location during insertion loss testing of Active Noise Reduction hearing protectors in active and passive modes

The Microphone in Real Ear or MIRE protocol, standardized as ANSI S12.42-1995, does not specify a standardized location for measurement microphone placement in the human ear. Prior research is mixed as to the potential benefits and/or shortcomings of placing the measurement microphone outside versus inside the ear canal. This study captured and compared spectral attenuation data at three different microphone locations: in the concha of the outer ear, in the ear canal-shallow, and in the ear canal-deep (with a probe tube microphone positioned near the tympanic membrane), using human test participants across five active noise reduction (ANR) headphones of differing design. ANR headphones were used because this paper is an excerpt of a larger research effort designed to assess attenuation measurement methods and instrumentation. Research goals were to determine whether spectral sound pressure levels (SPL) differed significantly at the three microphone locations tested and whether any changes in frequency significantly affected SPL measured at each location; the results showed small, but statistically significant differences between microphone locations, but with frequency-specificity. The results provide assistance to practitioners in determining where to place measurement microphones to best suit their particular needs when using MIRE for hearing protector attenuation testing.