Real-Ear-Attenuation-at-Threshold (REAT) Measurement Method
The ANSI standard method of measuring the noise attenuation of hearing protectors and earphones is to compare the quietest sound that can be heard with the earphones in place to the quietest sound that can be heard with the ears open. This Real-Ear-Attenuation-at-Threshold (REAT) method is the basis for all OSHA Noise Reduction Ratings (NRR). Etymotic earphones and headsets were measured with the REAT method at E-A-RCAL Laboratory, Indianapolis, IN. Virtually identical results were obtained on these eartips by the TNO laboratory in the Netherlands.

Probe Microphone Measurement Method
Active noise-canceling earphones cannot be measured with the REAT Method because their circuit noise interferes with the measurement. Etymotic earphones and headsets were compared to active noise-canceling earphones using a probe microphone method: A calibrated 84 dB SPL broad-band pink noise was generated in a reverberation room using four uncorrelated noise sources. The sound pressure developed in the ear was measured with a 2.5 mm microphone located deep in the ear canal. The noise reduction of each earphone was calculated as the difference between the noise level in the open ear and the noise level with the earphone in place.