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In the midst of performing some lab work yesterday, I finally decided to take a few minuts and quanity the noise insulation performance of my Bose QC-1 noise cancelling headsets.
The setup is simple: I used a calibrated ¼-inch microphone (Radio Shack 33-3028, calibrated with a GenRad 1986 OmniCal) connected to a TerraSonde ATB Pro. I placed the microphone outside of the left QC-1 earcup, and played pink noise through my audio system to measure the baseline level. I next placed the ¼-inch mic inside the earcup (inside the pinna, outside the ear canal) with the noise-cancelation circuitry switched off to measure the passive noise reduction caused by the cups. Finally, with the mic still inside the ear cup, I switched on the noise cancellation circuitry.
The measurement results are 10-sec averages (Leq). The overall sound level from the pink noise outside the earcups was ~75 dBA. The pink noise sound level was at least 10 dB higher than the background noise level in the range of 31.5 Hz to 16 kHz.
The results are fairly consistent with those measured in this report on ANR headset performance measurements. Keep in mind that these measurements are typically performed with microphones inside the ear canal, so there will be some differences between my quick-and-dirty tests, and official measurements. Also, I think the negative insertions losses that show up below 40 Hz may be caused by a poor seal around my ear (my glasses got in the way).