**4. Noise level in flight**

The noise level within the helicopter was measured using a Bruel and Kjaer model 2270 sound level meter. The average noise level (LAeq) was measured over the entire 30-minute flight. An example LAeq from a 30-minute helicopter flight in this project was 107 dBA. An A-weighted decibel (dbA) is a scale for measuring loudness corresponding to hearing thresholds of the human ear.
