Tonality Perception Lessons Learned

The future is expected to bring Advanced Air Mobility (AAM) vehicles, including small unmanned aerial systems (sUAS), urban air mobility (UAM) vehicles and regional air mobility (RAM) vehicles. These manned and unmanned vehicles are propelled by rotors. Rotors tend to generate tonal sound as their blades interact periodically with airflow features. Since people are more sensitive to tonality, including tones, than broad band sound, AAM generated tonality is expected to be an important consideration for design. In this paper several tonality metrics are examined for their ability to explain perceived annoyance of AAM flyover noise as measured by NASA's Rotorcraft Sound Quality Metric 1 (RoQM-1) test. The various investigated metrics use one-third octave band, narrow band, and autocorrelation analysis. It is observed that tonality influences but does not control perceived flyover noise annoyance due to other sound qualities like roughness, consistent with previous work. The metrics are also examined for their ability to explain perceived tonality of sounds generated by IT equipment. The metrics based on autocorrelation are observed to best explain perceived tonality while also being among the best for explaining flyover annoyance. The plausibility of the auditory nervous system as a physiological auto-correlator is discussed.