Establishing an audibility sensitivity rule for low-frequency tone complexes in noise

Predicting the audibility of a sound over ambient noise is an important step to assess its impact on observers. One anticipated need for such predictions is the desire to operate aerial vehicles in urban surroundings close to the public. The increased audibility of a complex set of low-frequency tones due to the recruitment of a number of independent auditory channels has been investigated in this work. Three experiments were carried out to establish a sensitivity rule for these signals. During the main experiment, the masked threshold for signals consisting of one, two, and three tone components (55, 120, and 200 Hz) were measured. As expected, the thresholds identified during this experiment point to an increased sensitivity relative to a single-tone case. However, these increases were greater than similar findings by other researchers at higher frequencies. Two follow-up experiments examined the effect of larger frequency separation between tones and their intermodulation. The findings of these experiments attribute part of the audibility improvement to biased estimation of the thresholds of multitone sounds due to an error in the estimation of thresholds for single tones. Additional details may be found in a forthcoming NASA Technical Memorandum entitled “Audibility of Multiple, Low-Frequency Tonal Signals in Noise.”Predicting the audibility of a sound over ambient noise is an important step to assess its impact on observers. One anticipated need for such predictions is the desire to operate aerial vehicles in urban surroundings close to the public. The increased audibility of a complex set of low-frequency tones due to the recruitment of a number of independent auditory channels has been investigated in this work. Three experiments were carried out to establish a sensitivity rule for these signals. During the main experiment, the masked threshold for signals consisting of one, two, and three tone components (55, 120, and 200 Hz) were measured. As expected, the thresholds identified during this experiment point to an increased sensitivity relative to a single-tone case. However, these increases were greater than similar findings by other researchers at higher frequencies. Two follow-up experiments examined the effect of larger frequency separation between tones and their intermodulation. The findings of these experim...