Detection of complex sounds in quiet and masked conditions by a California sea lion (Zalophus californianus) and a harbor seal (Phoca vitulina)

Standard audiometric data, such as absolute detection thresholds and critical ratios, are often used to inform noise-exposure limits for marine mammals. However, these data are traditionally generated using simple stimuli, such as pure-tones and flat-spectrum noise, while natural sounds tend to have more complex structure. In this experiment, detection thresholds for complex stimuli were obtained in (a) quiet and (b) masked conditions for one California sea lion and one harbor seal. For part (a), three stimuli types were synthesized, each isolating a common feature of marine mammal vocalizations: amplitude modulation (AM), frequency modulation (FM), and harmonic structure. Detection thresholds in quiet conditions were then obtained for these stimuli at frequencies spanning the functional hearing range. For part (b), the same complex signals were combined with flat-spectrum noise or shipping noise. To test how well standard hearing data predict detection of complex sounds, the results of parts (a) and (b) were compared to a priori predictions based on previously obtained audiogram and critical ratio data. Preliminary results indicate that absolute detection thresholds for AM and FM stimuli are reliably predicted by audiogram data, but that thresholds for harmonic stimuli are lower than predicted, in some cases by more than 10 dB.