Understanding Auditory Masking: The Auditory Filter Shape at 4 kHz in a Harbor Porpoise (Phocoena phocoena)

Auditory masking, where the perception of a target sound is reduced by the presence of another, non-target sound, occurs when both sounds fall within the same critical band. In other words, the masking sound falls within the auditory filter bandwidth that is centered on the target sound’s frequency. Both bandwidth and shape of the auditory filter determine how sharply the filter is tuned around its center frequency, which is essential information for modeling the masking effects of anthropogenic noise. This study provides information on frequency tuning in the hearing of the harbor porpoise (Phocoena phocoena) at 4 kHz, a lower frequency than previously investigated in this species. A tone-on-tone masking method was applied in which masked behavioral hearing thresholds were established for a 4-kHz pure-tone target sound. Masking tone frequencies were varied (between 1 and 6 kHz) at a fixed sound pressure level of 115 dB re 1 μPa. Masked hearing thresholds at 4 kHz were plotted as a function of masking tone frequency to provide an approximation of the auditory filter shape at a high input level (relative to the basic hearing threshold). The resulting curve is asymmetrical, with thresholds decreasing relatively steeply above and less steeply below 4 kHz. The sharpness of frequency tuning was further quantified by estimating filter bandwidths and accompanying Q factors (i.e., the center frequency divided by the filter bandwidth) at three attenuation levels. At 10, 20, and 30 dB below the highest hearing threshold measured, auditory filter bandwidths were 0.7, 1.6, and 2.7 kHz, respectively, resulting in Q factors of Q10 = 5.7, Q20 = 2.5, and Q30 = 1.5. This information on harbor porpoise frequency tuning around 4 kHz may contribute to estimation of the masking potential of noise sources spectrally overlapping biologically relevant target sounds around frequency.