White noise and pure tone masking of pure tone thresholds of a harbour seal listening in air and underwater

Abstract

Background noises mask the detection of sound throughout a limited frequency range termed the critical bandwidth. Critical bandwidths of a harbour seal (Phoca vitulina) were measured, using behavioural psychophysical techniques, by indirect (critical ratios) and direct (two-tone masking) methods underwater and in air. Underwater critical ratios were determined at 4, 8, 16, and 32 kHz, using white noise spectrum levels of 50, 56, 60, and (or) 70 dB re 1 μPa. The critical ratios (pooled data, threshold ±SD) were 19 ± 9, 22 ± 7, 25 ± 7, and 27 ± 5 dB for the respective frequencies. In-air critical ratios were determined at 2, 4, 8, and 16 kHz, using white noise spectrum levels ranging from 23 to 50 dB re 20 μPa. The critical ratios (pooled data) were 25 ± 8, 23 ± 10, 21 ± 15, and 23 ± 16 dB for the respective frequencies. The arithmetic mean of the critical ratios in both media was 23 dB. This suggests that the seal is equally sensitive to pure tone signals in the presence of broad band noise in both air and water. Direct measurements of the critical bandwidth underwater were determined at 4, 8, 16, and 32 kHz, using a pure tone masker ranging from 96 to 120 dB re 1 μPa. In-air direct measurements of the critical bandwidth were measured at 2, 4, and 8 kHz, using a pure tone masker set at 80 dB re 20 μPa. The bandwidths, estimated at 23 dB below the masking level, were all under 2.25 kHz and become proportionately narrow at higher frequencies. These results show a narrow critical bandwidth for the harbour seal, thus indicating high frequency resolution in both media. The directly measured critical bandwidths from the two-tone masking study were not 2.5 times the critical bandwidth estimated from the critical ratios, as previously reported in some other mammals.