Masking can reduce the efficiency of communication and prey and predator detection. Most underwater sounds fluctuate in amplitude, which may influence the amount of masking experienced by marine mammals. The hearing thresholds of two harbor seals for tonal sweeps (centered at 4 and 32 kHz) masked by sinusoidal amplitude modulated (SAM) Gaussian one-third octave noise bands centered around the narrow-band test sweep frequencies, were studied with a psychoacoustic technique. Masking was assessed in relation to signal duration, (500, 1000, and 2000 ms) and masker level, at eight amplitude modulation rates (1–90 Hz). Masking release (MR) due to SAM compared thresholds in modulated and unmodulated maskers. Unmodulated maskers resulted in critical ratios of 21 dB at 4 kHz and 31 dB at 32 kHz. Masked thresholds were similarly affected by SAM rate with the lowest thresholds and the largest MR being observed for SAM rates of 1 and 2 Hz at higher masker levels. MR was higher for 32-kHz maskers than for 4-kHz maskers. Increasing signal duration from 500 ms to 2000 ms had minimal effect on MR. The results are discussed with respect to MR resulting from envelope variation and the impact of noise in the environment on target signal detection.

The loud, impulsive, broadband underwater sounds produced during offshore pile driving are known to have auditory and behavioral effects on harbor porpoises (Phocoena phocoena) in the areas around piling sites. Thresholds to prevent behavioral effects have not yet been set, and it is unclear whether or not auditory frequency weighting of piling sounds, as used for criteria to protect hearing (Southall et al., 2019), is also useful for predicting behavioral responses and therefore required to set safety criteria and develop mitigation measures. A harbor porpoise in a pool was exposed to playbacks of piling sounds, and her behavioral responses were quantified in comparison to baseline periods without piling sounds. The full-spectrum playback piling sound was recorded at 100 m from a piling site for an offshore wind turbine. For comparison, five low-pass filtered (6.3, 3.2, 1.5, 1.0, and 0.5 kHz) versions of the sound in which the bandwidth decreased were played back at the same duty cycle (46 strikes/min) and similar single-strike sound exposure levels (power average in the pool: 135 dB re 1 µPa²s; t90: 90 to 100 ms). As the bandwidth of the piling sounds decreased, the porpoise’s behavioral response became weaker. Although these results are based on only one porpoise, they indicate that harbor porpoises respond most strongly to the higher frequencies in piling sounds. Therefore, frequency weighting of the sound exposure level (SEL) will improve prediction of behavioral responses, and behavioral response threshold levels for criteria should also be expressed as weighted SELs. However, it is unclear whether the weighting for predicting auditory effects is also the best weighting to predict behavioral effects. Mitigation of the effects of piling sounds on harbor porpoise behavior should be focused on reducing the high-frequency part of the spectrum.

To determine the frequency-dependent susceptibility of California sea lions (Zalophus californianus) to noise-induced temporary hearing threshold shift (TTS), one of two subjects were exposed for 60 minutes to two continuous one-sixth-octave noise bands (NBs) as fatiguing sounds: one centered at 0.6 kHz, at sound pressure levels (SPLs) of 168 to 174 dB re 1 µPa (sound exposure levels [SELs] of 204 to 210 dB re 1 µPa2s), or one centered at 1 kHz, at SPLs of 144 to 159 dB re 1 µPa (SELs of 180 to 195 dB re 1 µPa2s). Using a psychoacoustic technique, TTSs were quantified at 0.6, 0.85, 1, 1.2, 1.4, and 2 kHz (at the center frequency of each NB, half an octave higher, and one octave higher). When significant TTS occurred, higher SELs resulted in greater TTSs. In the sea lion that was tested 1 to 4 minutes after exposure to the fatiguing sounds, the largest TTSs occurred when the hearing test frequency was half an octave higher than the center frequency of the two fatiguing sounds. The highest TTS levels elicited were 8.7 dB at 0.85 kHz and 9.6 dB at 1.4 kHz. When their hearing was tested at the same time after the fatiguing sounds stopped, initial TTSs and hearing recovery patterns were similar in both sea lions. These findings will contribute to the protection of hearing of species in the Otariidae family from anthropogenic noise by facilitating the development of an evidence-based underwater sound weighting function.