Masking release at 4 kHz in harbor porpoises (Phocoena phocoena) associated with sinusoidal amplitude-modulated masking noise

Abstract

Acoustic masking reduces the efficiency of communication, prey detection, and predator avoidance in marine mammals. Most underwater sounds fluctuate in amplitude. The ability of harbor porpoises (Phocoena phocoena) to detect sounds in amplitude-varying masking noise was examined. A psychophysical technique evaluated hearing thresholds of three harbor porpoises for 500–2000 ms tonal sweeps (3.9–4.1 kHz), presented concurrently with sinusoidal amplitude-modulated (SAM) or unmodulated Gaussian noise bands centered at 4 kHz. Masking was assessed in relation to signal duration and masker level, amplitude modulation rate (1, 2, 5, 10, 20, 40, 80, and 90 Hz), modulation depth (50%, 75%, and 100%) and bandwidth (1/3 or 1 octave). Masking release (MR) due to SAM was assessed by comparing thresholds in modulated and unmodulated maskers. Masked thresholds were affected by SAM rate with the lowest thresholds (i.e., largest MR was 14.5 dB) being observed for SAM rates between 1 and 5 Hz at higher masker levels. Increasing the signal duration from 500–2000 ms increased MR by 3.3 dB. Masker bandwidth and depth of modulation had no substantial effect on MR. The results are discussed with respect to MR resulting from envelope variation and the impact of noise in the environment.

Mechanisms of auditory masking in marine mammals

“…11 ). The same pattern was displayed in an AM masking study by Kastelein et al ( 2021 ); however, at very different AM rates. In Kastelein et al ( 2021 ), MR did not occur for AM rates above 20 Hz.…”

“…The same pattern was displayed in an AM masking study by Kastelein et al ( 2021 ); however, at very different AM rates. In Kastelein et al ( 2021 ), MR did not occur for AM rates above 20 Hz. In Branstetter and Finneran ( 2008 ), the CMR effect was still noticeable with AM rates up to at least 500 Hz (Fig.…”

“…One of the most common features of natural ocean noise is amplitude modulation caused by a reverberant environment, or the animals themselves. Kastelein et al ( 2021 ) measured the harbor porpoise’s ability to detect a 4 kHz tone embedded in sinusoidal amplitude modulated (SAM) noise. A release from masking up to 14 dB relative to Gaussian noise resulted for AM rates between 1 and 5 Hz.…”

“…5 Masking release (dB) as a function of AM rate with SAM maskers and a 4 kHz tonal signal. The two colors represent two different harbor porpoise participants (Adapted from Kastelein et al 2021 ) …”

“…However, for tonal signals, integration times are frequency dependent and on the order of tens to hundreds of milliseconds. For example, the harbor porpoise has an integration time of approximately 277 ms at 4 kHz (Kastelein et al 2021 ). If a 277 ms integration window were applied to amplitude modulated noise, AM rates above 4 Hz (4 Hz period = 250 ms) would likely be smeared, suggesting that different mechanisms govern the integration of tonal signals and the resolution of AM noise.…”