Top scale view of the outdoor pool, in which the porpoise was exposed to fatiguing noise, and the indoor pool, in which hearing tests were conducted. Note in the outdoor pool the location of the aerial camera, the two underwater cameras, the underwater transducer emitting the fatiguing noise, and the listening hydrophone, which was used to monitor test stimuli (not for calibration). The outdoor research cabin to the south west housed the equipment producing the fatiguing noise. The harbor porpoise is in position for hearing tests at the listening station. The dashed line indicates the swimming path during a hearing trial.
Background noise in the pool under hearing test conditions (only researchers involved in the study within 15 m of the pool, water circulation system off, no rain, and Beaufort wind force 4 or below). Measurements were conducted using 1/3-octave bands and are converted to a pressure spectrum level (PSL). Above around 3 kHz the noise is dominated by the electronic noise of the measurement system.
The spectrum of the OBN centered at 4 kHz (SPL at 2 m from the transducer: 149 dB re 1 μPa) which was used as the fatiguing noise. A 10-s measurement from one hydrophone at 2 m from the transducer, at a depth of 1 m was analyzed in pressure spectrum level (PSL). The grey bar indicates the defined width of the calculated octave-band, and shows that the produced OBN had the intended width.
The mean (n = 4 per location) SPL distribution of the fatiguing noise in the outdoor pool (231 measurement locations; 77 per depth). Per location, the SPL did not vary much per depth, but an SPL gradient occurred in relation to the distance to the transducer. These data were used to calculate the mean fatiguing noise SPL that the animal experienced during noise exposure. The mean received level in this figure is 136 dB re 1 μPa.
Mean threshold shift in dB (±standard error—for SDs see Table III; n = 5 or 6 sessions per period) measured after the harbor porpoise was exposed to an OBN centered at 4 kHz at a mean received SPL of (a) 124 dB re 1 μPa, (b) 136 dB re 1 μPa, (c) 148 dB re 1 μPa for 7.5, 15, 30, 60, 120, and 240 min. Also shown are the results of control tests without fatiguing noise, i.e., with normal (low) background noise in the pool. PNE and PAE thresholds were measured 1–4, 4–8, 8–12, 48, and sometimes 96 min after the OBN or ambient noise exposure stopped.
Effect of mean received SPL (124, 136, and 148 dB re 1 μPa) on the mean TTS1–4 levels in dB (± standard error—for SDs see Table III; n = 5 or 6 hearing sessions) measured in a harbor porpoise exposed to a continuous OBN centered at 4 kHz for periods of 7.5, 15, 30, 60, 120, and 240 min.
Effect of exposure time (fatiguing noise duration—log scale) on the mean TTS1–4 levels in dB (± standard error—for SDs see Table III; n = 5 or 6 hearing sessions) measured in a harbor porpoise exposed to a continuous OBN centered at 4 kHz at mean received SPLs of 124, 136, and 148 dB re 1 μPa for periods of 7.5, 15, 30, 60, 120, and 240 min.
The observed TTS1–4 (dB) in the present study as a function of the sound exposure level (SEL) (in dB), for fatiguing sounds at 124 dB re 1 μPa (●), 136 dB re 1 μPa (◻), and 148 dB re 1 μPa (▲). The horizontal dashed line indicates the approximate level (2 dB) above which TTSs were statistically significant (see Table III).
Selection of hearing test signal frequency: the mean TTS (± SD; n = 6 for 4 kHz and 5 for 5.7 kHz) in a harbor porpoise measured with hearing test signals centered at 4 and 5.7 kHz, after the study animal had been exposed for 120 min to a continuous OBN centered at 4 kHz at a mean received SPL of 136 dB re 1 μPa (SEL = 175 dB re 1 μPa2 s).
The prestimulus response rate (based on all prestimuli during both signal-present and signal-absent trials; signal-absent trials are ended with a whistle stimulus) of the harbor porpoise during baseline (BL), post-noise exposure (PNE), and post ambient exposure (PAE) periods (all periods were 12 min).
The mean threshold change in dB (± SD; n = 5 or 6) in a harbor porpoise exposed to a continuous OBN centered at 4 kHz for 7.5, 15, 30, 60, 120, or 240 min at three mean received levels (124, 136, and 148 dB re 1 μPa). The post-exposure thresholds were measured for a narrow-band up-sweep centered at 4 kHz (3.9–4.1 kHz) in five periods: 1–4 min (TTS1–4), 4–8 min (TTS4–8), 8–12 min (TTS8–12), 48 (TTS48), and 96 (TTS96) min after the noise exposure stopped. The exact P-values for the Bonferroni post-hoc tests (conducted after the 18 ANCOVAs on hearing thresholds with factor period and covariate session number) are shown in parentheses [where significant; otherwise shown as not significant (NS)]. Bonferroni tests are used to compare the hearing thresholds in the test periods with those quantified in the corresponding baseline periods. The control hearing sessions were conducted after the porpoise had been exposed for approximately 60 min to the normal (low) ambient noise level in the pool (n = 8). ANCOVAs showed that no TTS took place during control sessions. #: Hearing test not conducted.
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