Top scale view of the outdoor pool, showing the study animal, the location of the aerial camera, the two underwater cameras, the underwater transducer emitting the sweeps, and the listening hydrophone. Also shown is the research cabin which housed the equipment and the operator.
Block diagram of the signal generation and control systems, and the listening and recording equipment, used in the porpoise startle response study.
(a) The 1/3-octave spectrum of the SPL of the 1–2 kHz up-sweep without harmonics at 6 dB below the highest test level (solid line). (b) The 1/3-octave spectrum of the 1–2 kHz up-sweep with harmonics at 6 dB below the highest test level (solid line). (c) The 1/3-octave spectrum of the 6–7 kHz up-sweep without harmonics at the highest test level (solid line). In Figs. 3(a)–3(c), levels were averaged over the sweep duration, and the dashed lines represent the 1/3-octave spectra of the background noise level (averaged over 1 s), determined from the recorded sound just before the sweep signal, including electronic noise of the measuring equipment. Recordings were band-pass filtered between 25 Hz and 32 kHz.
(a) The waveforms of the 1–2 kHz up-sweep without harmonics, (b) the 1–2 kHz up-sweep with harmonics, and (c) the 6–7 kHz up-sweep without harmonics.
The SPL distribution of the three up-sweeps (in dB re 1 μPa) as a function of the linear distance to the transducer. (a) 1–2 kHz up-sweep without harmonics (mean exposure SPL 122 dB; 12 dB < highest level presented), (b) 1–2 kHz up-sweep with harmonics (mean exposure SPL 100 dB; 6 dB < highest level presented), and (c) 6–7 kHz up-sweep without harmonics (mean exposure SPL 104 dB; 6 dB < highest level presented). Note that (a) has different y axis limits than (b) and (c). In the close vicinity of the transducer, the direct field of the source dominated the reverberant field. The distance to which the direct field dominated decreased with increasing high-frequency content of the signals (about 12 m for the 1–2 kHz sweeps without harmonics, about 6 m for the 6–7 kHz sweep without harmonics, and about 3 m for the 1–2 kHz sweep with harmonics). The mRL of the up-sweep and down-sweep in each sweep pair was similar.
The mRL versus the startle response rate for up-sweeps and down-sweeps in the three sweep pairs. Each data point is based on 24 sweep emissions. ▪ = 1–2 kHz up-sweep and □ = down-sweep, both without harmonics; • = 1–2 kHz up-sweep and ○ = down-sweep, both with harmonics; ▴= 6–7 kHz up-sweep and ▵ = down-sweep, both without harmonics.
The mean received broadband sound pressure level (SPL), in dB re 1 μPa, of one of the five transmitted Source Levels (SPL at 1 m from the transducer), of the three sweep pairs tested on the harbor porpoise, and the mean (± SD) SPL in the pool (mRL, n = 231 locations). All sweeps were hyperbolic in shape and had 50 ms rise and fall times and total durations of 1 s. Within each pair, the levels of the up-sweeps were identical to those of the down-sweeps.
The 50% startle response mean received levels (mRLs) for each sweep type, derived from the psychometric functions (Fig. 6); the 50% broadband detection threshold levels of the study animal for the signals used in the present study (Kastelein et al., 2011); and the difference between the 50% startle response mean received level and the 50% broadband detection threshold level (in dB, calculated as startle response level minus detection threshold level).
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