Page 45 - Summer2020

Why can BAS measurements be conducted with such a low SL, whereas BBS measurements require such a high SL? BAS measurements are subject to one-way transmission loss (TL) between the source and the hydrophone. TL is defined as the loss in signal level between one meter and another range. One-way TL is 60 dB at 1 km (due to spherical spreading), whereas BBS measurements are subject to two-way TL between the source to the fish and then from the fish to the sonar. The resultant TL is twice as large, 120 dB at 1 km. So, detection of fish with BBS at 1 km requires a SL, which is 60 dB louder than with BAS.
If there are no fish present between the source and the receiving array, then signal levels recorded by the hydro- phone array will be relatively loud and measured levels will be in accord with theoretical levels derived from TL models. TL models account for geometrical spreading loss, chemical absorption loss, and loss in signal level due to sound transmission into the bottom.
If a large number of fish are present between the source and the hydrophone array, then the fish will cause excess (biological) attenuation. Biological attenuation will be maximum at the resonance frequency of the fish. The biological attenuation coefficient (A), in decibels per kilo- meter, may be derived from measurements of signal level versus range by towing the source. If the source is fixed, then biological attenuation coefficients may be calculated by comparing measured levels with calculated levels of received levels that account for all causes of attenuation except biological attenuation.
Discovery of Biological Attenuation at Resonance Frequencies
David Weston (1967) discovered that biological attenua- tion peaks at the resonance frequencies of fish fortuitously as a result of routine measurements of TL in support of engineering tests of an experimental Navy sonar. To his surprise, sound attenuation was generally much higher at night than during the day and that transitions occurred during morning and evening twilight throughout the year (Ching and Weston, 1971). During months when sardines were present, differences in TL between night and day were generally about 15 dB and occasionally as high as 40 dB at some frequencies. During months when sardines were absent, the difference between night and day was essentially zero at all frequencies.
Ching and Weston (1971) attributed the diurnal vari- ability in attenuation to diurnal changes in fish behavior. Pilchard sardines, the dominant species in the Bristol Channel where he worked, generally disperse at night and school during the day. Weston speculated that the low levels of attenuation during the day may be due to interference between reflections from nearby fish in schools, a phenomenon that would dampen the reso- nance of individual fish. Attenuation at night tended to peak at frequencies of 0.7 and 3.5 kHz. Weston attributed the resonance at 0.7 kHz to pilchard sardines with a mean length of 24 cm and the resonance at 3.5 kHz, which was mostly evident a few months after the spawning season, to juvenile sardines (Ching and Weston, 1971).
Biological Attenuation: Day Versus Night
Inspired by Weston’s (1967) compelling acoustic observa- tions, Diachok (1999) conducted a BAS experiment. The objectives were to demonstrate that resonance frequen- cies of dispersed fish at night were due to the dominant species at the measurement site and to determine the cause(s) of the difference in biological attenuation during night and day. Concurrent trawls revealed that the 16-cm European pilchard (Sardina pilchardus) was the domi- nant species at this site. A ship-mounted echo sounder provided measurements of the depths and schooling behavior of this species. Concurrent echo sounder data showed that fish were dispersed at night at a depth of 20 m, descended to 65 m during dawn, and formed schools at 65 m a few minutes after sunrise.
TL measurements were made at many frequencies between 0.7 and 5 kHz along a track with constant depth, parallel to the shoreline, to simplify data interpretation and modeling.
Figure 5 shows measurements and theoretical calcula- tions of resonance frequencies of dispersed European pilchard (f0), and schools (F). The diameter of the data points is proportional to the magnitude of biological attenuation at the resonance frequencies. Measure- ments of resonance frequencies at night of 1.2 kHz at 20 m and at sunrise of 2.7 kHz at 65 m are consistent with theoretical calculations of the resonance frequencies of dispersed 16-cm sardines. The increase in frequency from 1.2 kHz at night to 2.7 kHz at sunrise is driven by the decrease in the effective radius of European pilchard swim bladders as they descend from 20 to 65 m. The measurement of the resonance frequency at 1.6 kHz at
Summer 2020 • Acoustics Today 45