| Summary: | 碩士 === 國立中興大學 === 農業機械工程學系 === 89 === In this research, the hydrophone is used to receive the sound-wave message from the cultivating tank in a water recirculating aquaculture system. The message is transmitted to a dynamic signal analyzer and then proceed the Fast Fourier Transform(FFT) to get the power spectrum characteristics of underwater sounds. The statistic test methods are applied to analyze the difference of the power spectrum characteristics under various underwater conditions. The results obtained can be described as follows:
(1)The loudest sound of background noise comes from the water jet areator
in the cultivating tank. When the water jet areator opened, the sound
pressure would be enhanced for the frequencies between 0 and 1500Hz,
and demonstrated a peak located at a frequency between 280 and 300Hz.
(2)The schooling fish would influence on the sound-wave message
measured.The effects may strengthen or weaken the background sound
pressure due to fish swimming behaviors, and can reduce the sound
pressure caused by the decay phenomenon. The sound pressure is
enhanced for frequencies under 120Hz. However, the influence is only on
the wave-peak and —valley as the frequency higher than 120Hz.
(3)Two indexes, the root mean square(RMS) and the average sound pressure
are obtained using statistic analysis methods are utilized to gauge the
feeding competition of schooling eels. The root mean square(RMS) is
computed for the full measured frequency range and for the subrange
between 0 and 100Hz. The average sound pressure is calculated for the
frequency range below 400Hz.
(4)The result of the two stages feeding experiment indicates that the
increases of the above two indexes are dropped as eels’ appetite
decreasing.
(5)The index of the average sound pressure in a frequency range between 0
and 400Hz is confirmed to be the criterion for the judgement of feeding
stop using the way of several stages feeding model. The thresholds of this
index for the cultivating tank A、B and C are about 2.0、1.5 and 2.0dB,
respectively.
(6)Comparing the results of different cultivating tank, a single background
noise from one tank cannot be used for all the power spectrum analysis to
evaluate scale of sound pressure changes due to feeding competition, yet
should use the background noise of each cultivating tank accordingly.
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