Abstract
The window size (WS) of the fast Fourier transform (FFT) is one of the important parameters when calculating target strength (TS) spectra from broadband echo waveforms. However, the effects of the FFT WS on TS spectra have not been thoroughly investigated. Before studying fish echoes, we thus focused on the echoes of tungsten carbide (WC) spheres, which are typically used as calibration spheres. A well-calibrated Simrad EK80 echosounder with 55–90 and 90–170-kHz linear frequency-modulated transmit signals were used for the echo sampling of WC spheres with different diameters. We calculated the TS spectra for different WSs defined by the pulse length determination level for each ping and each sphere. The calculated (measured) TS spectra were then compared with the predicted TS spectra based on the exact modal series solution. When the FFT WS was narrower, the differences between the measured and predicted TS spectra tended to increase because the ripples on the measured TS spectra became large, while the differences became small and stable above the FFT WS of approx. 0.5 m. We finally concluded that a 0.6-m FFT WS was reasonable for all cases examined herein.
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Acknowledgements
We thank Dr. Kouichi Sawada for coordinating the tank experiment. We give special thanks to Lars Nonboe Andersen for sharing the MATLAB code examples to read the raw data of Simrad EK80. We also acknowledge to our laboratory students (Tetsuya Nagashima, Ryo Sudo, Yuka Gomibuchi, Soichiro Nagahama, and Tsubasa Kano) for their kind assistance during the tank experiment. This work was supported by a JSPS KAKENHI Grant Number JP20K21329.
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Liu, J., Saygili, B., Iwasa, A. et al. Effects of fast Fourier transform window size on the target strength spectra of tungsten carbide spheres. Fish Sci 89, 147–157 (2023). https://doi.org/10.1007/s12562-022-01653-7
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DOI: https://doi.org/10.1007/s12562-022-01653-7