Effects of fast Fourier transform window size on the target strength spectra of tungsten carbide spheres

• Published in: Fisheries science Vol. 89; no. 2; pp. 147 - 157
• Main Authors: Liu, Jing, Saygili, Burak, Iwasa, Akira, Yamamoto, Natsuki, Imaizumi, Tomohito, Amakasu, Kazuo
• Format: Journal Article
• Language: English
• Published: Tokyo Springer Japan 01.03.2023; Springer Nature B.V
• Subjects: Biomedical and Life Sciences; Broadband; Calibration; Diameters; Echo sounding; Echoes; Echosounders; Fast Fourier transformations; Fish; Fish & Wildlife Biology & Management; Food Science; Fourier transforms; Frequency dependence; Freshwater & Marine Ecology; Life Sciences; Mathematical analysis; Original Article; Spectra; Spheres; Target strength; Tungsten; Tungsten carbide; Waveforms; Linear frequency-modulated signals; Broadband scientific echosounders; Pulse compression processing
• ISSN: 0919-9268; 1444-2906
• DOI: 10.1007/s12562-022-01653-7

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.