Design and Convergence Performance Analysis of Aspherical Acoustic Lens Applied to Ambient Noise Imaging in Actual Ocean Experiment
In this study, an aspherical lens with the aperture diameter of 1.0 m was designed for utilization in an actual ocean experiment of ambient noise imaging (ANI). It was expected that this ANI system would realize directional resolution, which is a beam width of 1° at the center frequency of 120 kHz....
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| Published in | Japanese Journal of Applied Physics Vol. 50; no. 7; pp. 07HG09 - 07HG09-6 |
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| Main Authors | , , , , |
| Format | Journal Article |
| Language | English |
| Published |
The Japan Society of Applied Physics
01.07.2011
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| Online Access | Get full text |
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| Summary: | In this study, an aspherical lens with the aperture diameter of 1.0 m was designed for utilization in an actual ocean experiment of ambient noise imaging (ANI). It was expected that this ANI system would realize directional resolution, which is a beam width of 1° at the center frequency of 120 kHz. We analyzed the sound pressure distribution focused by the designed lens using the 3D finite difference time domain method. The frequency dependence of a $-3$ dB area was then compared between 120 kHz and the higher or lower frequency. The analysis results suggested that the designed lens has fine directional resolution over the center frequency of 120 kHz. We had measured the directivity of the designed lens in an actual ocean experiment in Uchiura Bay in November of 2010. It was verified that the ANI system with this lens realizes a beam width of 1° at 120 kHz. |
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| Bibliography: | Ray diagram for the designed lens. Shape of the designed lens. Analysis domain of the 3D FDTD method for obtaining sound pressure field focused by the designed lens. (a) $x$--$z$ plane at $y=0$, (b) $y$--$z$ plane at $x=0$, and (c) $x$--$y$ plane at $z=0$. Examples of the sound pressure fields focused by the designed lens. (a) $\theta=0$° at 120 kHz and (b) $\theta=7$° at 120 kHz. The range between the point source and lens is 50 m. Examples of the beam patterns. (a) $\theta=0$° and (b) $\theta=7$°. The frequencies are 80, 120, and 160 kHz. The range between the point source and lens is 50 m. These beam patterns correspond to the extracted pressure distributions through the maximum pressure point parallel to the $x$-axis. Examples of the $-3$ dB areas. (a) 80, (b) 120, and (c) 160 kHz. The range between the point source and lens is 50 m. Determination of hydrophone positions. (a) Comparison of the image points at various refraction indexes and object ranges, (b) normalized image points so that the maximum of the $z$-axis is arranged at $z=0$. The legends of "zemax 50 m, $n=0.54$", "zemax 25 m, $n=0.54$", and "zemax 10 m, $n=0.54$" indicate that the media are the pure water (1482 m/s) and acrylic resin (2727 m/s), the temperature is 20 °C, the refraction index is 0.54, and the object ranges (ranges between point source and lens) are 10, 25, and 50 m. The legends of "zemax 50 m, $n=0.555$" and "zemax 25 m, $n=0.555$" indicate that the media are the pure water (1509 m/s) and acrylic resin (2706 m/s), the temperature is 30 °C, the refraction index is 0.555, and the object ranges are 25 and 50 m. The legends of "zemax 50 m, sea", "zemax 25 m, sea", and "zemax 10 m, sea" indicate that the media are the sea water (1523 m/s) and acrylic resin (2731 m/s), the mean water temperature of November is 20.9 °C, the refraction index is 0.557, and the object ranges are 15, 30, and 50 m. Angle beam patterns at 120 kHz measured in an actual ocean experiment. |
| ISSN: | 0021-4922 1347-4065 |
| DOI: | 10.1143/JJAP.50.07HG09 |