This paper describes a method of designing a multi-beam transducer capable of simultaneous detection in six fixed directions around a transducer and in the direction perpendicular to its radiation surface, using a total of seven beams. Beams in multiple directions around the transducer are realized by appropriate modifications to arrange the positions of the transducer elements and wiring of the elements inside the transducer. Compared to the normal phased array in which each element is connected to electric circuits with a signal line, this method ensures a substantial reduction in the number of signal lines, as it uses 14 lines regardless of the total number of elements. Therefore, it also offers a great simplification in the scale of the beam forming circuit in both the transmitting and receiving modes. We made an experimental model of the seven-beam transducer to verify the validity of the method and measured the basic acoustic characteristics. As a result, we confirmed the desired performance. Studies also were made to determine a method of realizing a transducer with more than seven beams.

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In this study, we used a new integrated measurement system that combines the acoustic imaging sonar of DIDSON (Dual-frequency IDentification SONar) with concentrator lenses, motion sensors, and GPS receivers to find and identify different species of aquatic plants. Two types of experiments were performed at two lakes in Japan: Lake Yamanaka and Lake Yunoko. In the first experiment at Lake Yamanaka, the image was captured with a one-degree concentrator lens. Multi-beam image processing was used to generate 3-D images of aquatic plants. The lens used in this experiment concentrated the vertical beam width within 1 degree. The second experiment at Lake Yunoko DIDSON with a 3-degree concentrator lens was applied, and histogram and spatial spectrum analyses were performed for plant species classification. Three species of aquatic plants, Myriophyllum spicatum, Chara globularis, and Elodea nuttallii, were classified by the parameters obtained from the current methods. The ability to visualize the features of each aquatic plant for species classification, e.g., leaf and branch dimensions, was attributed to spatial spectrum and scattering analyses of the acoustic images. The high spatial resolution of the integrated DIDSON measurement system will contribute protection of endangered species in rapidly changing underwater environment.

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To gain new insights into how dolphins communicate, we created an extremely broadband speaker called the “Dolphin Speaker”. To communicate or to detect their surroundings and prey, dolphins rely on a combination of a variety of vocalizations and vastly better acoustic abilities than humans have. Acoustic studies of dolphins have focused mainly on recording vocalizations and assessing hearing abilities; there have been relatively few playback experiments, which are very important for gaining a better understanding of dolphins' acoustic abilities. This is because there have been no speakers that could project broadband sounds in the range that dolphins use, from several Hz up to more than 150 kHz, although some could project low-frequency sounds or parts of dolphin sounds. In 2011, we succeeded in developing a prototype broadband transducer for an echosounder by using new types of piezoelectric elements that had never been used for underwater acoustic transducers. We applied this technique to our Dolphin Speaker, and it can now project sounds from 8 to 170 kHz within ±6 dB. The Dolphin Speaker's beam width is 17.5° at 120 kHz. The structure of the Dolphin Speaker and the dolphins' playback sounds by using it are also described.

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