1. 1) T. Ito, Acoustic Engineering (in Japanese "Onkyo Kogaku") (Denki-shoin, Tokyo, 1977).
2. 2) H. F. Olson, Acoustical Engineering (Van Nostrand, New York, 1957).
3. 3) M. Kobayashi and Y. Takahashi, "Selective distance recording using acoustic lens," Acoust. Sci. & Tech., 41, 654–662 (2020).
4. 4) Y. Ouchi and Y. Yamasaki, "Characteristics of acoustical horn with ring-shaped aperture," J. Acoust. Soc. Jpn. (J), 73, 5–11 (2017).