1. 1) T. Yamada, ``Wide spectral sounds from dental drilling equipments,'' J. Acoust. Soc. Jpn. (J), 65, 52-57 (2009) (in Japanese).
2. 2) H. Takahashi and K. Ashihara, ``Report of recent research on ultrasonic exposure,'' J. Acoust. Soc. Jpn. (J), 67, 189-193 (2011) (in Japanese).
3. 3) M. Hiroe, T. Satoh, S. Kuwano, S. Ogata, M. Toida, K. Kurakata and T. Kitagawa, ``The survey of the high-frequency sounds above 10 kHz radiated from the Japanese high speed and conventional railways,'' Proc. Inter-noise 2011, Wed-2-3 (2011).
4. 4) K. Kurakata, M. Hiroe, S. Ogata, T. Satoh and H. Takinami, ``Survey on high-frequency noises in living environments,'' Proc. 2010 Autumn Meet. INCE/J, pp. 253-256 (2010) (in Japanese).
5. 5) ISO 7779, Acoustics – Measurement of Airborne Noise Emitted by Information Technology and Telecommunications Equipment (International Organization for Standardization, Geneva, 2010).