1. 1) M. Hamamura and S. Iwamiya, "Survey on the use of portable audio devices by university students," J. Acoust. Soc. Jpn. (J), 69, 331–339 (2013) (in Japanese).
2. 2) K. Kurozumi and K. Ohgushi, "Quantitative expression of spatial impressions for sound image," J. Acoust. Soc. Jpn. (J), 40, 452–459 (1984) (in Japanese).
3. 3) R. Takakura, M. Okemoto and M. Miura, "An effect of environmental sounds on music listening using a headphone," Tech. Rep. Music Acoust. Acoust. Soc. Jpn., 39(8), pp. 43–48 (2021) (in Japanese).
4. 4) K. Saberi, L. Dostal, T. Sabralobai, V. Bull and D. R. Perrot, "Free-field release from masking," J. Acoust. Soc. Am., 90, 1355–1370 (1991).
5. 5) R. Takakura and M. Miura, "Effect of music listening using a headphone to the perception of vehicle's approaching sound," Tech. Rep. Noise Vib. Acoust. Soc. Jpn., 2022-02, pp. 1–7 (2022) (in Japanese).