Synthesized head related transfer functions to super near field and psycho-acoustical experiments for closeness

Head related transfer functions (HRTFs) are generally measured by located loudspeakers apart from a listener in equal distance around 1 m, because the loudspeakers should be far enough to be assumed as point sources. When the HRTFs in near field are measured, it is hard to reduce the size of sound source maintaining the output in low-frequency range. To solve this problem, a study proposed resynthesized procedure for near-field HRTFs (distance from center of head: 25 cm) by directional equalization of far-field datasets. In this study, we propose envelope modulation procedure to transform the binaural impulse responses in the near-field to super-near-field (12 cm). The sound convolved with the super-near-field HRTFs (SNF-HRTFs) in horizontal angle 90° can be perceived as if whispered in the ear. An attenuation factor in the envelope played a role in distance variation function. Therefore, we conducted psycho-acoustical experiments to adjust the attenuation factor of the synthesized SNF-HRTFs to measured ones using a small spherical wave speaker in anechoic chamber. As results, the rapidly attenuated SNF-HRTFs were perceived closer to the listeners’ target ears. Therefore, the combined method of the directional equalization and envelope modulation can transform any voice to whispered one.