Chapter 7
Spatial Sensations of Binaural Signals
Spatial sensations include the apparent location, apparent source width, and subjective diffuseness (envelopment) of sounds. These are described by the multiple spatial factors extracted from the IACF for the signal arriving at the two ear entrances. Neuronal correlates of spatial factors (LL, IACC and τIACC) observed in SVR, EEG and MEG signals (Section 5.6) were predominantly associated with the right hemisphere.
7.1 Sound Localization
7.1.1 Cues of Localization in the Horizontal Plane
The perceived direction of a sound source in the horizontal plane can be expressed in terms of the spatial factors extracted from the IACF, such that
LHorizontal= SR= fR( ll(0), rr(0), IACC, τIACC ,WIACC) |
(7.1) |
where ll(0) and rr(0) signify sound energies of the signals arriving at the leftand right-ear entrances. Interaural delay time, τIACC. is the most significant factor for horizontal localization among the five spatial factors in Equation (7.1). A second major factor, is the interaural level difference, which is a function of ll(0) andrr(0) based on the difference between the energies at the two ears. A well-defined, clear direction is perceived when the normalized IACF has one sharp maximum with a large value of IACC, and with a narrow (small) value of WIACC. These conditions are typically produced by high frequency components above 2 kHz. On the other hand, subjective diffuseness ranging from the lack of a clear direction to the absence of any impression of spatial direction corresponds to a low value of IACC, e.g. IACC < 0.15 (Damaske and Ando, 1972) and a wide set of associated delay times WIACC. These conditions are typically produced by low frequency components.
Y. Ando, P. Cariani (Guest ed.), Auditory and Visual Sensations, |
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DOI 10.1007/b13253_7, C Springer Science+Business Media, LLC 2009 |
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7 Spatial Sensations of Binaural Signals |
7.1.2 Cues of Localization in the Median Plane
Apart from these five spatial factors of the localization in the horizontal plane given by Equation (7.1), of particular interest is the localization in the median plane. In this plane, these spatial factors from the IACF are not significantly changed as a function of elevation (i.e., due to almost symmetric shape of the head and pinnae), so that IACC is almost in unity, τIACC is zero, and ll(0) = rr(0) = constant, and WIACC = constant, which depend only on the spectrum of the source signal. It has been believed for a long time that the cues must be found in the spectrum of the sound signals. However, it is hard to find distinct cues in the head-related transfer function (HRTF) in the spectrum as shown in Fig. 7.1 (Mehrgardt and Mellert, 1977). In fact, the temporal factors extracted from the early delay range of the ACF of a sound signal arriving at the ear entrances might act as cues (Sato et al., 2001).
Fig. 7.1 Measured head related transfer function (HRTF) for the different incident angles η in the median plane