Fig. 7.10 Relationship between the measured individual scale values of apparent source width (ASW) and the scale values of ASW calculated by Equation (7.5) for each individual. Correlation coefficient r = 0.90 (p < 0.01). Different symbols indicate data obtained by different individual subjects

Table 7.2 Coefficients a and b in Equation (7.5) for estimating the apparent source width (ASW) for each individual listener in Fig. 7.10 and the correlation coefficients between measured and estimated ASWs

7.3 Subjective Diffuseness

Scale values for the subjective diffuseness of sounds are described by the representative spatial factor, the interaural crosscorrelation magnitude IACC.

In order to obtain scale values for subjective diffuseness, paired comparisons were conducted using 1/3-octave band-pass Gaussian noise and by varying the horizontal angle of two symmetric reflections (Ando and Kurihara, 1986; Singh et al., 1994). Listeners judged which of two sound fields were perceived to be more diffuse. A remarkable finding is that scale values S of subjective diffuseness are

Fig. 7.11 Scale values of subjective diffuseness as a function of the IACC (calculated). Different symbols indicate different frequencies of the 1/3-octave band-pass noise: , 250 Hz; , 500 Hz;, l kHz; •, 2 kHz; , 4 kHz. (____): Regression line by Equation (7.6)

inversely proportional to interaural correlation magnitude IACC and may therefore be reformulated in terms of the 3/2 power of the IACC in a manner similar to that for other subjective preference values (see Section 3.1.4), i.e.,

where α = 2.9 and β = 3/2.

The results of scale values obtained through paired comparisons together with values calculated using Equation (7.7) are shown as a function of the IACC in Fig. 7.11. There is great variation in the data in the range of the IACC < 0.5, however, no essential difference may be found in the results for different frequencies between 250 Hz and 4 kHz. The scale values of subjective diffuseness, which depend on horizontal angle, are shown in Fig. 7.12, for 1/3-octave band-pass noises with the center frequencies of 250 Hz, 500 Hz, 1 kHz, 2 kHz, and 4 kHz. The scale values for each individual listener are shown in Fig. 7.13. Clearly, the most effective horizontal angles of reflections depend on the frequency range (Fig. 7.14). These are about ±90◦ for the low-frequency range of less than 500 Hz, around ±55◦ for the 1 kHz range (the most important angle for music), and smaller than 18◦ for the 2 and 4 kHz bands. Such directional reflections for each frequency range can be controlled by using a fractal structure for the wall surface, for an example see (Ando, 1998).

Fig. 7.13 (continued)

Fig. 7.14 The optimal horizontal angles of reflections to a listener for each frequency range for the purpose of decreasing the IACC and thus increasing subjective diffuseness. : Angles obtained by the calculated IACC; /: angles obtained by the observed IACC