A new method of analyzing the stereophonic sound field has been developed. The characteristic function of this analyzing method is the possibility both to evaluate the quality of a sound image and to calculate its direction, by paying attention to the relation between the amplitude ratio (ΔP) and the phase difference (Δφ) of the signals at the entrance of the external auditory canals of both ears of a listener (See Fig. 3, 4, 5, 6 and Eq. 1, 2, 3, 4). Fig. 4 shows the relation between ΔP and Δφ represented on the ΔP-Δφ plane when a real sound source (f=500 Hz) moves along the circumference of a listener. The representational relation of amplitude ratio and phase difference between two loudspeakers and both ears is shown in Fig. 6, obtained by Eq. 1, 2, 3, 4. The propriety of this analyzing method is discussed by experiments of subjective evaluation. Results of the subjective evaluation of sound image quality and the sound image direction are shown in Table 1, Table 2 respectively, which show no clear tendency of the sound image quality and sound image direction. The ΔP-Δφ analysis, however, can obtain clear tendency shown in Fig. 9 and Fig. 10. As the first example of application of this analyzing method, the "180゜natural sound image calizer" has been developed by which a natural sound image can be obtained outside of two long speakers (See Fig. 13, 14, 15, 16). As the second example of application, the conventional methods of using microphone systems so as the MS microphone system, the spaced mecrophone and the dummy-head system, are analysis in quality and direction of a sound image, for which it can be said that the conventional method of using microphone systems is proper (See Fig. 17, 18, 19, 20, 21, 22, 23)
