5. Conclusions
In this paper, we focused on the parametric loudspeaker that is known as a super directivity loudspeaker. The characteristics of the sound localization in the vertical direction are clarified in the listening tests. The sound localization in the vertical direction using the upper and lower parametric loudspeakers is confirmed by listening tests and physical measurements. The vertical direction of sound localization can to be controlled not only when the acoustical axis was set to the right ear but also when it was set to rightward 5 deg from the right ear. The results are similar to those observed when ordinary loudspeakers are used. However, it is found that the left-right sound localization could be realized only with the upper and lower parametric loudspeakers. By setting the parametric loudspeaker the right ear, that is, by setting it only 3 deg to the right, the direction of sound localization moved approximately 10 deg rightward. The dependence of the parametric loudspeakers on the level difference between the upper and lower loudspeakers was weaker than those of the ordinary loudspeakers. Moreover, by setting the parametric loudspeaker 5 deg to the right, the direction of sound localization moves approximately 20 deg rightward. The main cues of sound localization in the horizontal plane are ILD (Interaural Level Difference) and ITD (Interaural Time Difference). In considering this listening test scheme, as the loudspeakers are set in the vertical plane, the ITDs are estimated to be almost zero. Only the ILD plays the main role in perception of sound localization in the horizontal direction. The measured ILD (Interaural Level Difference) of the parametric loudspeaker using the dummy head are analyzed. The obtained ILDs of the parametric loudspeaker are similar in spite of the level differences of the radiated signals between the upper and lower loudspeakers. The ILDs were larger than those of the ordinary loudspeaker. The ILD of the parametric loudspeaker depends strongly on the angle of the acoustic axis of loudspeaker. The dependence of the ordinary loudspeaker is notably weak. Because the higher frequency sound is harder to diffract, the cross talk level from the loudspeaker to the left ear becomes smaller. This finding helps to explain the measured result that the ILD of the higher frequency becomes larger. These listening test results and the measured ILDs with the dummy head lead to clarifications regarding the interesting characteristics of sound localization with parametric loudspeakers.
