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:''See sound from ultrasound for a beam of ultrasound that makes audible sound in a restricted target area without a receiving set.'' Directional Sound refers to the notion of using various devices to create fields of sound which spread less than most (small) traditional loudspeakers. Several techniques are available to accomplish this, and each has its benefits and drawbacks. Ultimately, choosing a directional sound device depends greatly on the environment in which it is deployed as well as the content that will be reproduced. Keeping these factors in mind will yield the best results through any evaluation of directional sound technologies. Systems which guide evacuees during an emergency by the emission of pink noise to the exits are often also called "directional sound" systems. == Basic theory == In all wave-producing sources, the directivity of any source, at maximum, corresponds to the size of the source compared to the wavelengths it is generating: The larger the source is compared to the wavelength of the sound waves, the more directional beam results . The specific transduction method has no impact on the directivity of the resulting sound field; the analysis relies only on the aperture function of the source, per the Huygens–Fresnel principle. The ultrasonic devices achieve high directivity by modulating audible sound onto high frequency ultrasound. The higher frequency sound waves have a shorter wavelength and thus don't spread out as rapidly. For this reason, the resulting directivity of these devices is far higher than physically possible with any loudspeaker system. However, they are reported to have limited low-frequency reproduction abilities. See sound from ultrasound for more information. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Directional sound」の詳細全文を読む スポンサード リンク
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