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.
Sound waves have dimensions of inches to many feet, which roughly corresponds to the sizes of most loudspeaker systems. At high frequencies, however, the wavelengths are quite short, which can result in a narrow distribution of sound from the tweeters in a conventional loudspeaker system. By making the speaker larger, either through the use of a dimensionally large speaker panel, speaker array, or dome, higher directivity can be obtained at lower frequencies. However, the maximum directivity of any reasonably sized traditional loudspeaker is still quite weak.
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.
Read more about this topic: Directional Sound
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