Editing Directional sound (section)

== 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 {{cn|date=March 2015}}. 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.