The directivity (narrowness) of any wave producing source depends on the size of the source, compared to the wavelengths it generates. Audible sound has wavelengths ranging from a few inches to several feet, and because these wavelengths are comparable to the size of most loudspeakers, sound generally propagates omnidirectionally. Only by creating a sound source much larger than the wavelengths it's producing can a narrow beam be created.
Clearly, having loudspeakers twenty metres wide is not very useful.
therefore ...
to make a narrow beam of sound from a small acoustic source, we instead generate only ultrasound.
The ultrasound, whose wavelengths are only a few millimetres long, are much smaller than the source, and consequently travel in an extremely narrow beam.
