Why can ultrasound be formed into a narrow beam more easily than ordinary audible sound?

Ultrasound can be formed into a narrow beam more easily than audible sound because of the relationship between frequency and diffraction.1

Sound waves naturally exhibit diffraction, which is the tendency to bend around corners or spread out after passing through an opening. This phenomenon is most pronounced when the wavelength of the sound is comparable to the size of objects in its path. Because audible sound has relatively long wavelengths, it diffracts readily, making it difficult to maintain a focused, narrow beam.12

In contrast, ultrasound consists of very high frequencies, which results in extremely small wavelengths. As frequency increases, the ability of a wave to diffract decreases. Because these short wavelengths do not diffract strongly, they behave more like light, allowing them to be directed into precise, narrow beams. In these cases, the principles of ray optics can be applied, which is why ultrasound is highly effective for medical imaging and other specialized applications.13