Ultrasonic sound refers to sound waves that have frequencies above the upper limit of human hearing, typically above 20,000 Hz. The small wavelength of ultrasonic sound contributes to its high directivity through a phenomenon known as diffraction.
Diffraction is the bending or spreading of waves as they encounter obstacles or pass through openings that are comparable in size to their wavelength. When a wave encounters an obstacle or passes through an opening, it tends to spread out in all directions. However, the extent of diffraction depends on the size of the obstacle or opening relative to the wavelength of the wave.
For ultrasonic sound, which has a small wavelength, the obstacles or openings need to be relatively large in comparison to cause significant diffraction. This means that ultrasonic waves can travel in relatively straight paths and exhibit a high degree of directivity.
Due to their small wavelength, ultrasonic sound waves can be focused into narrow beams, similar to how light can be focused with a lens. This property is particularly useful in applications such as ultrasonic imaging, non-destructive testing, and industrial processes like ultrasonic cleaning. By focusing ultrasonic waves, it is possible to direct them precisely to specific locations or target areas, enhancing their directivity and enabling more precise control and application of the energy carried by these waves.
In summary, the small wavelength of ultrasonic sound allows for high directivity by reducing diffraction effects, enabling the waves to travel in relatively straight paths and be focused into narrow beams.