What characteristic distinguishes ultrasound from other types of sound?

Ultrasound is characterized by its frequency, which is higher than 20,000 Hz. This distinction sets it apart from regular sound, which typically falls within the human audible range of about 20 Hz to 20,000 Hz. Frequencies above 20,000 Hz are considered ultrasonic and cannot be heard by humans.

Ultrasound has various applications, particularly in medical imaging (such as sonar and echolocation) and industrial uses (like cleaning and welding). By utilizing these high frequencies, ultrasound can produce detailed images and effectively penetrate various materials. The ability to operate above the threshold of human hearing is what allows it to be employed in scenarios where lower frequencies would either be unsuitable or not effective.

Other options present characteristics that do not accurately define ultrasound. For instance, ultrasound is not limited to being heard only by animals nor is it fundamentally inaudible to all humans, as certain individuals might perceive very high frequencies. Understanding this frequency range is key to grasping the unique properties and applications of ultrasound in both natural and technological contexts.