Ultrasound is not included in the electromagnetic spectrum because it is a type of mechanical wave rather than an electromagnetic wave. The electromagnetic spectrum consists of waves that propagate through space without the need for a medium, such as light waves, radio waves, and X-rays.
Ultrasound, on the other hand, involves the vibration of molecules in a medium (like air, water, or body tissues) to create sound waves. These sound waves have frequencies higher than the upper limit of human hearing, typically above 20 kHz, but they require a material medium for transmission. This fundamental difference in propagation is the key reason why ultrasound is categorized separately from electromagnetic waves.
In summary, while ultrasound utilizes waves for communication and imaging, its reliance on a medium for propagation sets it apart from the electromagnetic spectrum, which encompasses waves that can travel through a vacuum.