Which range of frequencies are used in ultrasound equipments? Explain various uses of ultrasound.

The ultrasound equipment, familiar for investigating and tracking many medical problems, works at frequencies higher than 20000 Hz. In industry, ultrasound is used for detecting flaws and cracks in metallic and other structures. Some animals can here sounds of frequencies higher than 20, 000Hz. Dogs can also hear sound of frequencies higher than 20, 000Hz. Some people use special high frequency whistles, which dogs have this ability. Therefore, the police uses high frequency whistles which dogs can her but human cannot.