Here's a breakdown:

* 20 kHz is the upper limit of human hearing.

* 20 MHz is the typical upper limit for medical ultrasound.

However, the specific frequency used can vary depending on the application. For example:

* Medical ultrasound: 2-18 MHz

* Industrial ultrasound: 20-40 kHz

* Sonar: 20 kHz - 100 kHz

Therefore, it's inaccurate to say there's a specific number of hertz used in ultrasound. It's a range of frequencies depending on the purpose.