The sound waves used to sonar are?

infrasound
ultrasound
audible

Sonar is, in fact, ultrasound, and works in the same way as an ultrasound device can create an image of a developing fetus. It is not audible, as we cannot hear it, and not infrasound, as infrasound falls into the category of audible sound (we can hear it, although it is at the low-frequency limit of our hearing).