1.   One actually uses ultrasound, i.e. the frequency is far above the audible range.

2.   The sound, below the audible range, is generated by a high-velocity stream of air being forced through an organ pipe.

3.   These in turn produce a lower frequency within the audible range that is amplified and converted into sound by a loudspeaker.

4.   Ultrasound frequencies lie above the audible range, and like X-rays they penetrate solids.