sound wave

Passage says " The 4 strings of a violin all have the same length of roughly 12 inches. in an experiment students studied the 4 strings of an old violin and the four strings produced these frequencies string I : 196 Hz string II: 294Hz string III: 440 Hz string IV: 659 Hz The question asks "...

I am studying from TBR and I'm not sure I understand what the shock wave is. I know that shock waves occur when vobject > vsound but I don't understand what's happening to the pressure why is it increasing and then decreasing? Thanks!

I thought that less dense media made sound travel more slowly, e.g. sound travels slower through water than through a solid. Why, then, would a less dense medium (helium) produce a higher pitch (i.e. faster) sound wave vs. normal air?