The frame is constructed that the Mics listen in the thin air/smoke

..
so the sound of impact will travel trough the air, it is not measuring the surface
vibration..
Depending on the target material the sound should be a hard thumb,
this i say based on me being on a military spec shooting range where you
sit bellow the targets in a bunker, and there i could hear a very distinctive thumb
sound on impact..

But yes, i did not smoke anything on the speed of sound :-)
the calculations maybe.. :-)