Recently I found out that if you hold an object, in my case a piece of cardboard, close in front of the pop-up flash (1 - 2 cm), the flash makes a "pop" sound when it fires.
I also felt a slight tap on the cardboard. The flash is not touching the cardboard, it's just very close.
I've tried recording the sound, but it's not very well audible (or visible on a spectrometer). I would describe the sound as being lower (in frequency) than the shutter click and it sounds as if you say the letter "p".
How does the flash give the cardboard such a tap and why does it make the "pop" sound?
Addition: Is it bad for my flash when I hold objects so close to it that the "pop" is heard?