[Tig] OT: audio Highs speed different than Lows?

[Jim Houston]

On Jan 2, 2008, at 11:09 AM, Rob Lingelbach wrote:
>
> I notice the CRRaaackkkKKs start out with high frequencies, then
> reverberate at the low end.  Is this only because of the directional
> nature of the difference in frequencies, or is it because the high
> end travels somewhat faster than the low end?  If the freq. range
> were fully simultaneous, I would think the high cracks would be
> synchronous with the low rumbles, at least for lightning that is
> directly overhead, as it is now.
>

Most of the sound differences are because of the formation method
for lightning with some effects due to distance or travel time.
The lightning strike starts near the ground as particles form
an ionized plasma near an electrical ground.  If you are standing
near the start, you will feel a lot of static electricity in the air  
and your
hair raises on end.     The ions combine and merge in
the air very rapidly (the crackle) to form the main strike.  The main
strike is both very hot and ionizes air itself traveling at supersonic
velocities upwards towards the main ionized air mass.  Side strikes  
follow
along ionized air paths to give branches, many of which also
contribute their own sounds.    The main
strike causes a sonic boom (the low frequencies), which then echo
off of the ground, cloud cover, and buildings, to give you the rumble
effect.    I didn't find any of this out by personal experience, so i'll
just trust the reports of the people who have :-)

The speed of sound is relatively lightly affected by the frequency,  
however,
frequencies attenuate differently with high frequencies falling off  
faster
and more directionally (they don't have as much energy to move the  
air mass).


Jim H.