I was just outside with my phone and a cool app called PitchLab Lite (for guitar tuning primarily). I was using it to figure out what frequency the thunder was and i was getting readings between 17hz and 29hz. Interesting tidbit if you are into that. I believe it was f0# at 17hz, g0 at 24hz and a1# at 29hz Edit. Just figured out the 17hz wavelength would be roughly 67ft long, 24hz about 47ft and 29hz about 39ft.
Tricky question, im not sure exactly what you are looking for. Closer to the source you hear more in the higher frequency range, thus the snap of a close lightning strike. As the sound wave travels it is dampened (decreased amplitude/quieter) but i dont think the soundwave itself changes in frequency over distance. I could be wrong im not expert i am just fascinated with sound and waves.
Trust me i wish more would be interested and have fun with things like that than the norm. I was wondering just how accurate that app must be and how close one would have to be if they wanted the true measurement. It was only a polite curiosity, really
I wonder too, a phone microphone cant be that great. If you have android check out the app its pretty fun. Test the range of your ability to whistle!
" Usually, the ï¬nite amplitude propagation causes a doubling in the wavelength of the positive pulse within the ï¬rst kilometer, but beyond this range, the wavelength remains approximately constant" Source 'Audible thunder characteristic and the relation between peak frequency and lightning parameters' OuYang Yuhua and Yuan Ping