Originally Posted by
helispotter
Juan Browne has recently also uploaded the video but now with the audio apparently synchronised with the video (presumably by matching the fuselage striking the water with the sound of such an impact):
https://youtu.be/PFesmc48JNY?si=9srGyELlF--ml_Eg
The caption at the start suggests the frequency of the sound heard during the lead-up to the breakup matches the rotor RPM. Is that really the case? In any case, it certainly reduces in frequency over time suggesting what ever it is is slowing down. I am not sure if video or audio are at 'live' speed. Not even sure the audio is from same location as where this video was taken? To me the sound seems more like a jack hammer being used in the bowels of a ship. The regular sound of a 206L can't be heard beforehand either, why? Was sound of the breakup so loud that it dominated anything else picked up by the microphone in a busy city?
Once the source of audio is validated, should be straightforward for NTSB to analyse its dominant frequency and whether that matches main rotor RPM, tail rotor RPM or tail rotor drive shaft RPM (if different) or anything else, including harmonics.
With the sound synchronisation, the "banging" is already starting before the helicopter emerges from behind the building.
I saw the original video, with the speed of sound delay. There's a sort of dog bark sound very early on, it might be nothing, it doesn't sound mechanical. I'm not sure why the microphone so clearly picks out the diabolical thwopping noise, but the processing on some security cameras does normalise sound, especially distant loud noises, and it can make them sound much closer.