While watching practice 1 for MotoGP this afternoon, I heard a boom/rumble over the sound of the TV. Our dogs were instantly distressed as well.
Checking the Shake and Boom it is likely that it was a large atmospheric boom rather than an earthquake, as the low frequencies in the seismic signal are quite weak. I checked with a friend in town that nothing had exploded at one of the factories in town. He later reported back to me that another friend had heard the boom also in Oberon which is 10kms from my location.
Ruling out earthquakes and mineblasts as well as local explosions, it is possible it’s a sonic boom from a large meteor/bolide or re-entering space junk.
Only the first boom at 3:04:47UTC was heard. The following two peaks look like infrasonic echoes, but could simply be the tail end of a longer signal as the source moved away.
After a lot of research, it turns out this is just lightning. It turns out, this is the first time I’ve detected lightning since I changed the mechanical filter on the Boom from 1s (1Hz) to 20s. With the 1s filter each lightning strike is quite distinct. With the 20s filter it appears there may be low frequency infrasound generated from both pre- and post-strike charge movement. i.e. the infrasound begins to build before the sound of the lightning strike arrives (indeed before the strike occurs!) and continues afterwards. Afterwards could be the result of echoes, etc but before is hard to explain any other way.
Lightning strike times and positions were obtained from LightningMaps.org. The closest strike was at 03:04:17.558UTC. Lightning strike times are very accurate but positions have a significant error circle plotted around them. So the plotted position from the map in LightningMaps.org is used for the first pass calcs in the report and then the true distance to the strike calculated from the sound travel time.
The sound travel time was calculated at 29.442s, which at 320m/s corresponds to 9.42 kms.
The cluster of lightning strikes on the Newnes Plateau ~100kms away are also at the right time to coincide with the peaks later - at +250s and +430s. Again the low frequencies are not attenuated much so the signals all appear linked at low frequencies.
I have the 1s filter on my BOOM and never thought to change it to the 20s (not that I can experiment with thunder, as it’s pretty rare where I live) to see what differences may pop up. Maybe I’ll do some trials when I have a bit of time.
