Recording rocket launch

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Michaelk2423
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Recording rocket launch

Post by Michaelk2423 » Mon May 27, 2019 4:52 pm

I work for a certain aerospace company and record the launches/landings(for my own personal interest) from a couple hundred feet away from the launch pad with a Gopro 5 Black. While the gopro works amazing for video/audio, my intention is to record better quality audio with an H4N Pro I just purchased for some nature field recording. I realize there are probably much better devices to accomplish this, but this is what my budget allows for and I would rather not invest a lot of money into something that may easily get destroyed. The launches are, of course, very loud and violent. I'm new to the audio recording field and have no experience in microphones/recorders other than 2 recent recordings with the H4N Pro of some morning nieghborhood birds which sound amazing.
So, my questions are:
1... Would an external microphone connected to the zoom be advisable and if so which type? (Believe it or not the gopro audio of the launch is actually pretty decent)
2... How can I avoid the audio from being overexposed from the sudden increase in decibels from the launch and the sonic booms when the first stage flies back to the pad?...while still picking up the fainter subtleties from a fueled groaning beast
3...I can't be on site once the rocket is being fueled up, so it needs to be either turned on before I leave or be on some sort of timer that powers it on(may be 8 hours before rocket lifts off). Is anyone familiar with a way to rig up the zoom to power on at a specific time and start recording? I have a device that is programmable for the gopro and turns it on at a desired time, records, and turns it off. I doubt there's a product like that for audio at this point in time and I have spent a considerable amount of time researching for something that can, but no luck. If I have to rely on just leaving it turned on and recording for possibly 8 hours, batteries wont last and not sure how large of sd card would be necessary. So, I will need to get it set up with an alternate portable power source. Any suggestions for portable power? Thanks in advance for any help on this
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Re: Recording rocket launch

Post by Wulfraed » Tue May 28, 2019 4:20 pm

Michaelk2423 wrote:
Mon May 27, 2019 4:52 pm
1... Would an external microphone connected to the zoom be advisable and if so which type? (Believe it or not the gopro audio of the launch is actually pretty decent)
What audio exactly are you after (though given #3, probably not environmental/surround). A mono shotgun, or M/S type shotgun, might be desirable to focus more on the launch itself... Problem, AIM -- you might have to angle the microphone upwards some to handle the tracking of the rocket.

Shotgun type microphones have a narrower angle, so will be less affected by sounds to the sides... But that also means if you aimed straight at the pad, they'd fade away much sooner as the rocket climbs.
2... How can I avoid the audio from being overexposed from the sudden increase in decibels from the launch and the sonic booms when the first stage flies back to the pad?...while still picking up the fainter subtleties from a fueled groaning beast
There is a limited dynamic range available (AGC is going to lose the "subtleties" as it will attempt to make all inputs record at near the same level).

You will basically have to learn what gain level is needed to keep the PEAK around -12 to -6dB (this gives a small bit of headroom for transients that don't show on the meter). Recommend recording in 24-bit mode, to provide a bit more dynamic range (though you might start getting white noise from pre-amps).

Post-process with an audio compressor adjusting the rate and threshold for best effect, with post effect gain added. For example, threshold -24dB, rate 6:1 would convert -24dB to -24dB, but 0dB would convert to -20dB -- flattening the loudest sounds (lower dynamic range). You then add 20dB gain moving -24dB to -4dB, -20dB to 0dB -- and raising everything below -24dB, making the lowest level "subtleties" up into a more audible range.
3...I can't be on site once the rocket is being fueled up, so it needs to be either turned on before I leave or be on some sort of timer that powers it on(may be 8 hours before rocket lifts off). Is anyone familiar with a way to rig up the zoom to power on at a specific time and start recording? I have a device that is programmable for the gopro and turns it on at a desired time, records, and turns it off. I doubt there's a product like that for audio at this point in time and I have spent a considerable amount of time researching for something that can, but no luck. If I have to rely on just leaving it turned on and recording for possibly 8 hours, batteries wont last and not sure how large of sd card would be necessary. So, I will need to get it set up with an alternate portable power source. Any suggestions for portable power? Thanks in advance for any help on this
If it is to be synched with video, you probably want to use 48kHz.

You'll likely also want to fit out a large 6V external battery as a power source if it has to sit long. (2.5AH @ 3V -- 2 NiMH AA cells -- is rated for 6-8 hours, I'd probably recommend a 6AH 6V).

2channel * 24bits/sample * 48000samples/second / 8bits/byte => 288000 bytes/second

32GB card should hold about 30 hours. But ensure the H4n starts new files seamlessly (and doesn't need manual restarts), as any single file is limited to 2GB
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Re: Recording rocket launch

Post by Jim_Fogle » Wed May 29, 2019 11:41 am

Most audio recorders will record for only an hour to an hour and a half per recording because of the way the device operating system manages audio files.

You'll want to run a test recording to discover how the H4N Pro manages long duration recordings.
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Re: Recording rocket launch

Post by Wulfraed » Wed May 29, 2019 1:42 pm

For the H4nPro, page 63 of the manual (AUTO REC) may be applicable. You'd have to set the start level to something above ambient noise (which could mean losing those "subtleties"). Not sure if you'd want to also use "auto stop". I don't know if this function would do multiple start/stops (which, if it did, could be useful if there is a gap after launch and before descent/return that doesn't have anything worth recording).

I'd also recommend NOT using the built-in compressor -- unless you know for a fact that the parameters are useful

Don't user "REC LEVEL AUTO". That mode reduces gain whenever the input is higher than -6dB. It ensures you don't overload the input, but will give you a false recording that recorded level variations are not 1:1 with actual input signal). It only reduces gain, so in that aspect is better than a full AGC (which will move the gain in both directions trying to keep the overall recording at -6dB).


Suggest running a test with wall-power... Just set the recorder parameters (bit-depth/sample rate -- note: I presume WAV format) and see what happens to the recording after a few hours: does it close one file and start a continuation file or just stop totally.

Of course, if you don't need the best quality, you can record in MP3 format, wherein the 2GB file limit can record much longer. 24@48k stereo => 2304000 bps, vs MP3 at 320000 bps means MP3 can hold seven times as much in a 2GB file.

The H4nPro manual doesn't say anything other than having a 2GB file limit. In contrast, the H2n manual explicitly states that it can fill a 32GB card in one session (and supposed to run up to 20hours on a set of batteries in 16/44.1 WAV stereo) -- it will start a new file every time it fills a 2GB file.
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