Posts about: "Audio Analysis" [Posts: 51 Pages: 3]

The visual evidence of the RAT being deployed (or not) is a bit dubious, but the audio evidence is pretty solid. It's also corroborated by the best eye witnesses we have, the pilots, explaining that they had lost power in their mayday call. And corroborated again by the second best eye witness we have, the passenger who survived, commenting on the 'green flickering lights' of the emergency exit signs indicating power loss. And corroborated again by the (now confirmed) posts on social media from the previous set of passengers complaining that aircon, entertainment, lights etc on the plain weren't working.

The idea that the flaps weren't set for take off also seems incredibly dubious given that in several frames of the original footage you can see the engines through the wings, on both sides, something not explainable by grainy footage or compression artifacts, and only possible if the flaps are down.

Edit to add: the reports on social media from the previous passengers complaining about non-functional electronics have been confirmed

Regarding the sound believed to be the RAT: Some people suggest that the sound might come from the street, not the plane. I'd say the apparent doppler effect heard would indicate otherwise. It would be interesting to do a spectral analysis to check if the frequency drop is indeed compatible with the expected doppler effect. I wouldn't be surprised if several readers have the software and know-how to perform a simple analysis.

What else could it be? A motorbike\x85no. A power drill or other construction noise\x85no. An air conditioning unit\x85no. A cement mixer\x85no. I\x92m struggling to think of anything else you could find in an Indian urban area that sounds almost exactly like a RAT and just happened to be making a sound as an aircraft flew over at low level.

Occam\x92s Razor is in play here.

I hesitate to chip in in these accident threads. Keep them clean. However, as as a few comments above brushes my audio expertise, I will comment.

A very simple audio analysis give me this:
The 3 segments horisontally, are of different videos of B787s passing overhead/landing. The vertical drop you see is the doppler effect.
In other words, these are spectrograms over time which makes these distinctions easier than a simple static spectrogram.
1. B787 landing with RAT extended.
2.Air india crash
3. B787 landing without RAT


It's a 5 minute laptop job, and it would look much prettier and clearer if I spent some time with it, (Gain to color match, and spectrally match to compensate for microphone placement and type),
but it is 85% conclusive even when done as simple as this IMO.
(I do have legal forensic audio experience)
The RAT was out judging from the audio evidence. You can see the the equally spaced overtones of the propelller match when passing overhead resulting in the Doppler effect, the difference in length of the doppler is caused by distance and the slightly varying frequencies shown in the starting point is caused by a difference in speed. But the harmonic content match.
In the 3rd segment you see none of these overtones at all.

There has been much discussion here about RAT deployment. Various claims either way have been made, based on individual perspectives of available video and audio.

I am very mindful of just how awful a tragedy this is, and have significant misgivings about disproportionate interest in others misfortune where it carries no purpose, but also recognise that for some people knowing and learning what happened ASAP could be very important, particularly given the present circumstances.

Thus while I sincerely hope that early detail from investigators will give some clarity, in an effort to reduce needless speculation regarding RAT deployment I have taken:

(1) an audio sample from the video of AI171 passing by in which people claim to hear a RAT
(2) an audio sample from a 787 video with RAT deployed on test by Boeing
(3) an audio sample from a JAL 787 video with RAT deployed

And passed these through a FFT in order to gain a more quantitative view of the noise spectra from each event.

A spectrogram of the results is presented below. I hesitate to make any conclusions per se, but observe that there are similarities as well as divergences between them. In all three samples there is a relatively consistent signal roughly centered in the range 113-146Hz that could be what gives the characteristic 'buzz' sound of (apparently) a RAT in operation.

JAL ~141Hz
Test ~146Hz
AI171 ~113-134Hz (prob doppler variation here)


Spectral comparison AI171, B787 with RAT, JAL 787 with RAT



Spectral comparison #2 AI171, B787 with RAT, JAL 787 with RAT


These frequencies seem consistent(ish) with what I got from this video [[url]https://www.youtube.com/watch?v=O1r3CuRwjPc] in which a 787 RAT is being tested - albeit in this case the blades are hydraulically powered and not driven as a turbine. This test showed a fundamental frequency of 135Hz with relevant harmonics above (the second harmonic at 270Hz is higher SPL, no weighting):




It's important to note that the initial recordings are necessarily different; these are not controlled conditions, the recording equipment is probably quite diverse and almost certainly not ideal, and the environmental conditions will also be different. Moreover all of these audio samples have come from video files referenced here, one has no way of determining the provenance or veracity of these sources and, crucially, I have no prior experience of analysing/extracting RAT acoustic fingerprints (nor have I sampled 'control' audio of a 787 passing by /without/ RAT!).

Additionally it's been quite a long time since I did any work with [turbine] noise so given these and other variables I'm not prepared to make any declarations per se, but perhaps more knowledgeable people could. That said, my feeling from what I see is that RAT deployment is not dis proven, and that the apparent fundamental frequency difference between the samples may be explainable by - amongst other things - difference in a/c airspeed, bearing in mind that AI171 was on TO, the others were landing.

Ultimately what I've done here is extremely rudimentary and while it would be possible to go into much more depth I'd hope that more definitive answer would be forthcoming by then, however if anyone wants to discuss specific methodology etc off-line please PM, no wish to add to noise on this thread.

FP.

At the airspeed this stricken aircraft was at, was there enough airflow to spin the RAT sufficiently to produce a significant acoustic result?

It was hard to let it go, so I spent a bit more time with the audio, using filtering and matching to see if I could be even more sure.

It's a pity uploading audio to this site isn't as easy as uploading photos, but I can say it took very little filtering and matching to make the Air India audio become nearly indistinguishable from audio taken of B787 with known RAT extended during landing.

I can't see it in the photos either, but in these circumstances the audio is a lot more trustworthy, and from my audio point of view RAT deployment is 100% confirmed.

In the off chance that the audio I borrowed from a confirmed RAT event was somehow faked, I plotted the technical data I could find of the B787 RAT (4000rpm, 2 blades) combined with a height estimate and asked the O3 model with deep research to estimate doppler shift and speed.
The result matches the above documented 270-200Hz (in one of the harmonics) Doppler shift observed in 1.7 seconds.

I haven't managed to read every post in this thread, just a large percentage, so please excuse me if these points have already been raised. I'll post each separately in case the mods deem them irrelevant or redundant, in which case, my humble apologies.

First, the RAT. There has been a lot of discussion about this, and I suspect the audio analysis a few posts above pretty well confirms that the RAT was deployed. However, I don't think the following question has been asked, and I believe it's further weight in support of RAT deployment.

Q: Why does the video of the plane passing at low altitude, (potentially taken from an apartment window or balcony door), even exist?

I have lived near an airport flight path for many years, and would never bother to video a passing plane unless I perceived it was truly exceptional. Normally, I don't even look, unless it sounds off course, very loud or some such reason. I have yet to take such a video. Here, of course, I'm assuming that the video-taker lived in or near the building concerned, and was therefore equally used to the sounds of passing planes. I submit that the video was shot purely because the camera man (by the voice sound, but an assumption - maybe not the camera operator's) recognised that the sound he was hearing was indeed exceptional. The video begins with the plane out of the shot, so he 'must' (assumption) have heard it coming, and had time to get his phone and starting videoing. Of course I'm saying that he was hearing the RAT...

Secondly, I have found that different versions of the same video play differently (on the same hardware). In some, I cannot discern the RAT sound - unless I reduce the playback speed. Then, it seems to be very distinct - but whether this is an artifact of the reduced playback speed, I can't say. I suspect that different playback applications and hardware will also have a major influence on the RAT audibility. I am 100% convinced that the RAT sound was captured, but is not audible to all, for one or more of these reasons.

Oh, lastly, I'm guessing the (AI) RAT frequency in the audio spectrum analysis was probably lower because the plane was travelling slower than in the other, more controlled cases. Equally, the load on the AI RAT could have been greater, for any number of reasons.

I was initially going to post that a spectral analysis was a bit academic given that the similarity is plainly obvious just from listening to the audio, but there is something to be said for having visual evidence that 1) the sound had a pitch and distinct harmonics, rather than just being the noise you get from the roar of a jet, 2) the sound had the specific pitch and harmonic relationships of a RAT, and 3) that pitch fell exactly as you'd expect from the doppler shift of a plane flying away from you.

That's a lovely bit of analysis. Is it possible to work out at precisely which point the RAT deployed?

Also, while I'm barely a pilot, I am a qualified cognitive scientist with an interest in memory. Without going into any detail, even assuming no concussion or other insults, given what the single witness has gone, and is going through, I wouldn't trust a word he says about his experiences, especially about timings and orders of things happening. I'm not implying any dishonesty, just a very high risk of entirely innocent confabulation about such a short and intense experience that nothing could prepare him for. Flight crew, with vast amounts of training and years of trying to imagine their way through the impossible, would probably do better, but even then post hoc confabulation would be hard to avoid.

Enhanced audio files from 2 independent people, one of whom has experience in forensic audio evidence, and a screen grab from one of the videos, albeit very blurry. So both visual and aural evidence which given the circumstances I’d say is fairly compelling if not completely conclusive.

Would it be possible to determine from the video audio wheter the edit: engine fan noise (RPM) maches with a normal take off? I know that\x92s a long shot and I\x92m already very impressed by your work, thank you for sharing it.

We can debate over a poor quality photo but the audio evidence presented earlier today is pretty conclusive.. The RAT was out.

Extremely likely the RAT was out:

1. there are multiple frames of the video showing ' something' at the position where the RAT is located under the fuselage
2. the sound in the video which is very similar to a deployed RAT
3. the sound analysis posted here by multiple people
last but not least
4. the aircraft stopped transmitting ADSB data shortly after the rotating. This indicates an electrical failure. Other aircraft ADSB data was picked up by FR24 receivers indication there is a good reception of the signal in that area.

Agree with most of your comments, but Lauda 004, the B763ER/PW4060 aircraft, no evidence was developed to indicate a G/A sensing fault. The report is available on line,

Lauda Air B767 Accident Report

Lauda 004 taught that the flight test of reverse had some assumptions that were not accurate. The reduction in lift from the flow interaction with the wing was not recognised before Lauda 004, and before that was discovered, there were a lot of flight departments that were rather judgemental of the event due to their own ignorance. Just because our sims say it is so doesn't mean that the aircraft agrees.

Read the thread about the audio analysis. Its pretty conclusive. The RAT was almost certainly deployed. Plane crash near Ahmedabad..

Further to the audio analysis of a deployed RAT; given an audio print, known noise source [RAT] frequency, location of a receiver and relative location + direction of moving object, and using the principles of doppler effect etc, it should be possible to calculate [ground]speed of a passing aircraft.

Thus, having been informed that the source frequency of a RAT is 145Hz (thank you that person - please ID yourself if you wish), and with regard to the [assumed] RAT acoustic print available for AI 171 we are close to being able to determine an approximate airspeed during descent (given the video from which we get this acoustic data appears to start during the descent).

There are several variables in play here that we don't know exactly (the positions of observer and aircraft, height agl of observer and aircraft etc), and some I will ignore for the moment (effect of air temperature/pressure for example) but if one makes an educated assessment from the visual cues in the video I calculate an initial relative airspeed of circa 120kn. Necessarily this is an extremely rough number, in fact the range (making various positional assumptions etc) appears to be between ~100kn to ~150kn, but if it turned out be either side of this I wouldn't be completely surprised.

As with the initial audio analysis to determine whether a RAT was deployed or not I offer this for those to whom this sort of data may be important. Not sure if it's of any real use or not, and I wouldn't hang your hat on it, but here it is in case it assists. Should it matter I expect some sort of corroboration could be obtained by making an assessment of vertical speed from the variously available videos and extrapolating from there.

My previous caveats over veracity of source data etc remain.

FP.

No evidence of engine failure - Not true. No engine noise on video where you would expect TO or TOGA power to be in use. Mayday call stating No Power

No evidence of RAT deployment from a poor image . - You can argue Not from the Image but...: 2 independent audio analysis of the video audio shows the sound comes from a deployed RAT plus JB's video. Plus the guys who live in SEA having heard 100's of RATs deployed during test flight have stated that the sound is a RAT.

No evidence of electrical failure . - Not true. Reported cabin emergency lights going off, FR24 feed stopped just as in the 737 South Korea incident in December. APU intake door partially open at crash scene, suggesting an APU autostart.

Now you can call into question the above evidence but to state there is none, is simply not true.

Am I understanding that you are saying that the noise on the video identified as a RAT has a Doppler shift that matches plausible values for height and speed? That seems pretty conclusive to me that it is a RAT.

Correct. That was the original purpose of the calculation. In addition to the sound itself having the measurable harmonic signature from other rat videos.
What this plot also does however is tell you the speed if you know the height or height if you know the speed.

The iphone used to film this were pictured somewhere, knowing the iphone model, and thus the characteristics of the camera, and the dimensions of the airplane it wouldn't be impossible to calculate height from the video imo.

Just throwing it out there if anyone sees the use and feels the call.

My personal amateur speculation still centers around the cut off switches.
I have spilled coffee and sweet tea over complex electro/mechanical switches/panels before(large format audio consoles with 8000 buttons) and seen unexpected things happen.

I am sure the switches are spectacularly well built, but they are in close proximity and thus prone to the same external factors.
Does anyone know if these two cut-off switches in such close proximity has the exact same installation, or they differentiated in some way that makes a freak failure mode in one not neccesarily affect the other the same way?