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

I realise the discussion has largely moved on, but for completeness I have analysed, filtered and compared the audio from several 787 videos\x97both take-off and landing (the latter with and without RAT deployment; take-off footage with the RAT out is rare).

Using the same style of frequency plots employed in the RAT analysis\x97and drawing on speed of different engine components data supplied by other contributors\x97it is straightforward to identify the engines\x92 high-power acoustic signatures objectively, despite variations among clips and the presence of Doppler shift.

What is clear is that the engine noise in the recording bears no resemblance to a typical 787 take-off profile. Whether the engines were merely at very low thrust/idle or fully windmilling I cannot say with certainty, but they were certainly nowhere near take-off power.

Make of that what you will.

The audio analysis in this case isn't anything obscure or vague. The same software is used in courts all over the world every day. You can be pretty sure the same software in some iteration is used investigating CVRs all the time. It shows something emitting the exact same sonic signature as a B787 RAT went over the camera at the same speed as the Boeing 787 seen going over the camera. It isn't a broken fanblade, or a motorcycle passing by, that would give a completely different result. It literally means hundreds of thousands of datapoints fall into the exact same pattern. If it were anything else, that would be easy to observe. It is a much more accurate tool than watching heavily compressed video. It is the same software used in hollywood to surgically remove airplane noises from historical dramas, btw. At least until AI came along and took on that job about 2 years ago. . Just because you aren't familiar with it doesn't mean it isn't accurate.
Just putting this out there, since many might be unfamiliar with this, you included.

As one of the two authors who have offered an acoustic analysis of the AI 171 'RAT' video here I feel obliged to make some observations.

First of all Kraftstoffvondesibel and I did our work completely without knowledge of each other. We reached the same conclusions more or less, most likely using different software (I used four different methodologies) and in subsequent discussions between us we've found our specific results (frequency etc) match very closely - including the doppler shift that we've also both commented on.

While this doesn't absolutely prove the RAT scenario (I was, at least initially, somewhat more circumspect on this matter as anyone reading my first post would find) it does go some distance towards reconciling our respective methodologies and outcomes. Given the confluence and discussion I am now more satisfied that the RAT was deployed than I was before - although for the reasons espoused earlier I totally recognise the [scientific] challenges to this view.

In this regard I too would not 'mind' if our view regarding RAT deployment were proven incorrect. What Kraftstoffvondesibel and I have done is simply use a little science and apply a quantitative analysis to the available data that many had qualitatively argued over. One might hope that, amongst other things, it could have dispelled the RAT/no RAT question to some degree and reduce thread noise, but unfortunately it seems some haven't read the thread through, or perhaps lack analytical reasoning.

This site isn't the place for a full academic paper+peer review and for me the real subject of this thread is the tragedy of AI 171 and what might determinable from what we know in the hope it will be useful to those closer to the coal face. If you are a physicist, scientist or engineer and have the capability then the same data we used is also available to you to do your own analysis. Should you do so we'd welcome hearing the outcome - whether it concurs with our results or not - as this is the scientific way. If you want to include the full nitty-gritty detail of how you've gone about then for a very small percentage of us that might be interesting, but it'd be peripheral to the main issue - and bear in mind that RAT deployment itself is merely an indicator of other potentially more serious issues. It is not likely to be the cause per se !

To conclude, I understand it's a long thread but as far as the RAT question is concerned, I'd encourage you to read the relevant posts at least and if you remain unconvinced, and you have the skills, then why not conduct your own analysis and let us know the results? Otherwise perhaps as a group we could move on from this, remembering that this is a terrible event which at the very least deserves informed useful discussion rather than wild and/or repetitive speculation.

FP.

I don't believe I owe you anything, I believe this is done adequately previously and has already taken up enough time on this thread. I am of the opinion that we have shown the RAT being deployed satisfactory enough to be of use for speculation in this thread. I find repeated comments about the bad video being the only evidence a bit disrespectful, though. Even from a mere physicist. It is based on a spectrogram over time. The source file shows audio up to about 16 kHz, it is unknown whether this limitation is in the file format (ie. 32kHz sampling rate) or microphone. Doesn't matter much. The frequencies above 16kHz is not important in this context as it is not where the sound energy is anyway. The audio will have been lossy data compressed, but it does not affect these prominent properties of the audio. It does make me hesitant to draw conclusions from the parts of the spectrum with more broadband noise and several intersecting sounds. Noise floor suggests 16 bit sampling depth. Spectral resolution? N/A All samples are included. The spectrogram covers the entire frequency range recorded, It shows comparatively the same overtones of the fundamental expected from the technical specifications of the 2 bladed RAT running at it's intended RPM, the doppler characteristics fits completely with a reasonable range of passing speeds and distance to the passing source plotted out. Compareatively, All the harmonics are identical both in pitch and seperation to a recording of a known B787 landing with RAT deployed, while the Doppler fall shows a longer time frame in the landing video taken from a further distance. As expected. The overtones easily discernable in this recording falls in the 220-2700Hz range. Below that, there is other noise centered around 150Hz, which gradually fades towards the end of the recording. This, as far as I can find in available information, fits with an idling or even windmilling B787 engine, but this is not conclusive. This falls in a range of the spectrum where there are other noise sources and the signal/noise is low and of a broader band characteristic, these masking frequencies is where the lossy data compression might play tricks, so I do not weigh that heavily. Recordings of landing B787 without the RAT, shows none off the same characteristics, and completely lack the tonal components and exact overtones shown with the RAT deployed. More importantly, compared to videos of B787s taking off with normal take off thrust, the latter shows distinct tonal elements, but with very different overtones,, both in separation and composition, again possible to relate to known quantities of the rotational speed and elements of the engine at high power. The AI recording shows none of this.

The latest techniques let us separate such things as reverbration from the source, when superimposing the reverberation/ambience and background noise of the AI crash urban environment on the clean, dead open field recording of the known B787 w/rat, they do indeed sound exactly the same to this very skilled and experienced listener. Although this is not courtesy of the computer analysis. It is just another angle of confirmation.

All in all, i think this source audio is excellent. The source is an iphone, their mems based microphones, although noisy shows great spectral balance and is comparable to basic measurement microphones of professional application. There is plenty of information to analyse from in this sample.

And again, I can't see it in the video either, and until I put on some really expensive headphones and fired up the software I was of a different opinion. I bowed to the science.

Edit: I took an extra look, I am prepared to say the fall off at slightly above 16kHz is from the original recording, this is probably a limitation in the microphone, as it is not a hard cut-off before a 16kHz Nyquist frequency as it would be with a 32kHz sampling rate, there is dither noise from 16-20kHz fitting with the source being 16 bit.

While I get your light intent there is a reasonable question there, and one which was posited many posts ago.

There are various reasons why the signatures we obtained are not that likely to be from a motorbike, but to answer briefly - and pictures being 1000 words 'n all that - here's a quick comparative [visual] analysis. At the top is a motorbike passing by away from the observer, the other is AI 171 doing the same thing:




I expect the difference will be clear. That said one could level a number of criticisms at the method I've used to show this, the sources obtained etc (eg. this is motorbike, not a moped as I couldn't find one in the time available), and if you really wanted to fake it one could, but I doubt that's occurred here.

Would say more but I'm short of time and think this is sufficient clear for now.

FP.

Alright, just because that isn't very difficult. The first segment is a Moped/motorcycle whizzing past. Not from the same city, but it is in India, and I listened to a number of them to find a representative one, and the closest I could find in lenght/timing that isn't completely out of bounds with the streets in the area the AI video was filmed. it is taken uncompressed from a professional sound library and is a very high quality recording. The sound source is in direct line of sight, much like the AI recording when the aircraft is presumably just coming into the clear from above the roof. A moped/motorcycle in the AI video, would probably not be direct line of sight down on the street, and would have an even more ambient character, but shorter ambience. The second segment is the Air India video.
One thing we haven't discussed as a characteristic is how the AI recording moves from indirect sound with a shorter ambience, to direct sound, to a very much more ambient distant character in the same amount of time as the large aircraft moving overhead.
The moped/motorcycle in the sample, keeps about the same small street ambience throughout.
Notice how the harmonics are in different places with different spacing and with different relative intensity, the emphasis on the internal combustion engine is lower in frequency than the open Rat, but with added exhaust noise.
Thank you.



I also enclose a doppler distance

Maybe you should have kept sitting on your hands, theory a) in particular is about as risable as the last one you posted. Do you still think engine failure takes 30 seconds for the plane to react?! Thanks to rkenyon for alerting us not to take your post seriously. You still don't believe the RAT was out despite the in depth audio analysis done by an expert in the field. You still think its a motorbike?!

Given they took off at a perfectly normal point, at a perfectly normal speed, I don't see how you come to incorrect derate and low v speeds, or how on earth that would lead to a normal takeoff followed by sinking?

Hi all,

Sorry it’s going to be a long one but seeing the level of competency here, I though it would be the perfect place to get my answers. From the precious messages read and answers received, I have a scenario to run. I am more than happy to be told wrong from point 1). I don’t have the knowledge some of you do.

Please let me first start by saying that I am not trying to incriminate anyone. Hundreds of CRMS debriefs and accidents reports show us that unfortunately sometimes, the holes in that swiss cheese just do line up. It is far too easy for any of us, seating here, to judge any of the sequences happening in a Flight Deck. Mistakes happen, regardless of training and experience. We all do mistakes, every day, in every line of work.

DISCLAIMER:
I know that the consensus is a dual engine failure due to either TCMA bug or any sort of mecanical/software/wear and tear.
I do hear a RAT (I don’t see it though) and I do find the audio analysis quite compelling. It is at the top of my list as probable cause.
I am just exploring another scenario, based on the AC’s profile and state from grainy video and poor audio.

1- Let’s assume that we do all our perf calculation correctly. Is it possible that the OPT would spit up a F15/20 take off with the conditions on the day on a 787?
2- If so, let’s say we have performances for a F15/20 TO in the FMC. Now let’s assume we select F5 for TO (not in the FMC, physically). Would there be an FMC message, or would that trigger the T/O warning on the 787? If it doesn’t, we now potentially have an aircraft on the heavy side, with already a lift penalty on a high density altitude day.
3- Please bear with me, I know so far I have made an awful lot of suppositions and assumptions. Murphy’s law dictates that what can happen will happen albeit not on the same day. As it was answered to me by someone who was obviously seeing where I was going in a previous post, it’s a lot of swiss cheese to line up.
4- Take off roll goes on, Vr F15/20 comes and we rotate at a speed lower than we should for our actual F5 setting. My buddy calls for GEAR UP, I retract flaps to F1. Another lift penalty. Is there enough thrust now, or are we then already to deep on the back end of the drag curve ?

I do understand that this is not testable in a simulator. I am asking if someone with a 787 OPT and/or FCOM and knowledge views this scenario as possible or not (especially regarding the FMC message and the T/O warning). That is all.
thanks for the help !

For those postulating the RAT was not deployed, what counter explanations do you have for the following clues?

• Distinctive RAT sound in the rooftop video, audio analysis here .
• RAT visible in rooftop video, example in this image .
• APU door open suggesting auto APU start, suggestive of a full electrics failure (one of the criteria for auto RAT deployment)
• Loss of ADSB data suggestive of a full electrics failure (one of the criteria for auto RAT deployment)
• Unusual gear forward tilt position, suggestive of hydraulic failure and/or full electrics failure (one of the criteria for auto RAT deployment).
• Loss of all thrust, ie dual engine failure (one of the criteria for auto RAT deployment)

Great summary. I've already mentioned the first below, but I'd add another:

• The existence (and timing) of the flyby video by a young lad who apparently lived where the footage was shot from. With planes flying past every few minutes, why would he choose to film this one, before he could even see it? The video starts with the plane still approaching, out of view, and his position suggests it was unplanned, before he could move to a better vantage point. I say he already knew it was extraordinary - from the sound.
• Eye witness account from the mother of the lad who filmed the flyby, apparently said that the plane was "shaking". I'll assume she didn't know how to describe it properly, and that maybe it sounded like it was shaking, from hearing the noise from the RAT. Or it's a translation issue of a word/s with multiple meanings or used colloquially.

One question - are there two exterior doors to the APU compartment, one on top, one below, presumably inlet and outlet of cooling airflow? I've seen photos showing two open doors, but the lower one could be something else, and busted open during the crash.

On this matter, your numbers fall within the range I earlier calculated from doppler shift on the rooftop video's audio, so yes the numbers make sense and, given the circumstances, are reasonably close together.

I also placed the relevant positional data (last ADSB, video source, resting site) into a GIS application and used this along with the audio stream duration to calculate average speed. Obviously it is necessary to correct the speed of sound for environmental conditions but even with this I wasn't happy with the early results I got. At about this time I came to a view that this information wasn't really going to help anyone much so didn't go any further.

From detail that may be retrieved here the FAA noted that Boeing made the following 'Request for correction' (my bolding to highlight why I quote this):

"Boeing explained that the RAT will remain operational as the airplane decelerates through the minimum RAT design speed of 120 knots, not 130 knots. Boeing expressed that the performance of the RAT was shown to meet the Boeing Model 787 requirement that specifies 120 knots as the minimum RAT design speed. We agree that the RAT will remain operational as the airplane decelerates through the minimum RAT design speed of 120 knots, not 130 knots..."

Again I'm not sure this is of any particular utility now, but is included here in the interests of ensuring as much factual data is available as possible.

FP.