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Explosion Sounds and the World Trade Center - Twin Tower Collapses



version 1.45b


Introduction


Many witnesses to the collapse of the Twin Towers of the World Trade Center in New York City describe at least one "explosion" at the time of the collapses. A few eye-witness accounts specifically describe 3 explosions at the initiation of the collapse, whilst others some describe a "crackling" sound. Many people describe hearing a "pop-pop-pop-pop..." or "bang-bang-bang..." as the towers came down1.

The reality of there being some kind of explosive events coinciding with the demise of each building, as reported by eye witnesses is well documented. However, any word of explosions essentially disappeared from mass-media reports of the the attacks very quickly and to this day are not part of the official narrative. Mainstream scientific attempts to describe the collapses as unforeseen catastrophic engineering failures do not bother to take into account the widespread reports of explosions.

Though video clips of the collapses can be found all over the internet and on video releases, most people will not really hear evidence of explosions on the available footage. What most people would describe hearing from the available media is the "roar" of the buildings coming down. The main reason for this is probably because there is not much actual audio content available of the events. Most angles of the collapse are presented without any sound other than that of news anchors, reporters, interviewees, narrators, etc. speaking over-top. It is the preferred style of news-media to constantly have human voices "giving shape" to history as it unfolds. Creating meaning rather than reporting facts.

Some examples of video footage which includes audio can be found and this audio does include evidence of explosions, though it is not generally obvious for a number of reasons. Sounds of intense volume recorded at close distances will tend to overload and be distorted by the time they make it onto tape. If a very loud sound such as an explosion overloads the camera's sound circuitry and is followed very quickly by subsequent loud sounds, the individual sounds will be more difficult to identify because the shape of each sound, the attack and decay, will be masked as the audio circuits are completely saturated with signal. Results will vary depending on camera type, microphone type, specific settings, and proximity to the event. Some angles of the collapses are cut very short so they start playing only after the initiation of collapse sounds, perhaps because some videographers did not capture the first moments on tape or as a result of the news tending to only focus on the "juicy" bits in their presentations. Another problem with internet videos is that when there is sound, it is usually missing a lot of information and/or is distorted from various forms of data compression and/or sloppy transfers. Another problem with internet video in terms of close scrutiny can be audio which is encoded or played back slightly out of synch with the picture.

From listening to audio with evidence of explosions, it seem that the intense "roar" associated with the towers coming down actually begins before any signs of typical explosions occur. This would raise many more questions about the nature of the building collapses and what actually caused them to come down. The initial "roar"/"rumble" might have masked the initial sounds of explosions in audio recordings and even to the ears of witnesses to varying degrees.

The power of suggestion can have a very strong influence on what we think we are hearing and on what we remember hearing. Interpreting what we hear through audio playback equipment (and even in reality) will rely heavily on what our eyes see at the time and how the audio has been contextualized, pre-interpreted, and packaged for us. A dramatic example of this is how we can be made to hear hidden messages in songs which are played backwards; we will generally just hear nonsense in a backwards passage until someone points out to us what they want us to listen for, at which point we will hear what we are supposed to, clear as day, even though it may a vague, chance similarity. With this in mind, it is important to approach the evidence of audio recordings with very careful listening and analysis to try and be clear about what is actually there.

With all the available eyewitness testimonies and the fact that clear signs of explosions can be observed on the limited amount of available material, it should be apparent that some type of explosions did occur. Whether caused by bombs or not, these explosions would certainly seem to have something to do with the collapses of the towers.

What follows is a basic analysis of some audio clips of the collapses, taken from video clips which are available on the internet and video release. Reasonable full-range speakers or full-range headphones should be preferred for listening as computer speakers tend to have quite a limited frequency range.


Note: click images for larger file. embedded audio is mp4 format, click link for uncompressed.


Part I: The South Tower Collapse -- Audio Recordings




Trinity Church




**update** discussion of the authenticity of this clip's audio can be found on "this page" since the audio portion has most certainly be "dramatized" for the Dolby 5.1 format from whence it came.
The first video is screen-marked "10:28am" and is a very brief clip of the south tower collapse, recorded close by from the south east next to Trinity Church2. Though less than 5 seconds long, this clip offers fairly high quality audio, the most pronounced sound being that of 2 gigantic explosions. They are unmistakable and sound like 2 violent thunder claps.


video:
southtower.trinity.avi

640x384 - 900KB - AVI


audio only:
converted to 44.kHz 16bit - stereo - aiff - uncompressed
southtower-tinity1.aif
4.73 seconds - 820KB
preview mp4:




The explosions, particularly the second, bear a remarkable resemblance to those of a conventional building demolition within a city of other high-rise buildings. Footage of the explosive demolition of the J.L. Hudson Department Store Building, also a very large building which once stood in Detroit is given for comparison. Camera distance could be estimated at roughly 200 meters away from the building.


jlhudson.mpg

29 seconds - 4MB - MPEG


audio only:
jlhudson.aif
29 seconds - 4.7MB - AIFF
preview mp4:




When looking at visual content of the Trinity Church clip, starting at 2 seconds into the video, the South Tower mechanical floors at levels 75 and 76, where there were no offices, violently explode outwards. This is particularly noticeable on the east face (right) face of the building as the whole line of floors pops out. Material can be seen ejecting from the south and north faces also in this clip. Estimating the distance of the camera to these floors at about 450 meters, we would expect the sound to take about 1.3 seconds to reach the camera2, which in fact, it does.

The mechanical floors of the North and South towers (numbered 7-8, 41-42, 75-76, 108-109) had thicker concrete and steel floors and were supported by a much more robust construction than the normal office floors. They can be seen as darker bands on each tower's façade where there were vents rather than windows.



Thick steel I-beams were used instead of the criss-crossing floor trusses used in the most of the towers' floors because of the need to house heavy equipment and engineering considerations of over-all building stability. The fact that these floors offered no noticeable resistance to the progressive collapse of the buildings remains unexplained. The evidence of the Trinity Church clip by itself would suggest that at least the 75th and 76th mechanical floors were destroyed by high-explosives. This possibility has been widely discussed in other articles which visually analyse the same explosion as seen from different camera angles. Opponents of the various "demolition theories" contest that the placement of a large number of explosives within the building could never go unnoticed by building tenants, however, since there were no offices or even windows in the mechanical floors, the undetected placement of a large amount of explosives would not present such an extreme logistical problem as is imagined.



A useful tool for analysing audio is a spectrograph or sonograph which gives a visual display of frequency response over time.

A quick demonstration of how a device like this works can be seen here: http://www.documentexperts.net/video/voiceids3.wmv
An explanation of spectrograms here: http://en.wikipedia.org/wiki/Spectrogramhttp://en.wikipedia.org/wiki/Spectrogram


Sonograms (spectrograms) of the south tower collapse audio and the J.L. Hudson building demolition audio show numerous similarities. Sudden spikes of blast impulse noise running from the lowest frequencies up through the high frequencies can be seen in both sonograms. They are most intense in the lower registers and taper off as they go up. The similar manner in which these spikes cover a continuous range of frequencies from low to high should be quite telling.

Trinity Church clip sonogram: J.L. Hudson clip sonogram:

Immediately preceding these blast impulse noise spikes are blank spaces where the background noise is momentarily muted out. This could very well be an effect due to the explosive shock wave hitting the microphone before the sonic compression wave of the explosion is registered. Another similarity between the two audio files can be seen in the way explosion sounds decay as a curve from high to low frequency in quite the same shape.


closer views:

Trinity Church:


These next two sonograms are in the same time and frequency scale:

Trinity Church: J.L. Hudson:


The Trinity Church clip is much noisier than the J.L. Hudson clip over all. Besides the fact that there was of course much more background noise that day in New York, the Trinity clip was a handheld shot rather than being filmed from a fixed angle on a tripod.



West Street



The next south tower collapse clip was filmed from West Street between 1 World Financial Center and the Banker's Trust Building, placing it a fair bit closer to the base of the tower than the Trinity Church clip. It's a much longer clip and would seem to catch the first startling sounds of the collapse as can be seen by the reactions of the firemen who were initially being taped. The quality and clarity of the sound would seem to be less than that of the Trinity clip and the sound of explosions may not be immediately apparent. Never the less, repeated and careful listening does show there to be 2 obvious and 1 or 2 less obvious explosion sounds within the first 5 seconds of the clip. The audio is dominated by the "roaring" sound which is what most people would initially tend to only hear at first. Cutting and boosting different frequencies with the help of an equalizer can help with the discovery of the explosion sounds by offering a number of different, sort of sonic perspectives to the listener.

video:
southtower.weststreet.mpg

688x512 - 20sec. - 4.3MB - MPEG


audio only:
converted to 44.kHz 16bit - stereo - aiff - uncompressed
southtower.weststreet1.aif
19 sec. - 3.2MB
preview mp4:



beginning of clip with noise filtering, dynamic compression, and eq:
southtower.weststreet2NR.aif
9 sec. - 1.4MB
preview mp4:




The very first sound on the clip is a "tsss" or "click" which should be disregarded as it is likely just an audio file compression artifact which is not uncommon with certain types of compression for internet audio. Also right at the beginning of the file is a sort of "musical" sound which is probably an emergency vehicle siren in the vicinity (or maybe the tail-end of some incidental music on the video from where the clip was ripped). The first explosion can be heard at about 0.49 seconds and can be characterized as something like a "ka-boom". The second and less obvious sound is a "ka-thud" starting at 1.44 seconds and the 3rd is a "bang" heard at 2.45 seconds. The 4th and most obvious explosion sound is like a "ka-chüung" at 3.79 seconds and can be likened to the "ker-plunk" sound of slapping your hand into a bath of water.
Careful equalization reveals a sort of continuous "1-2-3-4..." / "boom - boom - boom - boom..." or "thumping" sound at a steady 1/2 second rhythm. It sounds mechanical like some sort of trash-compactor and could be the sound of "pancaking" floors, the echo of explosions off neighbouring, other smaller explosions, or perhaps some sort of recording / compression anomaly.

Though the major explosions can be heard with attentive listening, they are not as obvious as one might expect they should be. Compared to the Trinity clip, this might partly be a result of the camera being closer and being less obstructed from the tower by other buildings. Besides being masked by the over-all "roar" saturating the audio circuits of the camera, the following might also offer some explanation:

The rapid onset of intense noise may cause such noise to sound less loud than is indicated by its power spectrum and to act as if it has effects at high audio frequencies disproportionate to their representation in its spectrum (Coles 1980).
[...]
“Blast” noise is impulse noise from an explosion and results in shock waves with pressures (> 150 dB SPL) that no longer conform to the laws of ordinary acoustics (Henderson and Hamernik 1986). Blasts from military activities include the noise of detonating propellant from a gun (muzzle blast) and the noise of exploding shells or bombs at the target. Peak sound pressure level alone is often used to describe blast waves but is an inadequate descriptor for many purposes (Pater 1981).

--Effects of military noise on wildlife: a literature review, Ronald P. Larkin.

A sonogram of the West Street clip audio is seen to be noisier than that of the Trinity clip. The sonogram would seem to indicate many more explosive sounds than can be heard on its corresponding audio file. Some of the features might be attributed to the echo of explosions off of neighbouring buildings.

West Street left channel: West Street right channel:

Overviews of the left and right audio channels show signs of the first 4 explosions and also other similar features later on in the clip. Careful listening suggests that some these, the most pronounced ones, could be other explosions.

West Street left channel overview: West Street right channel overview:



7.6 seconds and 8.5 seconds show there to be what we might call the two largest discernible explosions of the whole recording. They are actually quite audible on the sound-track and can be described as a pronounced "slap!" or "bang!" sound, quite similar to that which can be heard at 3.79 seconds, only much stronger. The sounds coincide with shouts of "run!", or "go!" from someone near to the camera, and the second explosion is actually masked by one of these yells, making it more difficult to hear on the recording. Having the shouts of people coincide exactly with nearby explosions would seem like a pretty natural occurrence as people react to being hit by forceful shock-waves.

West Street - 2 largest discernible blasts:





A significant visual feature of the West Street clip is the appearance of "jets" of air and/or other material emanating from the çde, far bellow the zone of destruction above.



As has often been pointed out, the same sort of thing is quite a common sight with explosive demolitions, though they have also been explained merely the result of air and debris being pushed out of windows from the pressure of the collapse above.



The two isolated jets seen in on the south face in the West Street clip may also coincide with some smaller explosion like sound events which can be seen at 4.58 and 6.5 seconds. They are quieter than, and happen after the first 4 most prominent explosions. They are somewhat visible on the right channel overview sonogram.




Another apparent feature in the West Street clip is a cloud of white smoke hanging at the base of the south tower. Perhaps this had something to do with the collapse or was the result of a separate fire in the area. There are reports and some video footage of car fires near the base of the towers prior to the first collapse.



Varick Street



A rather famous view of the North Tower collapse comes from the documentary "WTC: The First 24 Hours". Also, a view of the South Tower collapse from the same angle is available. In this clip, the South Tower can be seen shrouded in smoke and standing behind the North Tower. It was filmed from somewhere off of Varick Street at a distance of approximately 1600 meters3. Though obscured by smoke, the top of the South Tower can be seen tipping over and falling down.

southtower.varickstreet.mpg

300x200 - 29sec. - 4.9MB - MPEG1



The audio quality is fair but distortion is heard in the high-end during the louder parts, a result of the levels being set too high on the camera or somewhere along the various stages of transfer from camera to editing system to DVD to internet video (generally this sort of thing happens in the last stage when being compressed to the internet). Listening to this particular clip at half volume helps the distortion problem a bit. There is a strange reverberance in the audio which can also be heard in the North Tower collapse recording from the same angle as transferred to internet video by multiple sources. This effect is probably because the microphone was located inside a room which caused a slight echo to the sound coming through the window. A similar effect can also be caused by post-production filters which take mono recordings and emulate a stereo sound by introducing a slight (a few milliseconds) delay between the left and right channels.

audio only:
converted to 44.kHz 16bit - stereo - aiff - uncompressed
southtower.varickstreet1.aif
29 sec. - 4.9MB
preview mp4:



The most noticeable sounds in the clip are that of people and vehicles on the street as well as a helicopter in the vicinity. Since the higher frequency sounds of explosions are attenuated by the air over longer distances, it is the lower frequency content which could be expected to carry the most evidence of explosions. A look at sonograms of this clip consistently shows everything below 170Hz to be generally quieter than the rest of the spectrum suggesting the low-end was rolled off by a low shelf-filter. This can be done during post-production but is very commonly facilitated by a switch which is built into many professional microphones. A low-frequency cut is very helpful in reducing unwanted bass from handling and wind noise. Further, the bands between 140Hz and 170Hz appear to have been notched out at some point during the journey from microphone to the example shown here or perhaps as a result of the microphone/camera's particular character or other factors.



Varick Street overview: Varick Street overview - low frequencies:


Sonograms of the Varick Street clip show a number of unique individual bursts of intense sound in the low registers. Some seconds before these bursts begin a low-frequency rumble can be seen.

Varick Street initial rumble:


Looking from the beginning of the clip, we see a low frequency rumble starting at 5.91 seconds centred around 55-75Hz. At about 8.1 seconds it appears to rise in pitch until centring around 85-108Hz at 9.6 seconds from where it would seem to continue on. This may be connected to the collapse of the building but it could also have something to do with the sound of the approaching helicopter heard some seconds later in the mid to high-end of the audio. At any rate, the rumble begins at 3.4 seconds before the first visual signs of collapse at 9.33 seconds (9 seconds and 10 frames in to the clip).

The following sound clip has been equalized and the low-frequencies expanded to make them more audible and show a bit of the character of the initial rumble. Full-range audio equipment will be needed to hear anything in this clip


converted to 44.kHz 16bit - stereo - aiff - uncompressed
southtower.varickstreet2.aif
13 sec. - 2.1MB


The first group of low frequency bursts are also centred around the 85-108Hz region, the first one appearing at 13.72 seconds, then again at 14.76, 15.9, 17.42, 18.05, and 19.1 seconds. Some of these bursts come come in small groups of 2 or 3 events, the consistent 0.3 second delay suggesting them to be some sort of echo.

Varick Street first group of low-frequency bursts:


By far, the most pronounced low frequency features are bursts at 21.01 and 21.90 seconds. This second group of blasts are quite audible in the clip as defined "thump" sounds. The "thumps" continue to be seen onwards in the sonogram with a lesser intensity at 22.7, 23.4, 23.87, 24.65, 25.38, and 26.05 seconds, most of these also being audible on the sound track at this point.

Varick Street second group of low-frequency bursts:


Though shot from much closer than the Varick Street clip, the following explosive demolition footage from Protec can offer something of a comparison as the camera is at a fairly long distance away.

protec100.mov

240x152 - 12sec. - 308KB - Quicktime/Sorenson 3


audio only:
44.kHz 16bit - stereo - aiff - uncompressed
protec100.aif
12 sec. - 1.8MB


Sonograms show similar amplitudes, duration, and frequency response in the low-registers caused by explosive detonations.

Protec100 overview: Protec100 - low frequency detail:



The occurrence of low-frequency sound bursts in the Varick Street clip temporally coincide with the major audio events pointed out from the West Street clip above. The that fact there is what appears to be the same major blast sounds found in the the West Street clip being recorded almost 2 kilometres away is extremely significant. It certainly suggests that we are dealing with very high energy sources and the recorded occurrences do happen to be consistent with air-blast from explosive shock-waves.

This figure compares the Varick Street clip (top) and the West Street clip (bottom). Both sonograms are in exactly the same time-scale:
(note, there are other coinciding peaks before and after the 2 loudest explosions which are not marked with arrows.)


In the time difference between the first 4 audio events, the Varick Street bursts begin to lag behind those of the West Street clip. If we assume that the sounds were coming from higher to gradually lower points in the building then we would expect such a lag because the sounds would be getting gradually closer to the West Street camera but remaining relatively the same distance from the Varick Street camera. Clocking the West Street times for these first 4 events at 0.49, 1.44, 2.45, and 3.79 seconds we get intervals of 0.95, 1.01, and 1.34 seconds. For the Varick Street clip we have times of 13.72, 14.76, 15.90, and 17.42 seconds making intervals of 1.03, 1.14, and 1.52. This makes the differences between intervals 0.08, 0.13, and 0.18 seconds.
Even with the very slight delays, the intervals between acoustic events act as a temporal signature which would not reproduce by chance coincidence. In both the West Street and Varick Street clips, the most energetic acoustic events happen towards the middle of the collapse sequence. This fact my shed some light on the overall sequence of events and the actual mechanics of the collapse.

Between 21 and 29 seconds the image is seen to be shaking. The same sort of thing can be seen some seconds prior to the North Tower Collapse as recorded from the same angle. The camera was obviously standing on a tripod as can be seen by the overall stability of the shots. Being zoomed in quite far would make the image much more sensitive to even slight movements. Most video cameras come with an electronic or optical image stabilization system ("steadyshot") which usually counters the effect of vibrations or small bumps. If switched on (as is usually the default setting) it means that smaller vibrations would not register in the image. It is unclear as to whether the shaking is caused by seismic activity or by something else.




If we want to see what an explosion at the South Tower should look like when recorded from the same distance and with the same camera, it may be instructive to analyse the South Tower plane impact included on the same documentary. This clip appears to have been recorded from a different location on the same building and possibly outdoors on a terrace rather than from through an office window. It is pretty safe to assume that the same equipment was used to shoot this footage given the circumstances and the low-frequency audio is identically notched out at 140Hz - 170Hz.

video:
southtower-plane.varick.mov

320x200 - 4.5MB - Quicktime Sorenson3, MPEG4 audio


audio only:
converted to 48kHz 16bit - stereo - aiff - uncompressed
southtower-plane.varick.aif
47 seconds - 8.7MB
preview mp4:


Varick Street plane impact overview:


For the moment, the area of interest would be around the plane impact / explosion / fireball portion of the clip's sonogram. Here we see the 2 most pronounced low-frequency sound bursts of the whole recording. These blasts are remarkably similar to the two biggest blasts seen in the Varick Street - South Tower collapse clip above. Unlike the blasts recorded well into the collapse footage which aren't particularly discernible above 145Hz, the plane-crash/explosions spread into the low-mid frequency bands as high as 480Hz with a lot of amplitude and as high as 700Hz with a lesser amplitude from where any possible higher mid-frequency response is obscured by the loud emergency vehicle sirens at 700Hz - 1750Hz.

Varick Street plane impact crash explosion: Varick Street South Tower collapse largest low-frequency bursts:


Varick Street plane impact crash explosion zoom-1: Varick Street South Tower collapse largest low-frequency bursts zoom-1:


Varick Street plane impact crash explosion zoom-2: Varick Street South Tower collapse largest low-frequency bursts zoom-2:



There isn't any real doubt as to whether these low-frequency bursts in the plane-crash clip were caused by some kind of explosion or not. The plane-crash / explosion / fireball and its sounds are of course well documented. The striking similarities in low-frequency response between the events of the South Tower collapse and the impact/explosion of the jet very strongly indicate that explosions occurred during the collapse of the tower. There are of course the visual similarities between the rapid expansion of the burning jet fuel at the time of the crash and the rapid expansion of the actual building at the time of its collapse.

For the purposes of a more thorough comparison these two clips, a few questions will have to be answered about what actually happened during the impact of the airliner. There are numerous claims of blasts at the base of the towers and even bombs at the top of the North Tower during the impact of the jet-plane4. Such claims aren't necessarily ruled out by this clip alone, in fact some of these claims are based on the visual content of this piece of footage. This paper will be later expanded with in-depth analysis of the various existing plane impact footage.

Varick Street plane impact, pre-crash sounds:


For now, with a quick look at the audio leading up to the crash, we see a steady tone at 385Hz starting at 18.5 seconds and stopping at 24.4 seconds. This sound is possibly a vehicle horn close by, whose harmonics are masked by the more dominant sound of the nearby emergency vehicle or perhaps it is even the sound of the approaching jet plane. Also, beginning from 5 or 6 seconds before the major fireball explosion, we see low-frequency content which might appear to suggest pre-crash explosions. However, this could be explained as the results of seismic activity from the crash somehow being registered by the camera before the sound of the crash. The camera, which was on a tripod, does start shaking at about the same time prior to the sound of the crash. It's not unreasonable to suppose the sound of the crash took up to or more than 5 seconds to reach the camera and the seismic waves as little as 0.5 seconds to reach the camera. The impact of the plane and the beginnings of the fireball/explosion are out of frame and anyway obscured by the foreground building making an accurate timing of the events rather difficult. Careful analysis of other plane-crash audio/video angles will be needed to ascertain whether the low-frequency audio and camera shaking is seismic activity from the actual crash/explosion, from pre-crash explosions, or perhaps even a freakishly coincidental gust of wind.


Other Factors


Whether they be from pre-planned terrorist bombs or freak accidents, the observed acoustic events very much appear to have been caused by large explosions. Minus explosions, an explanation which might be argued is impact-noise from the collapse. Though obviously falling debris would be extremely loud, it seems unlikely that so much material raining down in the consistent manner observed would cause the smaller number of clearly defined sounds over such a short timeframe. Further, the acoustic events can be seen to begin early in the collapse, before any material has even reached the ground. Twisting and/or breaking steel columns would also not seem to be a good explanation.
The sounds of "pancaking" floors can be ruled out because of the smooth and very quick wave of destruction. To be very general, we're talking about 110 floors in about 14 seconds so around 7.9 floors per second. That pretty much a drum roll speed and would appear more like a constant sound given the circumstances.

A square-wave at 7.9Hz is given as a rough example of the pancaking speed.


7.9Hz square-wave:
(format: mp4)
7.9hz.mov
11 sec. - 108KB






PAGE 2 >>>>>




















[1]

The New York Times released the 9/11 "oral histories" in August of 2005. Within these first-hand accounts from FDNY fire-fighters and EMS personnel are many accounts of explosions during and prior to the collapse of the North and South Towers. Excerpts from 40 different eyewitness accounts accounts can be found here.

Another collection of excerpts can be found here: 911research.wtc7.net

The official dogma of what actually happened doesn't leave any room for there to be any explosions during the collapse of the towers.. It is interesting to note that many of the accounts acknowledge the official story with something to the effect of, "we heard, saw, and felt explosions but given the official version of events I guess that it must have been something else".

For example:

I should say that people in the street and myself included thought that the roar was so loud that the explosive - bombs were going off inside the building. Obviously we were later proved wrong.
...
As I said I thought the terrorists planted explosives somewhere in the building. That's how loud it was, crackling explosive, a wall. That's about it. Any questions?

--ASSISTANT COMMISSIONER JAMES DRURY
Interview Date: October 16, 2001



[2]
The position of the Trinity Church camera was estimated by lining up building features in the foreground and background of the cameras field of view and drawing lines between these features on an aerial photo of the area, placing the camera somewhere on the corner of Rector Street and Broadway, about 355 meters away from tower 2. The mechanical floors 75 and 76 being about 2/3rds up the building or around 277 meters above the ground would make the distance from the exploding floors to the camera about 450 meters. Depending an the air temperature, sound should travel about 340 meters in 1 second.
The "10:28 AM" time on the screen is wrong. The south tower collapsed at 9:58 am. The North tower can be seen, still standing, behind the foreground tower in this clip.

The file was found here: http://www.terrorize.dk/911/wtc2dem1/index2.php



[3]
The North Tower collapse clip from the same angle is zoomed back a bit and shows a building in the foreground. The building's features identify it as 101 Avenue of the Americas located approximately 1400 meters away from the WTC. With this building in the foreground and the angle from which the towers are seen we can place the camera somewhere on Varick Street or on a side-street off Varick Street. Approximating the distance at about 1600 meters would have the sound taking about 4.7 seconds to travel from the WTC to the camera location.
This image approximates the distance of the "Varick Street camera" using Google Earth Satellite/Map images.

click for 1280x2232, 900Kb version.

link: http://maps.google.com/maps?ll=40.723323,-74.005773&spn=0.004828,0.007553&t=k&hl=en

note: The address of the production company who produced WTC: The First 24 Hours would suggest the camera to be about 50 meters to the right of the point on this map.



[4]

This video points to details in the same jet plane-impact footage which have been interpreted by the author, credited as OutoftheSky, as evidence of some kind of explosive charges going off in the North Tower at the time of the South Tower crash.





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