Explosion Sounds and the World Trade Center - Twin Tower Collapses

version 1.46

Part I: The South Tower Collapse -- Audio Recordings


The recently released documentary, "911 Eyewitness" ( provides a rare look at long stretches of unedited footage audio/video which was filmed from 2 piers in Hoboken. The DVD includes the North and South tower collapses as well as a few very brief fragments of the WTC 7 collapse. The South Tower audio shows not only the blasts at the time of the collapse but also 8 distinct explosion sounds which occur across the 4 minute time-frame prior to the tower's collapse.

With an approximate distance of 3.5 Km [footnote: map], it is the lowest frequencies which would tend to show evidence of explosions. An important aspect to this footage is the relatively quiet and unobstructed space between the towers and the camera. Furthermore, blast sounds which travel over water tend to be much less attenuated than those which travel over the ground [footnote]. The explosions are very audible though headphones or a decent full-range speaker system are needed to hear them. They sound much like what the thunder of a lightning-storm would sound like.

A portable radio on the location dominates the mid and low-mid frequency ranges with the sounds of a local AM radio broadcast, but this does not register much below 150Hz. The vocal reaction of bystanders close to the camera is also audible, as well as some jostling of the camera but these vocal and handling sounds are very easy to identify on a spectral analysis graph. The clicking of a 35mm photo-camera shutter can also be heard periodically. At some 40 seconds and again at 56 seconds after onset of the collapse sounds, there appears 2 small, low frequency noise events which extend up into the mid range of the spectrum. These are probably the localized sound of slamming car doors since the sound of an automobile engine which seems to have just arrived on the scene can be heard at this point. Between these 2 sounds, a 1Khz tone can be heard coming from the AM radio as it marks the exact time of 10:00 in the morning.

video of the South Tower collapse and proceeding 4 minutes
with scrolling audio analysis:
same clip without audio analysis:

480x360 - 32.9MB - Quicktime/Sorenson 3

480x352 - 17.3MB - Quicktime/Sorenson 3

audio only:
converted to 44.kHz 16bit - mono - aiff - uncompressed
6 minutes - 30.2MB

The dramatic nature of the blast sounds which occur in the 4 minutes prior to the collapse certainly raises some puzzling questions. Since other long stretches of unedited pre-collapse footage (with audio) from other angles has not yet been made public, it is not possible for us to hear if other recordings also caught these blasts. In other recordings from closer to the tower, though only short snippets are available, we don't see much of any indication that intense explosions have be registered by any people in the area.

It is perhaps possible, if these sounds are indeed the result of large explosions, that the blasts may not have been heard in the immediate area yet somehow have been audible on the other side of the river. One historical example of such an effect occurring is the violent explosion at the onset of the Mount St. Helens eruption. The explosion produced an air-blast which was heard as far as 800Km away but curiously not within a 100Km radius of the event. Some scientists in a plane who were monitoring the volcano from above captured photographs of the explosion but heard and felt nothing of the gigantic blast wave. This is assumed to be the result of the nature of the blast wave and specific meteorological conditions.

The 1993 truck bombing of the WTC sub-basements produced a large blast-wave but it is difficult to speculate what the acoustic effects would have been. Some occupants of the Twin Towers report that they did not actually hear the blast, they only felt the building shaking, yet some reports from other areas of the complex indicate that the explosion was heard. Members of the FDNY at a station on Liberty Street felt the rumble of the blast and responded to the incident, whether they also heard the blast or only felt it is not clear. The blast was reported to have at least been felt across the river in Brooklyn but NY area seismic records did not register the event, owing to the fact that the bomb was not coupled to the bedrock via WTC structural support columns.

The meaning of the pre-collapse explosion sounds in the Hoboken (South Tower) video is far from being understood. Though it would be fairly safe to assume that these sounds are coming from the towers, and probably from the South Tower as relating directly to its subsequent collapse, it is unknown if they would be coming from the crash-zone, the basement, or some other part of the tower.

Numerous theories contend that the towers must have been brought down some kind of pre-planted explosives. Such "demolition theories" cite not only the evidence of explosions, but also other factors such as the rapid nature of the collapses (less than 15 seconds each). There is also the highly suspicious spontaneous, symetrical, near free-fall collapse of Building 7 and the conspicuous absence of key issues from official government reports. Much of the government and mass-media response to the disaster gives the appearance of a large cover-up.

Aluminum Theory

One "non-demolition" theory for the demise of the Twin Towers, which actually acknowledges the explosiveness of these events, is offered in a recent paper (January, 2006) by Dr. F.R. Greening called "Aluminum and the World Trade Center Disaster"(PDF). Greening's theory suggests that molten aluminum-thermite reactions and low temperature "thermite sparking" were caused by a combination of aluminum from the 767 jet-plane, heat from the fire, the presence of lime in the towers' concrete floors, gypsum in the wall-boards, rust from the steel frame of the towers, and sources of water in the towers including concrete.

The theory states that the 767's aluminum air-frame would begin to melt a temperatures 500-550°C, flow down, and come into contact with the other ingredients "inducing violent thermite explosions". These explosions would in turn cause a partial collapse, like that observed in the leaning of the South Tower's top, allowing the flowing molten aluminum access to yet more reagents below, more explosions, the process repeating in something of a chain-reaction down the building, and gravity adding to the momentum of the collapse with the intact structure above.

The observed explosion of the South Tower's mechanical floors could lend some weight to this theory since these floors would quite probably house large water systems used for cooling of the building. Greening's paper quotes one B. Davy who says, "The aluminum-water reaction that occurs with molten aluminum is highly exothermic, and will cause the aluminum to detonate with greater energy release than an equivalent weight of TNT." In the case of the North Tower, a large source of water close to the crash could be the 5,000 and 10,000 gallon capacity holding tanks for the sprinkler and standpipe systems which were both located on the 110th floor.

Running contrary to Greening's theories are those of Steven E. Jones who proposes that "high-temperature cutter-charges such as thermite, HMX or RDX or some combination thereof", were the reason for the towers' explosive collapses. This is based on observations covered in his paper, "Why Indeed Did the WTC Buildings Collapse?"

To be continued...