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Before we get to the main event, lets look at the sound profile or "footprint" for this FT, which was captured on 28 April as it flew over Crystall and Cornet. Figure 14 compares the frequency spectrogram for the sound of the FT with that of five conventional aircraft, two helicopters, and the Harrier jump jet. In the footprints for four of the known aircraft (SR-71, DC-9, Blue Angel, and F-18) there is a clear increase in volume or decibel level as the aircraft passes the microphone. This is because the sound coming from the back of turbofan engines is much louder than that coming from the front, and the back blast is not heard until the aircraft passes the observer. In the frequency spectrogram for the Boeing 707 taking off from a standing position, the microphone is probably always behind the aircraft, and the increase in volume is due to the power curve. In all eight examples of known aircraft there is a considerable amount of white noise, which reaches 12 kHz in most cases, with significant white noise up to 8 kHz. This is important. For the three military aircraft (SR-71, Blue Angel, and F-18) the trail off or frequency dampening of white noise after the aircraft passes the microphone is rapid, with a drop below 4 kHz in all cases. This is not the case for the Boeing 707 and DC-9, which produce a high white noise signature further away from the microphone.
The sound profile for the Harrier GR-7 looks similar to that of the FT with most of the sound volume generated by frequencies below 2 kHz, but the Harrier sound does not contain the multiple frequency bands evident in the FT spectrogram. Like the FT, the white nose content of the Harrier is much lower than the other aircraft, falling below 6 kHz. But with these comparisons the similarites end. The sound spectrogram for the police helicopter (type unknown) shows the typical rotor blade slap of a helicopter at the beginning and end of the sound profile, which is produced by mini sonic booms created at the tips of the rotor blades. In this regard older type helicopters produce sounds unlike that of the FT. The spectrogram for the French Aerospatiale SA-366 Dauphin, however, does not contain obvious rotor blade chatter. It comes the closest of all the spectrograms in resembling that of the FT, contains five to six separate frequency band, and has a similar low white noise signature (below 6 kHz). Most of the sound volume for both the FT and Dauphin falls below 2 kHz, with red bands falling at or below I kHz. Sounds coming from both the advanced Dauphin and the FT were not audible until they were close to the microphone (less than 1,000 feet).
In three of the five spectrograms of conventional fixed-wing aircraft there is little or no separation of the sound into discrete frequency bands. This is typical of more modern jet engines with reduced noise emissions. There is some separation into frequency bands for the Boeing 707 as its turbofan engines increase rpms during takeoff, but there is a higher number of frequency band for the DC-9. From nine to ten separate frequency bands can be recognized in the DC-9 spectrogram. Both types of aircraft have been in service for a long time, and it is possible that they have more noisy engines compared to the military aircraft, where noise emissions are an important consideration in design - especially for the SR-71 supersonic stealth aircraft.
When comparing these spectrograms with that of the unidentified FT, the differences are significant (Fig. 14). First, the white noise content is noticeably lower than that of the military aircraft, falling below 5 kHz in general. Most of the noise produced is of lower frequency, below 2 kHz (Wav13b). Not only that, but the sound is divided into at least 15 different frequency bands, more than any conventional jet. In addition, these bands appear to be harmonically-spaced, and decrease in frequency as the FT approached the listener or microphone. The frequency bands for the DC-9 can also be seen to decrease in pitch, but this is due to the pilot throttling back as the aircraft touches down. It is highly unusual to have a sound violate Doppler's law, which states that the apparent frequency of that sound will increase as the source approaches the listener, and decrease as the source moves away. A normal Doppler effect can be heard in the recording of a French Aerospatiale SA-366 Dauphin helicopter passing in front of a microphone at close range (SA-366Da.wav). The first camera click marks the closest position of the microphone to the helicopter in the sound sequence.
As the FT flew over us, Crystall blurted out a sound. Immediately the volume or decibel level of the FT dropped dramatically, and then slowly recovered. As the FT sound level recovered, the apparent embedded frequency bands increased uniformly in pitch - once again violating Doppler's law if this was not due to a change in power output under pilot control. Remember: The loudest sound should occur after an aircraft passes the observer, not before. It is possible that Crystall's voice created some sort of impedance in the microphone-electrical-sound system of her camcorder, and this is what is responsible for the sudden drop in apparent volume for the FT. However, this is not the first time such a drop in volume has been recorded for a performance by one of these stealth craft. If it were the first time, I would be more willing to discount it as an artifact. But if you look at the sounds made by the crickets (at plus/minus 3 kHz), their volume shows no equivalent attenuation or dampening - some yes, but not as much. I suspect that the slight temporary drop in cricket volume after Crystall begins to speak may be due in part to her blurting out a loud sound and in part to the suddenness of the change in FT volume, which may have startled some crickets, causing them to momentarily stop their music. It is almost as if the volume of sound coming from this FT was intentionally cut so that Crystall's comments could be heard - but by whom? The Pilot? That possibility is too unlikely for me to seriously entertain.
The overall sound envelope and sound pattern produced by the FT is unlike that of any conventional aircraft I have heard or recorded. The audible character of the sounds produced by the Harrier jump jet (Harrier7.wav) and the Dauphin helicopter (SA-366Da.wav) are unlike that of the FT, even though all three have similar sound profiles. Because of the sonic similarities to aircraft which can hover, the FT powerplant may have been shielded and its exhaust deflected in some way to produce lift and/or stealth. The shielding might be responsible for the lower white noise signature and the low frequencies of the dominant sound. Such a sound would be harder to detect and would be more stealthy. Alternatively, the multiband harmonics of the sound envelope might indicate a possible synthetic sound source - a manufactured sound as a form of reverse stealth. Reverse stealth is where the engineering objective is to conceal the true identity of the aircraft by sounding like and looking like something else which is not a threat and for which it will likely be mistaken by untrained observers. If I were frequently flying illegally in someone else's airspace, and wanted to maintain a low profile, but couldn't make my stealth aircraft completely invisible, I would attempt to mimic one or more of the more common aircraft flying in that airspace. And that would require an engineer's bag of tricks. If the sudden drop in volume as the FT flew over Crystall and Cornet is real, and not an electronic artifact created by Crystall's voice, then this would be strong evidence that the sound was manufactured and controlled by the pilot.
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