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noise, also containing the shrill notes of higher frequencies. Apart from the evidence enumerated above, especially Smith Thunder is low-frequency sound, extending well into the infra- (2002), data on infrasound in jet planes appears to be scarce or sonic (inaudible) frequency domain. Various papers have been unavailable. It is, of course, true that there has been no general published indicating just this, including Petrovic (1980), Kehoe requirement to measure infrasound in aircraft to date, given an et al. (1994), Baklanov (2000), Scholz (2000), Alves-Pereira et exclusive regulatory focus on audible sound above 20 Hz in fre- al. (2001) and Smith (2002). quency. Infrasound was apparently not found at significant lev- The title of the Petrovic paper is apparently self-explanatory, els in passenger compartments of commercial jet planes by though it was not available. Kehoe et al. describe NASA ground Alves-Pereira et al. (2001), suggesting no compartment reso- vibration tests on a Jetstar passenger aircraft and an FA-18 nance. However, these measurements were not continuous, thus fighter jet which identified vibratory modes of basic structures intermittent air pressure oscillation of significant amplitude may such as wings and fuselage in the range of 5-18 Hz. If the fuse- simply have been missed. Perhaps these non-resonant levels are lage of a passenger jet is flexing at 11 Hz, even at small ampli- sufficient to account for the three syndromes in airline passen- tude, what is the effect on the passengers? Note that this fre- gers and crew mentioned in this article; alternatively, compart- quency is close to the theoretical air pressure (infrasonic) reso- ment resonance may be intermittent. The Swedish Defence nance in an airplane fuselage of length 15 metres. The exact Materiel Administration collected references in 1985-86 which dimensions of the small-to medium-sized Jetstar airplanes are are said to include papers on infrasound in airplanes; this collec- not known to this author. tion was not available to the author. Baklanov writes: "High limit of engine It is suggested that the phenomenon of behaviour as a solid body is in the range passenger compartment infrasonic reso- 25-30 Hz for a number of Tupolev trunk- nance, if present, may be intermittent, per- route jet aircraft. Disturbances in the engine haps dependent on particular engine speed gas flow duct are one source of noise." The aircraft's engines or atmospheric conditions. This is consis- Scholz writes. "Exhaust systems are one of tent with findings by Winck et al. (2002) of the most important noise sources in modern are connected toa an average 3.1 "rapid dips" in blood oxy- turbine power plants. In some cases resonant acoustic gen levels of up to 10 per cent in airline acoustic resonance can occur, producing passengers on long flights. In short flights, very high sound-pressure levels, usually at chamber, the passenger blood oxygen levels fell by only three per low frequencies." _ compartment, whose cent. This Suggests a cumulative response Alves-Pereira et al. measured infrasound . . . in the longer flights, presumably not in pilots’ cabins of commercial aircraft, this physical dimensions explained by reduced static cabin air pres- apparently being generated by airflow imply an infrasonic sure or quality. impact on leading edges of the air- plane, especially at lower altitudes. Smith found low-frequency noise lev- els between 5 Hz and 250 Hz sufficient to resonate aircrew upper torsos in sev- eral military aircraft -and amplitudes were higher aft of the engine exhaust outlet. 3. Random vibration excitation. "Aircraft, missiles and rockets are sub- jected to random vibration excitation. This is due to the extreme turbulence of jet exhaust downstream of the jet and rocket engines, and aerodynamic Findings mentioned previously of hypercoagulability in pilots propor- tional to logged flight hours also indi- cate a cumulative effect, in that case of infrasound from air impact on air- craft leading edges. As already demonstrated by Bendz et al., there is an immediate effect on blood chemistry of exposure to an air pressure decrease. It is suggested that infrasound exposure has the same effect; also, adverse health effects of exposure to infrasound are believed to be cumulative. (inaudible) resonance, but in which items such as loose plastic window blinds may rattle. buffeting," according to remarks on the Aircraft Design Inc. Winck et al. apparently did not measure air pressure or related website in 2001. changes during the flights in question, though decreased cabin air pressure in airplanes is mentioned as the reason for their PROPOSED MECHANISM: Infrasonic Resonance of research. Significantly, they did find different blood oxygen lev- Passenger Compartment els when people were tested in different parts of the airplane, The aircraft's engines are connected to a resonant acoustic e.g., the toilets, which would have similar static air pressure, chamber, the passenger compartment, whose physical dimen- suggesting that some other factor is involved, perhaps air pres- sions imply an infrasonic (inaudible) resonance, but in which sure oscillation at different rates, amplitudes or times related to items such as loose plastic window blinds may rattle. dimensional or other characteristics of the space or enclosure. An attached or coupled source of noise/vibration at 5-250 Hz Rapid "dips" in blood oxygen levels were not found at ground is perfectly capable of resonating a large enclosure at an inaudi- level (personal communication, Winck, 2002). ble frequency—for example, 6-8 Hz. An air-filled cylinder of The French company ONERA is researching aircraft cabin 30 metres in length resonates at around 6 Hz, in theory, like a noise generated by vibration from attached jet engines, but with giant organ pipe; lesser lengths resonate at higher frequencies. an apparent focus on audible noise above 20 Hz. There is cur- Acoustic resonance has a considerable amplifying effect on rently an ongoing international effort to quieten passenger jet the noise/vibration source. Noise/vibration was reportedly planes, including a focus on infrasonic engine noise at NASA sufficiently intense in early jet planes as to cause structural (e.g., the Advanced Subsonic Technology Noise Reduction failure. Program). These efforts appear to have achieved not much more The aircraft's engines are connected to a chamber, the passenger compartment, whose physical dimensions imply an infrasonic (inaudible) resonance, but in which items such as loose plastic window blinds may rattle. PROPOSED MECHANISM: Infrasonic Resonance of Passenger Compartment The aircraft's engines are connected to a resonant acoustic chamber, the passenger compartment, whose physical dimen- sions imply an infrasonic (inaudible) resonance, but in which items such as loose plastic window blinds may rattle. An attached or coupled source of noise/vibration at 5-250 Hz is perfectly capable of resonating a large enclosure at an inaudi- ble frequency—for example, 6-8 Hz. An air-filled cylinder of 30 metres in length resonates at around 6 Hz, in theory, like a giant organ pipe; lesser lengths resonate at higher frequencies. Acoustic resonance has a considerable amplifying effect on the noise/vibration source. Noise/vibration was reportedly sufficiently intense in early jet planes as to cause structural failure. 36 = NEXUS resonant acoustic www.nexusmagazine.com DECEMBER 2002 — JANUARY 2003