ACTIVE NOISE REDUCTION IMPROVES PROTECTION IN AVIATION HELMETS OVER MILITARY JET TRAINER FLIGHT
Rauno Pääkkönen, Dr (tech), Mikko Korteoja, Dr (tech), Pentti Kuronen, Col (med)
In addition to the design of each aircraft type, noise level in the cockpit depends on power settings required in different flight conditions, and by aerodynamic noise. Cockpit noise levels range from 95 to 105 dB so hearing impairment has been considered possible, especially during prolonged exposure (Gasaway 1986, Pääkkönen et al. 1996, Starck et al. 1990).
Radio communications noise, which is partly of impulse type, is known to cause an exposure rise of approximately 4 to 10 dB from the level of background noise (Pääkkönen et al. 1998). Impulse type exposure may increase the risk of noise-induced damage (Hamernik et al. 1993).
According to earlier studies the A-weighted sound pressure level (SPL) in the cockpit of jet trainer BAe Hawk Mk 51 has been in average of 98 dB during normal flights (Pääkkönen & Kuronen 1998), and at the entrance of an ear canal 89 dB. Finnish Air Force (FAF) usually operates the Hawks with conventional Alpha aviation helmets. The purpose of this study is to compare the properties of conventional and active noise cancellation (ANC) aviation helmets over a flight profile often encountered in air combat maneuvering (ACM) syllabus. At the frequency of 80-1000 Hz the noise attenuation for aviation helmets varies 0-30 dB (Pääkkönen 1992, Pääkkönen & Kuronen 1996). This frequency represents significant area of ANC. It is generally known that ANC can improve attenuation of headsets and helmets in the frequency range of less than 500-1000 Hz. However, the effect of ANC is not known so well at the frequency range of less than 100 Hz. Human speech area covers frequencies of 160-6000 Hz (ISO TR 4870, 1991). Then, ANC can influence to low-frequency spectra. On the other hand, headsets and helmets can attenuate well higher frequencies by their passive structural properties. For military pilots the speech intelligibility is vitally important for flight safety.
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