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Local Active Noise Reduction for MEDEVAC and CASEVAC


OBJECTIVE: Develop a litter-mountable active noise-reduction system that will reduce the level of noise in a Black Hawk MEDEVAC or CASEVAC helicopter to 80 A-weighted decibels (dBA) at the casualty"s head without interfering with monitoring of the casualty's condition or with in-transport medical treatment. DESCRIPTION: Medical evacuation of ill, injured, or wounded Soldiers often occurs in air and ground vehicle with very high noise levels in the compartment in which the casualty is being transported. For example, the sound levels in the right center position in the cabin of the UH-60A Black Hawk helicopter flying at 120 knots with doors open are recorded at 106 A-weighted decibels (dBA). In accordance with Army regulations and the operator"s manual (i.e., the"dash 10") of the UH-60A aircraft, all Army aircrew are required to wear both earplugs and the sound-protective flight helmet (HGU-56/P Aircrew Integrated Helmet System) in this environment. If a casualty has suffered a head injury, it is unlikely that traditional head-borne or insert hearing protection devices could be used, even if they were available. In this environment, a casualty without hearing protection exceeds his/her 100% noise dose in less than four minutes. Thus, it should be readily apparent that a casualty being transported in a vehicle with these noise levels will be at significant additional risk for permanent hearing injuries. However, the risk of hearing injury is not the only problem that excessive noise causes in the MEDEVAC environment. The noise levels in an Army MEDEVAC helicopter will completely eliminate the possibility of communication between the casualty and medical personnel, thus negatively affecting treatment. Furthermore, physiological stress responses are evoked by noise levels significantly lower than either experienced in the MEDEVAC environment or even the"safe"levels promulgated by industrial hearing conservation regulations (Babisch, 2002, 2003). This added stress may significantly impair the recovery of the ill, injured, or wounded patient (McCarthy, Ouimet, & Daun, 1991). The principles of active noise reduction (ANR) or active noise control are well known (Hansen, 2001; North Atlantic Treaty Organization, Research and Technology Organization, Human Factors and Medicine Panel, 2005). Using feed-forward (predictive), feedback, or hybrid techniques, an ANR system produces a sound ("anti-noise") whose compression and rarefaction phases are intended to cancel the rarefaction and compression phases of unwanted noise in a space, an earcup, or underneath an earplug. The proposed system will significantly reduce the noise exposure to the casualty being transported with consequent reduction in environmental stress that adversely affects the casualty as well as the risk of permanent hearing loss from excessive noise. PHASE I: Develop an initial concept design and model key elements for an active noise reduction system, mountable on a standard NATO litter, that will reduce noise at patient ear locations to less than 90 dBA in the presence of pink noise presented at 106 dB sound pressure level (SPL). Identify the key elements required for airworthiness testing for use in US Army MEDEVAC and CASEVAC operations. PHASE II: Based on Phase I results, construct and demonstrate the operation of a prototype MEDEVAC ANR system that will reduce the noise at ear locations to less than 80 dBA in the presence of pink noise presented at 106 dB sound pressure level (SPL). Demonstrate that the prototype system can pass the stringent requirements for airworthiness certification for medical devices by the US Army Medical Command. PHASE III: Medical evacuation in air or ground vehicles often includes an environment in which the presence of noise will interfere with patient assessment, triage, and treatment, produces additional stress on the patient and medical personnel, and may comprise a significant risk of permanent hearing losses and/or tinnitus. Road noise, engine and drive train noise, rotor noise all compromise the medical care given to the ill, injured, or wounded. The development and deployment of a local active noise generation system for air or ground ambulances should significantly improve the quality of care given during the"golden hour"following injury or wounding. REFERENCES: 1. Babisch, W. (2002). The Noise/Stress Concept, Risk Assessment and Research Needs. Noise Health, 4(16), 1-11. 2. Babisch, W. (2003). Stress hormones in the research on cardiovascular effects of noise. Noise Health, 5(18), 1-11. 3. Hansen, C. H. (2001). Understanding active noise cancellation. London ; New York: Spon Press. 4. McCarthy, D. O., Ouimet, M. E., & Daun, J. M. (1991). Shades of Florence Nightingale: potential impact of noise stress on wound healing. Holist Nurs Pract, 5(4), 39-48. 5. North Atlantic Treaty Organization. Research and Technology Organization. Human Factors and Medicine Panel. (2005). New directions for improving audio effectiveness nouvelles orientations pour l'amelioration des techniques audio, RTO meeting proceedings 123.
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