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14 The literature review started with the review that had already been completed for ACRP S02-48 (âAssessing Community Annoyance of Helicopter Noiseâ) and expanded to cover documents iden- tified in the references to that document and documents recommended by panel members, recom- mended by surveyed airports, and suggested by the authors of this report. A search of helicopter noise studies in general uncovered a large number of documents; the majority of helicopter noise studies that address the technical aspect of measuring, predicting, or controlling helicopter noise through helicopter design. These technical documents address the aero-acoustics, physics, and study of heli- copter noise generation and control. The technical documents that form the primary body of existing research are important but are not as useful as a practical guide on managing helicopter noise except to the extent they can be referenced as progress being made in the field of producing quieter heli- copters. The literature review focused on those studies that addressed human response to helicopter noise, mitigating helicopter noise, or managing helicopter noise, generally favoring peer-reviewed journal articles, government agency reports, and industry studies. The literature review for this project, as well as prior literature reviews such as those conducted by Molino (1982), Ollerhead (1985) and FAA (2004), documents research undertaken in the last half century to quantify and predict the individual and community annoyance of helicopter aircraft noise and evaluate ways to manage the impacts of helicopter noise. An annotated bibliography is presented in Appendix B. The reader is encouraged to review this material for a more detailed view of helicopter noise response research. The literature is most prevalent in the 1970s and 1980s, and appeared to peak in the 1990s, with most research done by government agencies in the United States and Europe, particularly by the mili- tary. After the year 2000 the research was focused more on the technical issues of aero-acoustics and design of quieter helicopters and tilt-rotors. Whether conducted under laboratory or field conditions, much of this research was intended, directly or indirectly, to inform decisions about aircraft noise regulatory policy. The early research searched for a noise metric that would correlate helicopter noise to response and create in effect a helicopter dosage- response relationship much like the fixed-wing, dosage-response relationship endorsed by the Federal Interagency Committee on Noise (FICON 1992). The reviewed literature did not identify any such âmagic bulletâ approach to assessing helicopter noise. A number of approaches were found that used helicopter âcorrectionsâ to account for the differences in response to helicopter noise; however, these generally were studies directed at large military aircraft or a single type of civilian helicopter. The findings of individual studies of the annoyance of helicopter noise disagreed about as often as they agreed. The main point of agreement in the technical literature is that helicopter noise is much more variable and complex than fixed-wing aircraft noise. This variability and complexity make it more difficult to accurately and credibly model helicopter noise exposure (other than under idealized conditions6), particularly in the vicinity of helipads. It follows, in turn, that predictions of the preva- lence of annoyance of exposure to helicopter noise are likely to be more uncertain than predictions of the annoyance of exposure to fixed-wing aircraft noise. A main point of disagreement is the degree to which main rotor impulsive noise controls the annoyance of helicopter noise. Some studies conclude that impulsiveness corrections are appropri- chapter five SUMMARY OF FINDINGS OF LITERATURE REVIEW
15 ate for predicting this annoyance; others found that conventional A-weighted noise measurements suffice for predicting the annoyance of helicopter noise. There were no studies done by countries, organizations, manufacturers, or communities that com- prehensively examined the rates of reported high annoyance with noise exposure for a variety of helicopter types. The lack of an accepted dosage-response relationship for helicopter noise is the single largest gap in the knowledge base for understanding helicopter noise response. Some of the findings of this literature review included: â¢ Neighborhood opinions about the difference in annoyance owing to helicopter and fixed-wing aircraft noise exposure likely differ for nonacoustic reasons and these differences may differ from community to community. Unless analytic means are employed to account for such community- specific differences, it may not be possible to reliably identify differences in opinions about fixed- and rotary-wing annoyance per se. â¢ The flexibility of helicopter flight lends itself to much more complex and widely varying flight paths than those of fixed-wing aircraft. The directivity of helicopter noise emissions and chang- ing noise levels during turns and acceleration and deceleration further complicate noise expo- sure predictions based on flight tracks alone. These factors contribute to the unpredictability of the noise, which is one cause of increased level of annoyance. â¢ Extensive efforts to confirm the utility of impulse noise adjustments have proved contradictory and inconclusive. Resolving how to account for the impulse characteristics and low-frequency characteristics is an area in need of research. One issue is that much of the early research on this topic was done based on noise from heavy military helicopters that do not have the same noise signature as lightweight civilian helicopters. â¢ Correlation analyses have shown that most of the noise metrics commonly used to quantify helicopter noise are so highly correlated that no one metric differs meaningfully from others in its ability to predict the prevalence of annoyance with helicopter noise. â¢ Questions about potential nonacoustic influences on the âexcessâ annoyance of helicopter noise are not readily answered in laboratory studies, whereas questions about the detailed acoustic origin of excess annoyance are not readily answered in field settings. Understanding the role of nonacoustic factors, including identifying the more relevant nonacoustic factors, is an impor- tant research need. â¢ Industry-developed noise mitigation measures focus on altitude, avoiding blade slap through control of forward speed and descent or climb rate, and avoiding the overflight of residential areas. The HAI Fly Neighborly Guide, while providing guidance to operators not airports, is a very useful summary of effective practices on operating the helicopter (subject to the limita- tions described later in the section on reducing high-speed impulse and blade slap noise). â¢ Air traffic issues can complicate the development and implementation of altitude or route restriction most identified by complaining communities, as described in the FAA report on the Los Angeles Helicopter Noise Initiative.