Assessing Community Annoyance of Helicopter Noise (2017)

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1 This report presents the findings of a study of the annoyance of helicopter and fixed-wing aircraft noise. This study developed and tested a series of hypotheses intended to determine whether helicopter noise is more annoying than fixed-wing noise. The request for proposal (RFP) cited a general lack of understanding of the relationship between helicopter noise and community response. In a 2004, FAA Report to Congress titled “Nonmilitary Helicopter Urban Noise Study,” it was suggested that “additional development of models for character- izing the human response to helicopter noise be pursued.” The RFP further raised the ques- tion of whether the assumed “excess” annoyance of helicopter noise was more appropriately attributed to purely acoustic factors, to nonacoustic factors, or to a combination of the two. The study began with a review of the technical literature that identified annoyance as the primary noise effect of concern, distinguishing between the direct annoyance of airborne noise and the indirect annoyance of secondary emissions (vibration and rattling sounds) that may be induced in residences by helicopters. The review included an annotated bibliography of a score of prior publications on the annoyance of helicopter noise as well as tutorials on the nature and aerodynamic origins of helicopter noise emissions. It also included an analysis of the correlations among noise metrics commonly used as predictors of community response and a description of a systematic approach to accounting for nonacoustic influences on the annoyance of helicopter noise. The literature review found inconclusive evidence from prior laboratory and field studies concerning half a dozen hypotheses about the origins of annoyance due to helicopter noise. The main point of agreement was that helicopter noise is much more variable and complex than fixed-wing aircraft noise. The main point of disagreement was the degree to which main rotor impulsive noise controls the annoyance of helicopter noise. Overall, the reviewed laboratory and field studies revealed little systematic, rigorous, or theory-based understanding of the annoyance of helicopter noise. Seven hypotheses were formed from the literature review about the origins of the annoyance of helicopter noise. In simplified form, the hypotheses were: 1. The prevalence of annoyance due to helicopter noise exposure in a community is greater than that associated with comparable levels of exposure to noise produced by fixed-wing aircraft; 2. The prevalence of annoyance due to helicopter noise is most usefully predicted in units of A-weighted cumulative exposure; 3. The prevalence of annoyance due to helicopter noise is strongly influenced by its impul- sive character, and thus requires an impulsiveness “correction” to A-weighted cumulative exposure; 4. The prevalence of annoyance due to helicopter noise is strongly influenced by indoor secondary emissions (rattle and vibration) due to its low-frequency content; S U M M A R Y Assessing Community Annoyance of Helicopter Noise

2 Assessing Community Annoyance of Helicopter Noise 5. The prevalence of annoyance due to helicopter noise is appreciably influenced by non- acoustic factors; 6. The prevalence of annoyance due to helicopter noise is more usefully attributed to prox- imity to helicopter flight paths than to helicopter noise emissions per se; and 7. Complaints lodged about helicopter noise are more reliable predictors of the prevalence of annoyance than measures of exposure to helicopter noise or proximity to helicopter flight paths. Telephone interviews were conducted with residents of three urban areas about their annoyance with exposure to helicopter noise. The interviewing sites were among those with the greatest concentrations of civil helicopter traffic in the United States. The range of helicopter-only cumulative noise exposure levels expressed in day-night average sound level (DNL) across the interviewing sites nonetheless ranged from about 27 dB ≤ Ldn ≤ 53 dB. A questionnaire consisting of 15 items was created to collect information relevant to these hypotheses in largely residential neighborhoods near three airports supporting fixed-wing and helicopter operations: Long Beach, CA [Long Beach Airport (LGB)]; Las Vegas, NV [McCarran International Airport (LAS)]; and Washington, D.C. [Ronald Reagan Washington National Airport (DCA)]. Interviewing sites were selected primarily for their substantial exposure—by civil aviation standards—to helicopter noise. A range of helicopter noise exposure levels was sought at each site, and when possible, a range of fixed-wing aircraft noise exposure as well. Because the primary site selection criterion was exposure to large num- bers of daily civil helicopter flight operations, only one of the three interviewing sites (DCA) was exposed to appreciable levels of noise exposure produced by fixed-wing flight noise. Modeling of these helicopter operations was undertaken to estimate the helicopter noise exposure. Representative random samples of both landline and wireless telephone- subscribing households at each site were then compiled into a sampling frame by first identifying geographic areas in proximity to helicopter flight tracks with similar noise exposure, and then by identifying households within them. Home addresses of wireless telephone subscribers were inferred from their billing addresses, or from address information associated with the wireless number in other proprietary databases. Computer-assisted, live-agent telephone interviewing was then conducted over a period of at least 1 week in each of the neighborhoods. A total of 2,372 respondents completed the interview: 1,189 in Long Beach, 741 in Las Vegas, and 442 in Washington, D.C. Field measurements to confirm the noise exposure predictions were conducted for a week prior to the start of interviewing and during interviewing at LGB and at LAS. Time series of sound pressure levels were collected at 1-second intervals, along with A-weighted 1-second equivalent continuous noise level (Leq), C-weighted 1-second Leq, and 1-second Leq in each of the one-third octave bands from 6 Hz to 20 kHz. Both A-weighted and C-weighted 1-second time histories of Leq values were also recorded. Due to high levels of fixed-wing aircraft noise in Washington, D.C., helicopter noise exposure levels were estimated by noise modeling alone. Helicopter flight operations at DCA were highly constrained by higher altitude fixed- wing approach and departure flight paths, and high-quality radar flight track information was available during the interviewing period. All of the neighborhoods in which interviewing was conducted had stable residential populations. Large majorities of respondents in Long Beach and Las Vegas described their neighborhoods as quiet. Nearly half of the respondents in Washington did as well. How- ever, nearly a quarter of the respondents in Long Beach described their neighborhood as noisy, and nearly a third of the respondents in Washington described their neighborhood as “quiet, except for aircraft noise.”

Summary 3 Only small minorities of respondents reported noticing helicopters more than a few times a day at any of the three study sites even though the number of flights per day at one site was nearly 10 times the number of flights at the other two sites. The mean level of exposure to helicopter noise of respondents who were annoyed in any degree by it was 44 dB. The mean level of exposure to helicopter noise of respondents who were not annoyed in any degree was 42 dB. The difference in exposure levels of respondents who were and were not annoyed in any degree by helicopter noise was unlikely to have arisen by chance alone, but accounted for very little variance in the relationship between noise exposure and annoyance. Likewise, a weak but statistically significant relationship between exposure to helicopter noise and high annoyance (self-description by respondents as “very” or “extremely” annoyed by heli- copter noise) was observed in the Long Beach interviewing area. No statistically significant relationship between helicopter noise levels and annoyance due to in-home vibration and rattling was observed at any of the three study areas. Less than 3% of all respondents reported that they had ever registered complaints about helicopter noise. Among the 1,937 respondents who reported no annoyance with helicop- ter noise, 1.3% registered complaints; of the 330 respondents who reported at least slight annoyance by helicopter, 9.4% registered complaints. No statistically significant difference was observed in the helicopter-only DNL for respondents who did and did not complain. At two of the three interviewing sites (Las Vegas and Washington), the prevalence of high annoyance with helicopter noise was statistically distinguishable from zero, but varied little with DNL. At the remaining site (Long Beach), the prevalence of high annoyance with heli- copter noise was also non-zero and invariant with DNL at low exposure levels, but increased modestly at levels exceeding about Ldn = 45 dB. The prevalence of annoyance with helicopter noise was not strongly related to noise expo- sure levels over the range of helicopter-only DNL values that were available for study. The present study could not determine whether respondents in the same communities differed in tolerance for fixed- and rotary-wing aircraft, because sites with comparable exposures to the two types of aircraft noise were not found. At the one interviewing site (Washington, D.C.) at which residents were exposed to both forms of aircraft noise, noise due to fixed-wing operations generated significantly higher annoyance, but the fixed-wing noise exposure was also considerably greater than noise exposure due to helicopter operations. The majority of survey respondents were exposed to helicopter-only DNL values between roughly 30 and 45 dB. These absolute levels of exposure to helicopter noise were low with respect to typical urban noise exposure, so that most of the observed prevalence rates of high annoyance with helicopter noise were correspondingly low as well. It was observed that individuals highly annoyed by fixed-wing aircraft noise were fifteen times more likely to be highly annoyed by helicopter noise than those not highly annoyed by fixed-wing aircraft noise. The relatively low levels of exposure to helicopter noise (with respect to other sources of cumulative urban noise exposure) are believed to be responsible for a general absence of strong helicopter noise effects in the current data set. The findings of the present study do not support construction of useful dosage-response relationships between exposure to helicopter-only noise and the prevalence of high annoyance. It also does not appear that further surveys along typical civil helicopter routes would prove to be any more useful in developing a dosage-response relationship. Additional study in communities with much higher helicopter DNL exposure values, such as around military facilities, might support development of a more definitive dosage-response relationship. However, such a relation- ship would be applicable primarily to heavy military helicopters whose impulsive noise signatures are more prominent than those of lighter civil helicopters.