Assessing Community Annoyance of Helicopter Noise (2017)

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96 This chapter discusses conclusions that may be drawn about the major hypotheses investi- gated in the current study. Hypothesis 1: Decibel for decibel, rotary-wing aircraft noise is more annoying than fixed- wing aircraft noise. No compelling evidence was found for the “excess” annoyance of civil helicopter noise with respect to that of fixed-wing aircraft noise. A likely reason for the absence of such evidence is that the study was conducted at interviewing sites with relatively low levels of helicopter noise expo- sure. If the study had been conducted in communities overflown by noisier military helicopters, the conclusion might have differed. Interviewing sites with relatively low levels of cumulative exposure to helicopter noise were not selected for study by preference, but rather because sites with greater levels of civil helicopter noise exposure could not be located, or were unsuitable for interviewing for lack of residential exposure. The majority of the urban residential population overflown by scheduled civil helicopter operations is exposed to helicopter noise during cruise conditions, during straight and level flight at altitude. Even though maneuvering helicopters can be more complex and variable noise sources than fixed-wing aircraft in the vicinity of landing pads, the character of their noise emis- sions in the cruise regime may not differ as greatly in character from that of fixed-wing aircraft. In the Washington, D.C., interviewing area, a notably greater rate of annoyance was observed for fixed-wing aircraft than for helicopters. Because noise exposure due to fixed-wing aircraft was considerably greater than that for helicopters in Washington, D.C., it was not possible to draw inferences about the relative annoyance of the two noise sources on a decibel-for-decibel basis. A greater annoyance prevalence rate for helicopters than for fixed-wing aircraft was observed only in the Long Beach study area, but the respondents in the study area were exposed to very little fixed-wing traffic noise. For the one site at which a reasonable dosage-response function could be inferred for annoy- ance due to exposure to helicopter noise, the DNL at which 50% of the population would be highly annoyed by helicopter noise was estimated at 69 dB. That is 4 dB less than the grand average for the 44 fixed-wing aircraft (Ldn = 73.4 dB, per Fidell et al. 2011). An indirect inference can therefore be drawn that helicopter noise is 4 dB less tolerable (quite likely for nonacoustic reasons) than the noise produced by fixed-wing aircraft. Hypothesis 2: Main rotor impulsive noise controls the annoyance of helicopter noise (and hence requires an impulsive noise “correction” to A-weighted measurements). A strong correlation between the prevalence of high annoyance and (A-weighted) DNL values was observed in only one of the three surveys in the interviewing area. Neither C-weighted C H A P T E R 6 Conclusions and Discussion

Conclusions and Discussion 97 measurements nor helicopter-adjusted LFSL measurements were any better at predicting annoyance prevalence rates due to dose. In Las Vegas and Washington, D.C., annoyance was not related to dose as measured by the A-weighted, C-weighted, or the helicopter-adjusted LFSL. In Washington, D.C., a public concern over relocated fixed-wing flight tracks might have made it difficult to discern any dosage-response relationship. It is also likely that the low range of doses of helicopter noise precluded observation of a strong relationship with annoyance. It would have been advantageous to have surveyed a community with a helicopter noise exposure greater than Ldn = 60 dB. To do that, a survey would have had to have been conducted around a military facility. The research panel restricted the surveys to civil helicopter routes, thus limiting the noise dose to DNL below 60 dB. Measurements of A- and C-weighted impulsive noise levels and non-impulsive A- and C-weighted levels differed only by a constant. However, the rotor disks of the civil helicopters that created the noise exposure measured in this study lack the heavy loading, larger diam- eter, and high tip speeds of military helicopters. The levels of impulsive noise to which respon- dents were exposed in this study were considerably lower than those produced by maneuvering, heavier helicopters. This hypothesis would be better tested at sites with heavy military helicopter operations so that the impulsive noises were more pronounced. No clear conclusion could be drawn from the present findings about this hypothesis. Hypothesis 3: Secondary emissions (rattle) induced by helicopter noise strongly influences its annoyance. The prevalence of high annoyance was regressed on reported in-home vibration/rattling as well as on BVI (thumping or slapping), buzzing, and whining noise. No statistically significant relationship was observed between annoyance due to in-home vibration and rattling and annoy- ance due to noise level alone. The dosage-response relationship between helicopter noise exposure and annoyance due to buzzing differed significantly from chance, and was unlikely to have arisen by chance alone in Long Beach, but not in Las Vegas or Washington, D.C. The regression of reported buzz- ing noises on helicopter noise exposure was the only one that was unlikely to have arisen by chance alone, but it accounted for very little variance in the relationship between annoyance and exposure. In the apparent absence of any strong association between helicopter noise exposure and annoyance at the low exposure levels that were available for this study, it is likely that nonacoustic factors had a greater effect than exposure levels on community response to helicopter noise. Hypothesis 4: The annoyance of helicopter noise is strongly influenced by nonacoustic factors. No acoustic factors can account for observed differences in the annoyance of exposure to heli- copter noise at the interviewing sites. Given the observed differences in response at the Long Beach and Las Vegas interviewing sites, it is likely that nonacoustic factors were more salient than noise exposure in determining community response. Respondents in Las Vegas were exposed to about 10 times the number of flights (albeit at a greater altitude), but a much smaller percentage of the respondents in Las Vegas than in Long Beach reported high annoyance. The higher altitude effect on DNL (about a 3 to 4 dB reduction) was much smaller than the 10 dB effect of a greater number of operations on DNL. Aircraft fleet mix cannot account for the difference in annoyance prevalence rates either. In Washington D.C., the concern over the change in fixed-wing flight tracks obscured the dosage- response effect for both fixed-wing and helicopter noise.

98 Assessing Community Annoyance of Helicopter Noise Hypothesis 5: Annoyance is better predicted by time-integrated proximity to flight tracks than by acoustic measures. Regression analyses showed that proximity to the flight path was as good a predictor of self- reported high annoyance with helicopter noise as helicopter noise levels. This is not a surprising finding, since proximity and sound level are highly correlated. It remains unclear, however, whether exposure to the noise of direct overflights was found to be more annoying than expo- sure to noise of overflights that pass to the sides of residents’ homes. Additional hypotheses examined: Complaints lodged about helicopter noise are more reli- able predictors of the prevalence of annoyance than measures of exposure to helicopter noise or proximity to helicopter flight paths. An analysis of variance revealed no statistically significant difference in noise exposure for respondents who reported complaining than for those who did not. Very few respondents indi- cated that they had ever registered complaints about helicopter noise, however. Nonetheless, a statistically significant relationship was observed between the likelihood of complaint and report- ing some degree of annoyance. Among the respondents who reported no annoyance from heli- copter noise, 1.3% complained; of the respondents who reported at least slight annoyance from helicopter noise, 9.4% registered complaints. The likelihood of complaining about helicopter noise is thus at least partially dependent upon some degree of annoyance. Additional observations: Noise exposure and annoyance, dosage-response relationship No compelling evidence was found other than at the Long Beach interviewing site of a dosage- related increase in the prevalence of high annoyance. That is, all data points were observed to lie on some non-zero asymptotic value. With the Long Beach data, the rightmost three data points in the dosage-response plot were assumed to be dependent on dose. The remainder were assumed to be independent of dose and lie at some asymptotic value. Similarly, for the distance relationship, the data points at 28 dB and higher were assumed to be dependent on reciprocal distance, and the rest independent. For Washington, D.C., there is no evidence of annoyance growth with increasing dose or reciprocal distance as shown in Figures 5-19, 5-21, 5-23 and 5-35. For fixed-wing aircraft the asymptotic value of annoyance is about 15%. The range of respondent DNLs is also the same (a 10 dB range from 50–60 dB). However, comparing asymptotic annoyance percentages between fixed- and rotary-wing aircraft, the numbers are 15%, 16%, and 4.75%, respectively—a 10.41 dB difference.