Assessing Community Annoyance of Helicopter Noise (2017)

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115 The entries in the following bibliography are not intended to be comprehensive but rather to summarize interpretations of findings of some of the better-known studies of the annoyance of helicopter noise. They exclude studies intended mostly to measure helicopter noise emissions, and some laboratory studies of rotor noise whose findings have little direct bearing on the design of social surveys of the annoyance of helicopter noise. Although preference was given to annotat- ing peer-reviewed studies, a number of technical reports are annotated as well. Atkins, C., Brooker, P. and Critchley, J. (1983) 1982 Helicopter Disturbance Study: Main Report. Civil Aviation Authority/Department of Transport/British Airports Authority. The authors report the results of a large-scale field study intended to evaluate attitudinal dif- ferences to fixed- and rotary-wing aircraft. Six interviewing areas were chosen with differing proportions of the two aircraft types, from none to exclusive. Areas near military installations were avoided in the belief that attitudes near such installations might differ from those of the general population. Each potential site received considerable pre-study qualification, including site visits to some and consultations with air traffic control and airport personnel. Exclusive heli- copter exposure was found in areas where aircraft served North Sea oil platforms and helicopter passenger service. Interviews were conducted in person. Interview areas were sized to encompass cumulative exposure ranges no greater than 5 dB. (All respondents within such areas were assumed to receive the same dose.) Questionnaire completion rates across interviewing areas ranged from 61 to 82 percent. Continuous sound level measurements were conducted for 10 or more days in each area. The measurements were largely unattended except in areas where varying source contributions or complex flight procedures were anticipated. The survey instrument was quite lengthy, as it sought information about a large number of variables that might relate to respondent attitudes. The main questionnaire item about bother or annoyance used a four-point category scale. This question was asked only of those respondents who in an earlier question responded positively that they heard aircraft noise. An average of 30 percent of respondents expressed fear that an overhead aircraft might crash. The attitudinal response of bother or annoyance to aircraft noise was found to be positively correlated with crash fear: “On the whole, residents who feared a crash were more annoyed by aircraft noise than those who did not.” The authors noted that the scatter of dosage-response points about their trend line exhib- ited greater scatter than expected by chance alone. This scatter was somewhat reduced when respondent socio-economic group was factored into the analysis. Some neighborhoods differed markedly in the age of the population, however no age effect was found in the dosage-response analysis. A P P E N D I X B Annotated Bibliography

116 Assessing Community Annoyance of Helicopter Noise Edwards, B. (2002) Psychoacoustic Testing of Modulated Blade Spacing for Main Rotors. NASA Contractor Report 2002-211651. Edwards reports the results of laboratory studies of the annoyance of noise created by a simulated 5-bladed main rotor with unevenly spaced rotors. Forty subjects assigned numeric ratings to the annoyance of various simulated blade configurations, and forty provided paired- comparison ratings. Edwards concludes that “No strong subjective differences among the pre- dicted helicopter test sounds were found in either test. . . .” and that A-weighted measures of helicopter rotor noise are “. . . not strongly indicative of subjective response.” Federal Aviation Administration (2004) Report to Congress: Nonmilitary Helicopter Urban Noise Study. Report of the Federal Aviation Administration to the United States Congress Pursuant to Section 747 of the Wendell H. Ford Aviation Investment and Reform Act for the 21st Century (AIR-21), Washington, D.C. FAA’s review of the technical literature on the annoyance of helicopter noise in its Report to Congress cites eight (mostly laboratory) studies supporting the imposition of a blade slap “pen- alty” on A-weighted measurements of helicopter noise, and seven suggesting that such a penalty is not justified. The FAA report also cites two studies of “heightened reaction” to helicopter noise—presumably not associated with blade slap—by Schomer (1983) and by Atkins et al. (1983). Despite the inconsistency and ambiguity of these findings, the report repeats the com- mon assertion that “helicopter noise may be more noticeable because of its periodic impulsive characteristic.” The report also cites “the possible phenomena (sic) of ‘virtual noise’ ” [see annota- tion for Leverton (2014) below], which it suggests may be due to attitudes and beliefs about the necessity of helicopter operations and fear of crashes. The FAA report also includes brief discussions in Sections 3.5.5 through 3.5.8 of contentions that “helicopter noise is more annoying than fixed-wing aircraft noise”; that “helicopter sounds may be more readily noticeable than other sounds”; that attitudes such as fear of danger, beliefs about the importance of the noise source, and invasions of privacy may influence the annoy- ance of helicopter noise; and that rotary-wing flight capabilities such as prolonged hovering and proximity to residences may also heighten the annoyance of helicopter noise. The primary conclusion of FAA’s Report to Congress is that “models for characterizing the human response to helicopter noise should be pursued.” The report also includes a wide range of recommendations, including some that are reflected in the current effort. For example, FAA recommends study of “nonacoustical effects,” among which includes vibration and rattle, and “virtual noise,” as described informally by Leverton (see below) and systematically by Fidell et al. (2011). The report also suggests that unique characteristics of helicopter noise emissions (nota- bly including blade slap) may heighten community annoyance with helicopters; that evaluation of noise metrics other than DNL should be undertaken; and that “operational alternatives that mitigate noise should be examined.” The latter specifically includes higher altitude flight and route planning to avoid noise sensitive areas. Fidell, S., and Horonjeff, R. (1981) Detectability and Annoyance of Repetitive Impulse Sounds, Proceedings of the 37th Annual Forum, American Helicopter Society, New Orleans, LA, pp. 515–521. The audibility of low-frequency rotor noise is of concern not only in residential settings, but also in military applications (where the element of surprise can be mission-critical) and airspace subject to special federal aviation regulations intended to protect natural quiet. In such applica- tions, the main concern is prediction of the audibility of wavetrains of repetitive acoustic impulses, rather than of individual impulses. Fidell and Horonjeff (1981) demonstrated that over a range of observation intervals (0.25 to 2.00 seconds) and repetition rates (5 Hz to 40 Hz, corresponding to

Annotated Bibliography 117 the range of fundamental and harmonics of blade passage rates of present interest) the audibility of impulse wavetrains is very closely predictable from the audibility of a single impulse. Under highly controlled listening conditions, participants determined when impulse wave trains of varying repetition rate and observation interval duration were just audible in white noise. The impulse was a 1000 Hz sinusoid. Test participants also listened for a single impulse randomly placed within a 500 msec observation interval. Equation 1 shows a derived relationship between the energy ratio of a wave train divided by single impulse (left side of equation) and the repetition rate and observation interval (right side). ( ) ( ) ( ) ( )− = + +10 log E N 10 log E N 5 log RR 8 log D 1.5 Eq. 110 ri 0 10 si 0 10 10 where: Eri/N0 = signal energy to noise power density ratio of impulse wave train Esi/N0 = signal energy to noise power density ratio of a single impulse RR = impulse repetition rate (Hz) D = observation interval (seconds) Figure B-1 shows the resulting clustering of data points (each an average over all test subjects) when the energy ratio is plotted against repetition rate and the energy ratios have been adjusted for the duration term, 8 Log10(D) in Equation 1. The tight fit of the data points to the line (plus or minus 0.3 dB) suggests a strong predictive relationship between repetition rate and observation interval (all for the same waveform) and the energy ratio of the wavetrain and single impulse. The positive slope of about 1.5 dB per 3 4 5 6 7 8 9 10 11 250 msec 333 msec 500 msec 1000 msec 1333 msec 2000 msec Tic 5 10 13 20 30 40 Impulse Repetition Rate (Hz) 10 L og [E ri/ N o] - 1 0 Lo g [E si /N o] ( dB ) Figure B-1. Observed relative signal-to-noise ratios (10 log10[En/N0] – 10 log10 [Esi/N0]) of equally detectable impulse wavetrains as a function of impulse repetition rate collapsed over observation interval duration by 8 log10[D].

118 Assessing Community Annoyance of Helicopter Noise doubling of repetition rate (or 5 dB/decade) indicates that greater signal energy is needed at increasing repetition rates to maintain constant detection performance, and that these slopes are effectively independent of observation interval duration over the investigated range. Fields, J., and Powell, A. (1987) Community Reactions to Helicopter Noise: Results from an Experimental Study. J. Acoust. Soc. Am., 82(2), 479–492. Noting the characteristically small numbers of helicopter overflights in many residential exposure settings, Fields and Powell focus on “the applicability of the equivalent energy assump- tions about the relative importance of noise level and number of noise events.” They devised a controlled-listening field study in which the same 330 respondents were paid $40 to complete repeated interviews on the evenings of 22 days about their annoyance with late morning and early afternoon weekday helicopter noise. The study area, in close proximity to an army helicopter training base, was a strip 500 meters long, containing 861 dwellings, in a “quiet, well-maintained, middle-class suburban area” with high military employment. The residents were thoroughly habituated to helicopter overflight noise. Large percentages of respondents considered helicopters “very important” (64%), believed that “pilots or other authorities” could not do anything to reduce helicopter noise (62%), and were not afraid that a helicopter might crash nearby (67%). The daily interview lasted only about four minutes and was confined to determining the times at which respondents were at home during the day, what noise sources they heard, and how annoyed they were by them. Noise measurements were limited to those made at one fixed site at the end of the exposure area, and two roving mobile sites. Fields and Powell found that respondents’ annoyance ratings of helicopter noise increased with both number and level of noise exposure. The average annoyance scores were almost all below 4 on a ten-point scale, indicating that few, if any, respondents were highly annoyed by helicopter noise in the target population. They also found only minor differences in annoyance scores for long-term exposure to more or less impulsive noise: “annoyance, in general, was slightly higher” for exposure to more impulsive noise (UH-1H). Correlations between noise exposure levels and annoyance scores accounted for less than 10% of the variance in the relationship. Leverton, J. Helicopter Noise: What is the Problem? Vertiflite, Vol. 60, No. 2, March/April 2014, pp. 12–15. (See also Leverton and Pike, 2007 and 2009) The standard measure of adverse public reaction to transportation noise exposure is the prevalence of a consequential degree of noise-induced annoyance (FICON 1992; ISO 2016). Leverton (2014) asserts that vigorous adverse community reaction to helicopter noise “is a little difficult to understand because most helicopters generate less noise than the noise certification standards [for fixed-wing aircraft]. . . .”20 He infers from this observation that “there appears to be something different about the way in which helicopters are perceived.” Leverton expands the concept of “something different” about the perception of helicopter noise into the concept of “virtual noise.” He offers somewhat contradictory definitions of virtual noise, however. On the one hand, Leverton states that virtual noise is nonacoustic in nature. This is a plausible belief, since the annoyance of an unwanted noise intrusion is, after all, a property of an unwilling listener, not of a noise source per se. A sound level meter measures sound pres- sures, not annoyance. Absent a reliable dosage-response relationship, useful inferences cannot be drawn from noise levels alone about the prevalence of annoyance with transportation noise in noise-exposed communities. On the other hand, Leverton believes that even though virtual noise is not directly related “either to the absolute level or to the character of the noise generated by helicopters,” it is

Annotated Bibliography 119 nonetheless “triggered by the direct acoustic signal.” As Leverton puts it, “Virtual noise is depen- dent on a wide range of inputs but is triggered initially by any distinctive feature of the acoustic signature and, to a far lesser extent, the absolute noise level.” In other words, adverse community reaction to helicopter noise is conditioned on two sets of factors other than the conventionally measured, A-weighted acoustic energy of helicopter noise emissions. The first component of virtual noise is the noticeability of distinctive features of helicopter noise emissions, such as HSI, tail rotor (TR) noise, main rotor/tail rotor interaction (TRI) noise, and BVI. In Leverton’s view, the second component of “virtual noise” is entirely nonacoustic. Leverton’s concept of virtual noise has several limitations. First, it does not consider the pos- sibility that certain characteristics of helicopter noise could be highly annoying at levels that do not control a helicopter’s total A-weighted noise emissions. Second, it does not clearly distin- guish between the influences of acoustic and nonacoustic factors on the annoyance of helicopter noise, nor offer any quantitative guidance about the relationships between them. Third, it does not provide any operational definition or methods of quantifying the nonacoustic aspects of virtual noise. The major contribution of this publication is that it reinforces the notion that factors other than those that can be measured with a sound level meter may somehow affect the annoyance of helicopters. Magliozzi, B., Metzger, F., Bausch, W., and King, R. (1975) A comprehensive review of helicopter noise literature. FAA-RD-75-79. The “comprehensive review” of Magliozzi et al. (1975) is more of a summary of early field measurements of helicopter noise than a critical review. It focuses more on noise emissions and noise control concerns than on the subjective effects of helicopter noise on individuals or communities. Some of the reasoning is specious, as for example, when the authors conclude “Spectrum analyses of helicopter noise show that the main rotor, tail rotor, and engine sources contribute significantly to annoyance.” Merely because rotating noise sources contribute con- spicuously to a spectrogram does not mean that they are “significant” sources of annoyance. Likewise, Magliozzi et al. (1975) repeat the views that a need for “a new noise unit” for mea- suring helicopter noise is required, and assert that a “modification of the Day-Night Noise Level (sic) . . . shows promise” for assessing community acceptance of helicopter noise. Molino, J. A., (1982) Should Helicopter Noise Be Measured Differently From Other Aircraft Noise?—A Review of the Psychoacoustic Literature, NASA Contractor Report 3609. Molino’s review describes the many differences between fixed- and rotary-wing aircraft noise but pays most attention to the impulsive nature of helicopter BVI noise (“blade slap”). He reviewed 34 studies of the noisiness of helicopter blade slap, many of which were non-peer- reviewed conference papers or technical reports, which yielded conflicting if not contradictory findings. His conclusion that “there is apparently no need to measure helicopter noise any dif- ferently from other aircraft noise” is based largely on the lack of consistent empirical findings about the “excessive” (with respect to the annoyance of fixed-wing aircraft noise) annoyance of impulsiveness per se. The zeitgeist of the early 1980s, particularly ISO’s attempts to recommend noise metrics appropriate for certification of helicopter noise, appears to have influenced Molino’s analyses. Several national helicopter industries had proposed methods for assessing the annoyance of heli- copter noise. Each disproportionately penalized the noise emissions of competitors’ products. Aérospatiale, for example, proposed a “correction” to helicopter noise that heavily penalized even slight short-term temporal variation in noise levels. “Corrections” proposed by British

120 Assessing Community Annoyance of Helicopter Noise sources, on the other hand, heavily penalized tonal components of helicopter noise, such as those produced by Sud Aviation’s (subsequently Aérospatiale, Eurocopter, and now Airbus Helicopters) high-speed, ducted fan (“Fenestron”) tail rotor. Molino’s report goes into considerable detail about the acoustic characteristics of helicop- ter noise emissions and into variability in noise emissions associated with various helicopter types and operating conditions. He notes that relationships between operating mode, engine power, and airspeed in helicopters are not as straightforward as they are for fixed-wing aircraft. For example, Molino observes that unlike fixed-wing aircraft, “helicopters generally produce a minimum sound level at some intermediate airspeed, with higher sound levels at lower and higher airspeeds.” He also observes that “for the same airspeed, helicopters often exhibit differ- ent sound spectra for approach versus level flight.” The psychoacoustic research reviewed by Molino consists mostly of 1970s-era studies, with a smattering of earlier and later studies. A major part of Molino’s review addresses the meth- odological advantages and disadvantages of varying forms of signal presentation, listening con- texts, and annoyance-rating scales for controlled-listening tests. He ultimately speculates that 1) “the source of . . . [discrepancies among empirical findings] . . . may lie in the methodolo- gies and approaches selected by the experimenters,” rather than in bona fide differences in the annoyance of helicopter noise and 2) that inadequate experimental treatment of the complexity of helicopter noise may obscure the annoyance of helicopter noise. For example, Molino notes “The presence of blade slap, in and of itself recognized as contributing to increased annoy- ance, produces changes in other acoustic parameters that can compensate for or account for the increased annoyance caused by the presence of blade slap.” Molino concludes from the contradictory and inconclusive nature of the findings of labora- tory studies about the annoyance of helicopter noise that “there is apparently no need to measure helicopter noise any differently from other aircraft noise.” The logic and universality of Molino’s conclusion are open to question given the limited nature of comparisons that Molino describes among the findings of different forms of laboratory studies of the annoyance of helicopter noise. Another major limitation of Molino’s review is that he confines his review to the direct annoyance of airborne acoustic energy produced by helicopters, and does not take into account the potential contributions to annoyance of secondary emissions (audible rattle and sensible vibration) produced by helicopter flight operations inside residences. To the extent that any excess annoyance of helicopter noise is related to the annoyance of secondary emissions, Molino’s conclusion about the sufficiency of A-weighted measurements is premature. More, S. R., (2011) Aircraft Noise Characteristics and Metrics. Purdue University Doctoral Thesis and Report No. PARTNER-COE-2011-004. More’s thesis reports the findings of laboratory studies of second-order effects, such as “sharp- ness” (spectral balance of low and high frequency energy), tonality (presence of prominent tones), slow fluctuations in loudness (fluctuation “strength”), and “roughness” (rapid fluctua- tions in loudness) on absolute judgments of the annoyance of single-event, fixed-wing aircraft noise presentations. (The reported work does not address the effects of rattle and vibration, or the annoyance of cumulative noise exposure.) Although More’s interests did not specifically extend to the annoyance of helicopter noise, some of the factors that he studied are more char- acteristic of complex rotary-wing noise emissions than those of simpler, broadband fixed-wing aircraft. The laboratory judgments did not demonstrate any clear contributions of sharpness, rough- ness, and fluctuation strength to judgments of the annoyance of aircraft noise. Loudness remained the major determinant of judged annoyance, with a clear contribution of tonality.

Annotated Bibliography 121 Munch, C. and King, R. (1974), Community acceptance of helicopter noise: criteria and application.National Aeronautics and Space Administration, NASA-CR-132430. Because assumptions made by the authors have not withstood the passage of time, the rea- soning in this 40-year old study—dating from the era prior to FICON’s recognition of the prevalence of a consequential degree of annoyance as a preferred measure of adverse impact of transportation noise—is largely irrelevant to modern analyses of the effects of helicopter noise exposure on communities. For example, the authors loosely define “community noise acceptance criteria” in terms of “a noise exposure acceptable to the average member of the community.” Further, they interpret EPA’s recommendation of a DNL of 60 dB as a level consistent with “requirements for human compatibility in the areas of annoyance, speech interference, and hearing damage risk” as a basis for regulating aircraft noise. They also assume that A-weighted noise levels 2 dB lower than ambi- ent levels are completely acceptable, and that ambient noise levels in inhabited places will decrease “over the years due to stricter controls on noise sources other than aircraft.” Neither assumption is correct. The audibility of aircraft noise cannot be reliably predicted from A-weighted noise levels, and Schomer et al. (2011) has shown that the slope of the relationship between population density and cumulative noise exposure has remained unchanged for about 40 years. The authors also report an informal study of the noticeability of blade slap, from which they estimate that notice of blade slap occurs at a crest factor of 13 dB. This figure is little greater than the crest factor of many urban ambient noise environments. Although the authors repeatedly emphasize that understanding of the annoyance of blade slap is “sketchy,” “inadequate,” “very limited,” “inconsistent,” etc., they nonetheless conclude that a “penalty” is required to account for the annoyance of repetitive impulsive aircraft noise. The magnitude of the recommended penalty in units of perceived noise level is 4 to 6 dB, or 8 to 13 dB in A-weighted units. Namba, S., Kuwano, S., and Koyasu, M. (1993) The Measurement of Temporal Stream by Hearing by Continuous Judgments—In the Case of the Evaluation of Helicopter Noise, J. Acoust. Soc. Jpn., 14, 5. Namba et al. (1993) suggest that the practice of calculating equivalent energy metrics for time-varying environmental noises (such as those produced in the course of helicopter flight operations) can misestimate their annoyance because they do not take into consideration the temporal context of noise intrusions.21 They propose instead a method of continuous judgment, such that the annoyance of helicopter and other “. . . fluctuating sounds [can be measured] by pressing a key on a response box . . .”, in real time. The authors found marked differences in the momentary annoyance of helicopter takeoffs, overflights, and landings. Ollerhead, J. B. (1982) Laboratory Studies of Scales for Measuring Helicopter Noise. NASA Contractor Report 3610. Ollerhead solicited absolute judgments from scores of test subjects of the annoyance of tape recorded helicopter sounds presented both over headphones and via loudspeaker in a series of laboratory studies. A set of preliminary investigations was conducted to pilot-test the annoyance-rating and signal presentation methods. A set of “main” tests followed, in which six undergraduates at a time rated the annoyance of the sounds of 89 helicopters (mostly level fly- overs) and 30 fixed-wing aircraft heard through headphones. The headphone presentation results were generally replicated in subsequent free-field testing at NASA Langley Research Center. Ollerhead concludes that tone-corrected effective (that is, duration-adjusted) Perceived Noise Level predicts the annoyance of helicopter noise better than does A-weighted sound pressure level, and that any putative effects of impulsiveness per se may be equally attributed to increases in helicopter noise level and duration.

122 Assessing Community Annoyance of Helicopter Noise Ollerhead, J. B., (1985) Rotorcraft Noise. Loughborough University of Technology, Leicestershire, England. Ollerhead’s review addresses “subjective impact” (individual and community response to exposure to helicopter noise), mechanisms of helicopter noise generation, and potential helicop- ter noise control measures, with greater emphasis accorded to the latter two topics.22 Like most other review articles, Ollerhead’s article deals at length with differences between rotary- and fixed-wing noise emissions. Among other salient differences, Ollerhead notes that unlike fixed- wing aircraft, “helicopters are usually confined to low altitudes,” and that “many helicopters radiate maximum noise in a forward direction,” so that “an approaching helicopter can often be heard for as long as five minutes.” Ollerhead’s review of subjective impacts of helicopter noise deals with statements attributed to Molino (1982). Like Molino, Ollerhead draws attention to contradictory findings and to apparent discrepancies between the findings of field studies and laboratory studies. Ollerhead notes, for example, that his own 1971 finding “that the very long attention-arresting sound of an approaching helicopter did not affect annoyance responses in the laboratory experiments” conflicts with “hearsay evidence of complainants near heliports that [duration of audibility] may be a particular source of aggravation to people at home.” Patterson, J., Mozo, B., Schomer, P., and Camp, R. (1977) Subjective Ratings of Annoyance Produced by Rotary-Wing Aircraft Noise. Bioacoustics Division, US Army Aeromedical Research Laboratory, Fort Rucker, Alabama, USAARL Report No. 77-12, May 1977. Patterson et al. (1977) describe an outdoor noisiness magnitude estimation test in which a panel of 25 audiometrically screened participants rated the sounds of actual rotary-wing aircraft passbys relative to that of a fixed-wing C-47 propeller-driven aircraft. The goals of the study were fourfold with regard to determining a metric that would best predict subjective annoyance: (1) which spectral weighting function(s) are most appropriate? (2) what type of temporal integration should be used? (3) is an impulsive blade slap correction factor necessary? and (4) do present fixed-wing annoyance predictors underestimate annoyance from rotary-wing aircraft? To evoke differing spectral and temporal characteristics, the listening test involved nine dif- ferent rotary-wing aircraft each flying six different flight maneuvers: (1) level flyover, (2) nap- of-the-earth, (3) ascent, (4) decent, (5) left turn, and (6) right turn. During each passby the sound pressure level signature was FM-recorded on magnetic tape for subsequent analysis into one-third octave bands. Observers recorded their noisiness rating relative to the C-47 at the end of each passby. In the subsequent analysis five broadband frequency-weighted metrics were considered: A-weighted sound level, B-weighted sound level, C-weighted sound level, D-weighted sound level, and tone-corrected perceived noise level (per Federal Aviation Regulation Part 36). For each, four different temporal treatments were examined: the maximum sound level, the peak sound level, the average sound level over the passby, and the time-integrated level over the passby. The Pearson product moment correlations (r), relating noisiness to all frequency weight- ings and temporal considerations are shown in Powell, C. A. (1981) Subjective Field Study of Response To Impulsive Helicopter Noise, NASA Technical Paper 1833. Figure B-2 plots the correlations in four groups of differing temporal considerations. Within each group the four different frequency weightings are shown. The figure reveals that the A-weighted and D-weighted sound levels and the tone-corrected perceived noise level all performed equally well as noisiness predictors regardless of the time integration method employed. The dashed horizontal line plots the average value of all the

Annotated Bibliography 123 coefficients for these metrics (0.81). In addition, the figure shows that B-weighted and C-weighted sound levels performed demonstrably more poorly. However, the maximum level was a better predictor of annoyance for both the C-weighted sound level and tone-corrected perceived noise level than was a temporal integration of these measures. These correlations notwithstanding, the authors found that on average the rotary-wing aircraft were rated an equivalent of 2 decibels more annoying than the fixed-wing C-47. This difference represents only about one-third of the scatter in sound level observed for any given relative annoyance rating but this difference is probably significantly different from zero (not determined by the authors). The authors note that the similar performance of the A, D, and tone-corrected metrics was largely due to the high correlation between the metrics themselves. The correlations (r) were largely independent of temporal consideration and ranged from 0.91 to 0.98. The authors thus concluded “The high correlation among these predictors of annoyance makes any attempt to show the superiority of one over another unlikely to succeed.” The authors also explored two measures of impulsivity to determine whether either improved the correlation. These were (1) the crest factor (peak minus root mean square) and (2) a novel adjunct to crest factor that measured the root mean square level between blade slaps and sub- tracted this value from the peak level. No improvement was found using crest factor. However, some modest improvement was found using the second method, but the authors concluded the method was too cumbersome to be used in practice. Powell, C. A. (1981) Subjective Field Study of Response to Impulsive Helicopter Noise. NASA Technical Paper 1833. Powell conducted two controlled-listening studies in which 91 test participants located both indoors and outdoors judged the noisiness of 72 helicopter and propeller-driven, fixed-wing Powell, C.A. (1981) Subjective Field Study of Response To Impulsive Helicopter Noise, NASA Technical Paper 1833. 0.84 38.038.0 0.84 0.50 0.70 0.63 0.70 0.38 0.55 0.59 0.63 0.79 0.85 0.83 0.85 0.72 0.82 0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0 Metric Integration P ea rs o n r c o rr el at io n c o ef fi ci en t A B C D PNLT Integrated Average Peak Maximum Figure B-2. Subjective noisiness correlations with four frequency weighting functions and four temporal integration measures.

124 Assessing Community Annoyance of Helicopter Noise aircraft flybys. After noting the “very diverse” character of helicopter noise, Powell comments on the inconclusiveness of studies intended to ascertain whether an impulsiveness correction is use- ful for predicting the noisiness of helicopter noise. One purpose of the current investigation was to determine whether highly impulsive helicopter overflights are judged to be noisier than less impulsive helicopter overflights at constant EPNL values. The other purpose was to determine the utility of ISO’s then recent suggestion of an impulsiveness correction to EPNL. Powell’s findings were counter-intuitive and in direct contrast to the common assumption (cf. Sternfeld and Doyle, 1978) that the impulsiveness of helicopter noise accounts for much of its annoyance. Powell found that “at equal effective perceived noise levels (EPNL), the more impulsive helicopter was judged less noisy than the less impulsive helicopter.” Powell also found that ISO’s proposed impulsiveness correction, based on measurements of A-weighted crest fac- tors, failed to improve the ability of EPNL to predict helicopter noisiness judgments. Powell concluded that “. . . some characteristic [of helicopter noise] related to impulsiveness is perceiv- able by subjects but is not accounted for by either EPNL or [ISO’s] proposed impulsiveness correction.” Schomer, P. D., Hoover, B. D., and Wagner, L. R. (1991) Human Response to Helicopter Noise: A Test of A-weighting. U.S. Army Corps of Engineers, USACERL Technical Report N-91/13. Schomer, P. D., and Neathammer, R. D. (1987) The role of helicopter noise-induced vibration and rattle in human response. J. Acoust. Soc. Am., 81(4), pp. 966–976. Schomer et al. (1991) describe this study as a continuation of a field study (“jury test”) con- ducted by Schomer and Neathammer (1987). The former study solicited individual paired- comparison judgments of the annoyance of helicopter flybys with respect to a single broadband noise from groups of paid test participants seated in a house, a tent, and a mobile home. Schomer and Neathammer (1987) concluded that A-weighted measurements of helicopter flyby noise did not adequately predict differences in annoyance between the flyby noise and the control signal, and that the level of secondary emissions (helicopter-induced rattle) in the listening environ- ment influenced the annoyance judgments. The annoyance judgments were solicited in a field setting rather than in a laboratory because “the very low-frequency sounds, the rattles, and the vibrations characteristic of helicopter noise would be too hard to simulate realistically in a laboratory. . . .” Neither A-weighted nor C-weighted measurements of helicopter noise were able to predict offsets between objective measurements of sound levels produced by helicopter flybys and the comparison sounds when heard at subjectively equally annoying levels. The differences between A-weighted and C-weighted levels of helicopters and equally annoying broadband noise varied from 10 dB (for helicopters with two bladed main rotors) to 8 dB for helicopters with greater numbers of rotor blades. In other words, Schomer et al. (1987, 1991) found that exposure to helicopter noise depended in part on its impulsive characteristics (blade passage frequency and/or repetition rate) and the rattle induced by repetitive impulsive signals in residences. This finding directly contradicts Molino’s interpretation a decade earlier of the (largely laboratory-based) research findings that “there is apparently no need to measure helicopter noise any differently from other aircraft noise.” Note, however, that the Schomer et al. (1987, 1991) studies included no direct comparisons of the annoyance of exposure to rotary- and fixed-wing aircraft sounds. Because these studies included no direct empirical comparisons of helicopter noise with fixed-wing aircraft noise, they do not clarify whether the observed “excess” (that is, greater than A-weighted) annoyance of helicopter noise also holds with respect to fixed-wing aircraft noise.23

Annotated Bibliography 125 Schomer, P., and Wagner, L. (1996) On The Contribution Of Noticeability of Environmental Sounds to Noise Annoyance. Noise Control Eng. J., 44 (6), 294–305. Schomer and Wagner provided modest numbers of paid volunteers at three locations with portable (palm-top) computers to self-report prompt annoyance judgments for naturally occurring outdoor noises that they noticed while at home. The computers administered a brief questionnaire that asked respondents to identify the source of the annoying sound (e.g., rotary- or fixed-wing aircraft) and their degree of annoyance with it. Unattended outdoor noise mea- surements were made at locations near the test participants’ homes. The authors analyzed both the per event annoyance ratings and the rate of notice of noise events. They found only minor differences in the per event annoyance ratings of fixed- and rotary- wing aircraft noise of comparable A-weighted SELs. In fact, for some of the test participants, the annoyance ratings varied little with SELs. Mere detection of noise events seemed to suffice to annoy these participants. However, the authors also found that the rate of notice of helicopter noise was three times as great as the rate of notice of fixed-wing aircraft noise. They speculate that the greater rate of notice of helicopter noise was due to the “distinct sound character” of rotary—wing air- craft. Since the participants were exposed to notably fewer helicopter than fixed-wing over- flights, it is also possible that they were less habituated to helicopter noise than to fixed-wing aircraft noise. Sternfeld, H., and Doyle, L. B. (1978) Evaluation of the Annoyance Due to Helicopter Rotor Noise. NASA Contractor Report 3001, NASA Langley Research Center Contract NAS1-14192. Sternfeld and Doyle conducted controlled (laboratory environment) listening tests in which 25 volunteer listeners adjusted the annoyance of three degrees of rotor impulsiveness, heard at four blade passage (repetition) rates, to the annoyance of a single broadband noise. Like vir- tually all other publications in this research area, Sternfeld and Doyle characterize helicopter noise as “unusually complex.” They assert, however, without further elaboration, “It is the more impulsive types of rotor noise which are responsible for most of the noise complaints against helicopters.” Sternfeld and Doyle did not match the annoyance of broadband noise with that of fixed-wing aircraft noise. The experimentation conducted by Sternfeld and Doyle was premised on the assumption that main rotor impulsiveness controls the annoyance of helicopter noise. The authors there- fore did not study the potential contributions of other sources of helicopter noise to annoyance judgments. Sounds presented to test participants for annoyance judgments were reproduced by headphones, rather than in free-field settings and consisted entirely of synthesized signals. On the continuum of compromise between face validity and precision of control, the work of Sternfeld and Doyle sacrifices nearly all claims to face validity to a desire for very high precision of control of signal presentation. The authors concluded that their findings permit designers of helicopter rotor systems “to trade off rotor design parameters” to minimize their annoyance, but note certain limitations of the generalizability and practicality of their findings. They were also puzzled (1) by an “apparent inconsistence that when different rotor sounds were adjusted to be equally annoying as a broad- band reference sound, subsequent subjective ratings of the rotor sounds were not equal to each other, or to the broadband reference sound,” and about (2) “the apparent relative insensitivity to the rotor blade passage period.” They conjecture that headphone presentation of signals for annoyance judgments deprived test participants of the sensations of high-level, near-infrasonic harmonics on body surfaces.

126 Assessing Community Annoyance of Helicopter Noise Sternfeld, H., Spencer, R., and Ziegenbein, P. (1995) Evaluation of The Impact of Noise Metrics On Tiltrotor Aircraft Design. NASA Contractor Report 198240. Sternfeld et al. (1995) introduce their indoor, controlled-listening study of the judged annoy- ance of simulated rotor noise by re-capping the inappropriateness of the A-weighting network as applied to rotary-wing aircraft noise, which characteristically includes large amounts of low- frequency, if not infrasonic, acoustic energy associated with the fundamental blade passage fre- quency of a main rotor and its harmonics. Although the work is motivated by concerns about noise produced by a hovering tiltrotor, the arguments apply generally to other rotary-wing aircraft. Forty test subjects rated the annoyance of 145 outdoor and 145 indoor simulated rotor noise sounds. The sounds varied in A-weighted and overall sound pressure level from 72 to 96 dB, and in fundamental blade passage rates from 15 to 35 Hz. The spectra and presentation levels of the test sounds were arranged such that the overall sound pressure levels of the test sounds always exceeded A-weighted levels by 6 dB. Sounds intended to represent indoor listening conditions were accompanied by a projection of an indoor scene, while sounds intended to represent outdoor listening conditions were accompanied by a projection of an outdoor scene. Sternfeld et al. (1995) concluded that A-weighted measurements of the sounds rated by the test subjects were inferior predictors of the annoyance ratings because they were insufficiently sensitive to low-frequency rotor harmonics. They also concluded: 1. That a combination of A-weighted and overall sound pressure level measurements provided improved prediction of the annoyance ratings; 2. That annoyance predictions based on a combination of the two metrics were at least as good as, if not superior to, predictions made from Stevens Mark VII method of predicting per- ceived sound levels; and 3. That including blade passage frequency as a predictor of annoyance judgments improves matters yet further. The differences in correlations between predicted and observed ratings for the various pre- diction schemes were quite small in some cases. For example, adding blade passage frequency to perceived level increased the variance accounted for in outdoor judgments by only 2%, from R2 = 0.87 to R2 = 0.89. Considering the marginal size of many of the observed differences, and that the ISO standard for low-frequency equal loudness curves has changed since the conduct of the Sternfeld et al. analyses, the authors’ conclusions are best regarded as suggestive rather than definitive. Sutherland, L., and Burke, R. (1979) Annoyance, Loudness, and Measurement of Repetitive Type Noise Sources. EPA 550/8-79-103. This report evaluated “subjective and objective aspects of moderate levels of noise from impulsive sources,” such as truck-mounted garbage compactors, drop hammers, two-stroke motorcycle engines, and rock drills. The report specifically excludes consideration of high- energy impulses (sonic booms, weapons fire, and quarry blasting), and treats helicopter blade slap as a special case. Sutherland and Burke’s summary of early findings about the annoyance of blade slap may be paraphrased as follows: • The mean observed blade slap correction or penalty factor was 3.3 ±2.7 dB for 11 (labora- tory) studies that measured this quantity directly. However, three of these 11 studies found essentially a zero or negative correction. The maximum correction for moderate blade slap (i.e., crest level of 10 to 15 dB) was about 6 dB. The maximum correction for severe blade slap (i.e., crest level about 20 dB) was 13 dB, comparable to the values measured for a variety of non-helicopter sounds.

Annotated Bibliography 127 • The methods proposed [by ICAO in the late 1970s] to objectively compute a blade slap cor- rection factor do not appear to agree consistently with the correction factors measured sub- jectively to account for annoyance of blade slap. • Improved results are obtained if [ICAO’s proposed methods] are modified to account for variations in the frequency of the blade slap. Adjustments of 2 dB (for a blade slap repetition rate of 10 Hz) to 7 dB (for a blade slap rate of 30 Hz] might be appropriate. (These findings are discussed above in the annotation for Fidell and Horonjeff.) The dependency on rep- etition rates in this frequency range suggests that a blade slap “correction factor” may arise from inherent errors in perceived noise level computations for signals with significant energy below 50 Hz. The latter inference is not fully consistent with the observations of Fidell and Horonjeff (see above.) • ICAO’s proposed methods for predicting a subjective correction factor depend on some means of measuring the relative impulsiveness. These methods vary from a simple measurement of the crest level of A-weighted noise levels to more complex procedures involving sampling the detected signal (e.g., instantaneous A-weighted level) at a high rate (~5000 Hz) and computing a measure of mean square fluctuation level from these samples.