Helicopter Noise Information for Airports and Communities (2016)

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12 Measuring noise is complex. It is not the intent of this synthesis to delve into this topic in great detail, but to provide a broad overview. When discussing noise metrics it is common to talk about weighting factors. The two most common weighting factors are “A” and “C” weightings. These are frequency weightings used to adjust measured sound levels to how the individual perceives loudness at differ- ent frequencies. Frequency is measured in hertz (Hz) and it represents the sound frequency in cycles per second. Middle C (the centermost key on a piano) is approximately 250 Hz. People hear very well in the so-called middle frequencies (centered at 1,000 Hz) and poorly at low frequencies; for example, sound at 125 Hz would have to be approximately 16 decibels (dB) louder than a sound at 1,000 Hz for an individual to perceive them as having the same loudness. The commonly used DNL metric used for noise policy is based on the A-weighted decibel. The C-weighted decibel does not adjust the low-frequency sounds in such a significant way until the frequency is less than 25 Hz. For helicopter noise, the measured C-weighted level is significantly higher than the measured A-weighted level; on the order of 10 dB (Schomer 1987). Note that this discussion of the frequency weightings is based on perceived loudness. There are other scales for measuring human response to noise based on perceived noisiness. Aircraft noise cer- tification tests are based on Perceived Noise Level (PNL). The differences between the two systems resulted from psychoacoustic studies where people were asked to rate different noises; some studies were based on ratings in terms of loudness and others based on ratings according to noisiness. To the extent that excess annoyance of helicopter noise is attributable to the annoyance of rattle and vibration (to which A-weighted noise metrics are insensitive), A-weighted noise metrics are unlikely to adequately predict the overall annoyance of helicopter overflights of residential populations. The two frequency weighting networks and families of noise metrics are commonly employed in the United States to express sound levels of both fixed-wing aircraft and helicopters. For aircraft noise certification purposes, FAA’s required frequency weighting is the Tone-Corrected Perceived Noise Level, abbreviated PNLT, developed in the 1950s. For predicting and assessing environmental impacts of aircraft noise exposure, FAA endorses the A-weighting network, which dates to the 1930s, and is the weighting used in DNL that forms the basis of current FAA noise policy.4 Each metric such as Sound Exposure Level, DNL, or Maximum Noise Level, can be expressed in terms of A or C weighting and from instantaneous through annual time frames.5 Concern about the appropriateness of noise metrics for predicting annoyance derived from expo- sure to helicopter noise has arisen several times since the 1950s. As discussed in Appendix B, a 1982 literature review by Molino (1982) compares the findings of 34 earlier analyses of the annoy- ance of helicopter noise, the earliest of which date to the 1960s (cf. Crosse et al. 1960; Robinson and Bowsher 1961; and Pearsons 1967). The findings of these early studies are neither consistent nor definitive. They and other studies (e.g., Powell 1982) do not fully support Molino’s conclusion that there is “no need to measure helicopter noise any differently from other aircraft noise.” The common belief that rotary-wing aircraft noise is more annoying, on a decibel-for-decibel basis, than fixed-wing aircraft noise has led to the practice of imposing decibel-denominated penal- ties on A-weighted (but not PNL-weighted) measures of helicopter noise for purposes of assessing environmental impacts of helicopter noise. chapter four NOISE METRICS FOR QUANTIFYING HELICOPTER NOISE

13 The tactic of assigning penalties treats the assumed excess of annoyance of helicopter noise as a simple problem of measurement, while ignoring the underlying causes of the supposed excess annoyance. However, since the evidence supporting the assumption of excess annoyance is not definitive, the issue may not simply be one of physical measurement. The supposed excess could be attributable to operational factors (the characteristic shorter slant ranges and relatively longer dura- tion of helicopter flyovers vis-à-vis fixed-wing aircraft operations) rather than inherent differences in noise-induced annoyance. The supposed excess could also be entirely attributable to nonacoustic factors. Although much has been learned about the mechanisms that generate rotary-wing aircraft noise in different flight regimes since Molino’s 1982 review, it is only recently that systematic means have become available to focus more closely on potential nonacoustic factors that influence annoy- ance judgments (Appendix A3 provides greater detail).