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Correcting the inappropriate noise impact conclusions of Massport's environment disclosures regarding Hanscom Field
Updated February, 2023
The noise data presented by Massport in their Environmental Status and Planning Reports (ESPR) is extensive and realistic but their findings of no impact of this noise on the historic and natural resources are arbitrary and without foundation in science, law, or common sense. This White Paper suggests a more appropriate criteria for assessing noise impact which, if applied, would substantially alter the conclusions of the ESPR. This paper was written in 2005, but the anaylsis and conclusions still hold.
Use of 65db DNL as a criterion for impact significance
The ESPR concludes that there currently is no "significant impact" on the natural and historic resources in the Hanscom area. In the scenario analysis, it further concludes that under the maximum 2015 High Growth Scenario that significant impact will be limited to only a single historic building. This entire conclusion rests on a single sentence in the report on page 10-14:
The entire conclusions of this report related to noise impacts are based on this one number, 65db DNL. Considering the overarching importance of this number, very little explanation of it is provided in the report. We believe this number is applied inappropriately and that the use of a more appropriate value would dramatically alter the conclusions of the report.
In the ESPR, Massport cites Federal Airline Regulations (FAR) Part 150 as the source for the 65db level. It is important to recognize the following points regarding this:
The Hanscom Field Noise Workgroup found that Massport's use of the 65db DNL level as a criteria to determine significance of impact in the prior GEIR was improper and recommended in the Workgroup report that this criteria not be used in the ESPR. In particular, the workgroup specified that other Federal and international impact levels be reviewed and considered, and that a clarification distinguishing impact level criteria for residential use from the levels for natural and historic sites was needed. No discussion or response is included in the ESPR.
For the first time in this ESPR Massport does provide some indication of the need to adjust impact levels based on ambient sound level. The graph D-1 in Appendix D does indicate that impact is a function of ambient sound level. However there is no discussion of this in the text, nor do the conclusions recognize this.
Massport's interest in the selection of the level to be used for assessing significance of impact is not dispassionate. The ability of Massport to develop Hanscom Field unimpeded by further environmental review or mitigation may depend on the selection of this criteria, placing great pressure on the Massport contractor creating the report to use a high value as the impact level used. The fact that conclusions of this report with regard to noise impact are inconsistent with the following public professional comments made by Massport's own noise consultant suggests that the selection of impact levels in the ESPR was not objective.
Background of 65db DNL criteria
The logical background of the 65db DNL value as a Federally recognized mitigation level for residential areas is described by Massport's noise consultant HMMH in the 1992 paper "LDN, Necessary but Not Sufficient". The following points are critical to understand and are summarized:
Related standards for impact significance
The ESPR omits the fact that 65db DNL is not the only, or even the most common, standard for noise impact used by the Federal government. Furthermore, no scientific or international body recommends this standard. Most Federal and International standards for the assessment of noise do use the DNL scale but they specify a lower standard for impact significance.
The following tables summarize the noise impact significance standards used by various Federal, scientific, and international bodies. In Table 1, the known agencies using the 65db DNL standard for residential use are shown.
1: Agencies specifying criteria of 65db DNL
Note that the USAF suggests the use of a normalization factor to account for low ambient conditions (USAF, 1957 Land Use Planning Guide). Note also that the FAA has a dramatically different standard for noise impact in a National Park. This standard is 3000 times lower in sound energy than the 65db DNL standard. The explanation and interpretation of this standard are provided in Appendix A.
In Table 2, agencies using lower standards for the assessment of significant impact are shown.
2: Agencies and Organizations specifying criteria less than 65db DNL
Note that this list contains 12 standard bodies that use a lower standard than the FAA 65db DNL criteria. The World Health Organization and the National Research Council should be considered the primary authorities on acceptable levels of pollutants because they are scientific organizations charged with this type of standard setting. Both of these agencies specify 55db DNL as a level of significant impact, and further they both suggest even lower levels for lower ambient conditions or sensitive noise receptors.
Placing DNL values in persective It is helpful to put the 65db DNL criteria into context. In Figure 1, a vertical scale of DNL values is annotated with various regulatory thresholds.
To appreciate the magnitude of the vertical scale, it is important to understand the following mathematical facts:
Hanscom would need to go to 1,600,000 operations before the sites currently experiencing 55db DNL would experience the 65db level that Massport claims should be used as the criteria for impact. This would be almost 3 times the number of operations of Logan Airport. Arguing that this is the criterion for "significant impact" is not sane or reasonable.
The ESPR uses a criteria which is only 5db away from the level which the WHO classifies as the level at which the onset of hearing damage begins. It is 15db, or 30 times higher in noise energy, than the 50db standard which is prescribed by scientific and regulatory bodies for rural residential environments.
Furthermore, the use of the 65db DNL criteria for significant impact in National or State Parks and Wildlife Refuges is completely inconsistent with any regulatory standard and is much higher than even the only FAA regulation for noise in a National Park.
Figure 1: Scale of DNL with regulatory citations and reference values
Appropriate standard for impact significance
In adopting an appropriate standard for assessing impact levels in the Hanscom environment the following factors must be incorporated:
The available data and standards suggest that the following criteria would be appropriate for assessment of impact significance in the Hanscom vicinity:
For residential and natural/historic impact assessment, two alternate methods are described. It is likely that the alternate methods will provide results similar to, but not exactly the same as, the described DNL method in the Hanscom environment. In particular, the DNL method is more sensitive to very loud events with a low frequency of occurrence, while the TA method is more sensitive to total event count.
Figure 2: Noise impact area using the TA method
Figure 2 shows that Massport's own data using the TA method show that the noise impacts on Historic sites will be increased signficantly under Massport airport growth plans. This figure was generated by the noise report in 2002. It is not current, but can still be generated any time Massport runs the noise model, but Massport has declined to produce it in subsequent reports.
The use of these criteria would not substantially alter the detailed data presented in the ESPR. However, it would dramatically alter the conclusions based on that data. We suggest that the report conclusions be deemed not valid unless adjusted using the above criteria for assessment of noise impact.
We have demonstrated that:
We have provided alternative criteria for the assessment of impact, which are based on science, law, and common sense. The application of these criteria would provide a truthful and accurate disclosure of the current impact and of the future noise impact of Hanscom Field on natural, historical, and residential sites.
Grand Canyon impact level
The impact levels established for the Grand Canyon National Park are not expressed in DNL, and some assumptions are needed to refer them to DNL values. Note that in the Grand Canyon it was found that DNL was not an appropriate metric, similar to the findings by the Hanscom Field Noise Workgroup that DNL is not an appropriate metric. Nevertheless, it is possible to make rough conversions between noise impact metrics, as was required to create Figure 1.
The Grand Canyon rules were established in accordance with a 1987 Congressional Act to:
A noise methodology to establish standards for measuring significance of impact for a National Park are established in 64 Fed Reg at 3971. In summary, two impact levels are defined:
Note that these standards do not specific impact levels, but establish levels relative to the ambient sound environment. The need to adjust the significant impact level with ambient background is consistent with the recommendations of the Hanscom Field Noise Workgroup and of most Noise Professionals, including Massport's own noise consultants HMMH . For a given environment, it is possible to estimate the DNL values that correspond to the impact levels established by this Federal regulation.
Using the Integrated Noise Model, the impact areas described by this regulation could be determined for Hanscom Field. Absent this analysis, it is possible to roughly estimate the DNL values corresponding to impact significance for a National Park environment.
The ambient sound level in the Hanscom area, as measured by L90, varies depending on location but in the rural areas is on the order of 30-35 db. Therefore, for significant impact in "remote" areas the appropriate noise contour would be the Time Above 28db 180min contour (180 = 25% of a 12hr std day as defined in the regulations) For significant impact in the "more developed" areas the appropriate noise contour would be the Time Above 38db 180 min contour. These contours are not available in the ESPR; however it is possible to roughly estimate their size using the data in the report. Comparison of the TA 65 and TA55 contours in the ESPR shows rough equivalence in terms of enclosed area for the year 2000 30min TA55 contour and the 90min TA65 contour. This establishes a rough relationship of 3x in duration for every 10db TA. Using this relationship the "remote" impact level TA contour area would be extrapolated to be approximately equal to a 13min TA55 contour, and the "more developed" impact area would be approximately equal to a 40min TA55 contour. While these contours are not in the draft ESPR, a 30min TA55 is included, from which the approximate size of the 13 and 40 minute contours can be estimated. The "more developed" impact area thus obtained would be significantly larger than 55db DNL contour, the lowest DNL reported in the ESPR.
Note that this analysis is based on shifting the impact levels up due to the higher background levels in the Hanscom environment. The actual dbA levels used to assess impact in the Grand Canyon are lower by at least 10db, and the corresponding DNL values used are also at least 10db lower.
The noise impact standards resulting from the National Parks Overflights Act express impacts relative to ambient sound level. These standards can be rationally applied to National Park sites and other noise sensitive sites surrounding Hanscom Field including Great Meadows National Wildlife Refuge, Walden Pond, and large conservation areas like Estabrook Woods. Such application would utilize Time Above contours, and not DNL contours, to describe impact areas. In a discussion of impact limited to utilizing the DNL metric, the Overflights Act standard can be approximated in two parts: a DNL approximately 5db greater than the background ambient sound level for remote areas, and a DNL approximately 15db greater than the background ambient sound level for more developed park areas (where background ambient is measured, as prescribed in the regulation, by L90).
The Integrated Noise Model is fully capable of predicting impact areas as defined in the Overflights Act. Such analysis would provide a more accurate description of the impacts than the estimation and extrapolation contained in this appendix. Nevertheless, absent this analysis the impact areas can be estimated, and these estimates are much better than the arbitrary use of 65db DNL for determining impact.
WHO impact levels
The WHO noise levels on Figure 1 are not exact DNL values because the WHO document expresses noise levels using the Leq scale, which is nearly, but not exactly, the DNL scale. DNL is computed by using Leq daytime values, but nighttime Leq values are given a 10db increase prior to averaging. Therefore, if the DNL is mainly due to daytime noise, then Leq and DNL are approximately equal. If the DNL were dominated by nighttime noise, then the DNL would be 10db higher than the Leq value. This means that placing Leq values on the DNL scale as in Figure one is only accurate if the DNL is dominated by daytime noise. Primary contribution to DNL from daytime noise is a realistic assumption for the Hanscom environment. However, the existence and expansion of nighttime operations at Hanscom changes this assumption. A detailed analysis using the Integrated Noise Model could be used to provide a more exact placement of the Leq WHO values on the chart of Figure 1. It is expected that such analysis would result in moving the corresponding WHO impact levels up by less than 2db. Summary: the WHO impact levels are not exactly expressed in DNL, but the error for Hanscom Field will be only a few db.