Alternative Aircraft Metrics – Useful or like moving the deck chairs on the Titanic?

Widespread criticism of the ANEF system to predict aircraft noise impacts has lead to the use of supplementary tools (N70, NXX, 10+,TA, daily ANEF). Do these tools work? Are they of benefit to the community or the aviation industry? Do they add confusion? Are they appropriate for military aircraft or general aviation? Australian Standard AS2021 [1] considers in the first instance the suitability of a building site in terms of an ANEF value. By use of Table 2.1 (building side acceptability based on ANEF zones) the site can be classified as “acceptable”, “conditionally acceptable”, or “unacceptable” for the relevant building type. If a proposed site is in an ANEF zone above “acceptable” the acoustical assessment of the proposed development (by the methodology in AS2021) disregards the ANEF value as the assessment is then conducted in terms of the maximum Aweighted level. Appendix A of AS2021 provides information as to the basis of the ANEF system and refers to the NAL study [2]. The NAL report recommended a different weighting system to the (then) standard NEF assessment procedure used in the USA. The last two pages of the NAL report were clear as to the outcomes of the study as to describing excessive noise limits and community reaction by stating: to describe 20 NEF as an excessive amount of aircraft noise is to offer a reasonable interpretation of the scientifically determined dose/response relationship. Whether or not areas within this exposure are incompatible with residential zoning is another matter. As scientists, the authors are charged with describing community reaction to aircraft noise. The task of prescribing regulations and standards relating to land-use around airports properly belongs to legislative and planning authorities. They must translate the findings of the present investigation into practical guidelines. This translation will necessarily involve reaching a compromise between what is desirable in terms of the quality of life in a residential area, and what is practical given the demand for housing and many other facets of urban community management. The dose-response curve set out in AS2021 is slightly different to the curve recommended in the NAL study. The proposal for a third runway at Sydney airport generated debate on aircraft noise impact with general criticism of the use of the ANEF system for determining noise impacts. There was widespread criticism by the community in relation to the Third Runway draft Environmental Impact Statement [4] and the draft Noise Management Plan [3] as reported in the Senate Inquiry report into the Third Runway [5]. Chapter 8 of the Senate Inquiry, titled “An inaccurate prediction of noise impact”, discussed the limitations of the ANEF system and noted various submissions repeated the position that the ANEF is only an average value and therefore does not provide a complete picture. The ANEF is subject to variations in noise level as a result of changes in aircraft composition, weather, and wind conditions prevailing at a time. If there are large periods of time during the day when there are relatively few aircraft then at other times (so as to obtain the required average over the whole day as part of the ANEF system) there will be either a much larger number of aircraft movements or a grouping of aircraft that give rise to significantly greater noise levels. Accordingly an ANEF value does not indicate variations of noise exposure that residents may receive from one day to the next and throughout any day. Criticisms were made in the Senate Inquiry as to the relevance of the NAL report in that the period in time where aircraft noise intrusion for the NAL study was no longer valid, vis: 1979 – 1980 using aircraft types in of B707, B727, B747, DC8, DC9, DC10, F27, F28 and (for Richmond Air Base) C130, CC08, C141, having a different mix to aircraft operations in 1994. The most common types of aircraft in service at the time of the NAL study were Boeing 727s and Fokker F27s. In critising the NAL study the issue of the accuracy of the predicted noise contours (from predictions in late 1970s) versus actual field measurements has never been addressed. If the predicted contours were wrong, then the basis of the 23-27 August 2010, Sydney, Australia Proceedings of 20th International Congress on Acoustics, ICA 2010 2 ICA 2010 dose-response curves must be wrong. Cooper considered that issue [6] and concluded: “The INM program that was available at the time of the NAL study was significantly less sophisticated than the model that is available to date. The source material in terms of flight tracks, aircraft numbers, movements and types were not appended to the NAL report that could permit one to take the original data and model it with the knowledge gained from experience in operating an INM (and more recent versions of that model that are purported to be accurate). The NAL report indicates an assessment of the population based on noise contours that had been determined where the population had been placed in the various noise NEF zones. As the NEF zones determined in the NAL report were NEF3 and not the NEF3,6 that NAL recommended, then the contours used do not directly relate to the percentage population affected that appears in the Australian Standard. The NAL report is silent upon the matter of noise monitoring that was utilised for verifying the noise exposure zones so depicted in the report with respect to the aircraft movements used for the purpose of assessment. Furthermore, the NAL report is silent on the distribution of the population surveyed with respect to the NEF contours. Is the NAL report dose-response wrong? I say the report could overestimate the noise impacts. However, the dose-response curve set out in the Standard is not the same curve as recommended by the NAL report and is a dose-response curve that is lower than that proposed by NAL. Accordingly, it would be correct to attribute questions of the accuracy of the dose-response curve to the Department of Aviation, who modified the NAL dose-response curve to provide the curve set out in AS2021.” Chapter 8 of the Senate Inquiry report recommended the ANEF system be re-evaluated and NAL explore the development of indices or other information for predicting the noise impact. But NAL is no longer the eminent research group that existed in the late 1970s and such studies cost a lot of money. Cooper [7] proposed a mini-NAL study but was denied completing such work by academic issues. As a result of criticism of the ANEF system that was brought before a Senate Inquiry into Sydney Airport, additional acoustic descriptors/metrics have been suggested to supplement the ANEF contours so as to better describe aircraft noise impacts [8]. But those descriptors whilst primarily applied to domestic airports are not based on socio-acoustic studies and in some circles are considered to be politically motivated to say there is no aircraft noise impact. Because military jet operations result in a different aircraft noise exposure to that typically encountered at domestic or international civil airports, the noise exposure zones are subject to intermittent high levels of noise throughout the day and night. And as such represent a different relationship of maximum level versus exposure levels to that obtained for domestic/international jet operations. As a consequence of military operations, residential development in unacceptable areas (with respect to aircraft noise) and proposed residential release areas in proximity to a military aerodrome, there is some confusion as to the use or the validity of these additional noise metrics that have been proposed to assist an understanding of the ANEF. In seeking to provide residential development in areas that by reference to Australian Standard AS2021 should not be used for residential purpose the cost ramifications in terms of implementing noise control measures to achieve a satisfactory internal environment are considered by some to be excessive, with a suggestion that such noise controls are not required, or such controls may be reduced with respect to the required degree of attenuation. AS2021 and the ANEF System Australian Standard AS2021 “Acoustics – Aircraft Noise – Building Siting and Construction” was originally prepared based upon the American noise exposure system identified as NEF (Noise Exposure Forecast). That system considered the cumulative impact of aircraft operations for an entire year averaged over a day using various acoustic parameters and reliance upon a computer program. The system indicated various noise exposure levels related to the acceptability of such noise and the use of the site, which in general is for consideration of residential premises. With the introduction of jet aircraft to Australia the early form of jet aircraft had a significant difference in the noise signature when compared to propeller aircraft that operated at that time. Accordingly the community response to the introduction of these new aircraft was somewhat vocal and ultimately following various Inquiries a socio-acoustic study was undertaken by the National Acoustics Laboratory to consider the noise impact at various airports around Australia [1]. That study involved a social survey of sample populations around airports and analysis of the community’s response with respect to predicted noise contours around airports based upon a computer model of the aircraft noise exposure levels. The social survey covered the major capital city airports in Australia and included one military base at Richmond which was predominantly (and still is) an aerodrome operating propeller aircraft. There were some Boeing 707 refueller aircraft operating from Richmond at the time. The results of the social survey were assessed against various acoustic descriptors utilising the data averaged across Australia to recommend the use of an energy averaged assessment base (similar to the NEF system) but utilising different weighting factors for night time operations to account for Australian conditions. The NAL assessment curve provided the dose-response for persons affected by aircraft noise which is slightly different to that contained in the Australian Standard AS2021. 23-27 August 2010, Sydney, Australia Proceedings of 20th International Congress on Acoustics, ICA 2010 ICA 2010 3 Figure 1: Dose/Response Relationship Diagram