This year the National Park Service (NPS) celebrates its centennial, having been established by the passage of the “Organic Act” in 1916. According to the Organic Act, a main purpose of the NPS is to conserve “parks, monuments, and reservations” in such a manner that “will leave them unimpaired for the enjoyment of future generations.”
Since 1916, it has become increasingly challenging to conserve the acoustical environment, or “soundscapes,” of NPS properties. When considering the development of a new project near an NPS-managed property, the potential impact to the acoustical environment will need to be examined using an approach that considers the natural soundscape – which is somewhat atypical relative to most environmental noise evaluations in the U.S. Unlike typical state, county, or municipal noise laws around the U.S., which often include quantitative limits to the environmental sound level, the NPS Soundscape Management policy simply states that the NPS “will preserve, to the greatest extent possible, the natural soundscapes of parks.” (See Section 4.9 of NPS Management Policies 2006 PDF.)
In order to preserve natural soundscapes, acoustical environmental impacts need to be minimized, possibly to a far greater extent than would be the case to meet a municipal sound level limit. In a quantitative sense, this means limiting – or possibly eliminating – increases to existing ambient sound levels.
Ambient sound levels in some National Parks could be very low during the nighttime, so preserving the natural soundscape could be quite challenging. Quantifying the existing ambient sound levels typically requires an ambient sound level survey. If the survey needs to take place on NPS land, a permit could be required to gain access to a measurement site (such as a campground) and set up sound level measurement equipment for a period of several days.
For projects where a survey has not been or cannot be conducted, the NPS has developed a rather unique workaround. For roughly the last two decades, the NPS has been measuring long-term ambient sound levels at many of their properties. The result of this effort was summarized in a May 2014 article in the Journal of the Acoustical Society of America (JASA) by Daniel Mennitt, Kirk Sherrill and Kurt Fristrup. Utilizing over 250,000 hours of ambient sound level measurement data from locations on 190 NPS properties, the NPS has developed a geospatial model of ambient sound levels. The model includes geospatial data for roughly 60 other independent variables that can affect sound propagation, including geographical/topographical, meteorological, ground cover, hydrological, anthropogenic, seasonal/temporal, and equipment variables.
The model results enable the NPS to quantify the ambient sound levels with a high degree of accuracy at any of the more than 400 properties they manage – even properties where they have not previously measured sound levels. And the data extend beyond NPS properties to cover the entire U.S. The JASA article generally demonstrates good agreement between modeled and measured sound levels, with the exception of very remote rural areas (modeled sound levels higher than measured) and dense urban areas (modeled sound levels lower than measured).
The model results for the U.S., including Alaska and Hawaii, are gradually being made available for public use through the NPS website. The data are seasonal, long-term averages of statistical sound level parameters, and the resolution is coarse (a roughly 1000 foot by 1000 foot grid), as would be expected for a national sound level map. As of this writing, the modeled summertime, daytime, median sound levels have been made available. The NPS plans to release wintertime and nighttime data in the future. The public availability will enable professional acousticians around the country to compare the modeled results to their own measurements.
Assuming agreement between modeled and measured generally holds up (or improves) over time, the data could eventually prove useful for early assessments of any project in the U.S. – even projects that are not situated near NPS properties. While it would not eliminate the need for site-specific sound level surveys, a review of the data could prove useful for a screening-level or site-selection assessment that takes place in the early stages of project development. The modeled ambient sound levels could be reviewed prior to site visits to gain an early understanding of the potential environmental noise impact from a project. In some cases, this could lead to early exclusion of potential project sites, potentially saving travel and assessment costs.
Published originally on Black & Veatch Solutions.
